<title>Locust handbook</title>
Natural Resources Institute (NRI)
Overseas Development Administration
<section>Acknowledgements</section>
© Crown Copyright
First published by The Anti-Locust Research Centre 1966
Second edition published by the Overseas Development Natural Resources Institute 1988
Third edition published by the Natural Resources Institute 1990
The Natural Resources Institute (NRI) is the scientific arm of Britain's Overseas Development Administration. NRl's principal aim is to increase the productivity of renewable natural resources in developing countries through the application of science and technology. Its areas of expertise are resource assessment and farming systems, integrated pest management and food science and crop utilisation.
Short extracts of this publication may be reproduced, providing the source is acknowledged:
STEEDMAN, A. (Ed.) (1990). Locust handbook. (3rd edn) Chatham: Natural Resources Institute, vi + 204pp.
Permission for commercial reproduction should, however, be sought from the Head, Publications and Publicity Section, Natural Resources Institute, Central Avenue, Chatham Maritime, Kent ME4 4TB, United Kingdom.
ISBN 0-85954-281-5
Price £30.00 including postage and handling
No charge is made for single copies of this publication sent to governmental and educational establishments, research institutions and non-profit making organisations working in countries eligible for British Government Aid. Free copies cannot normally be addressed to individuals by name but only under their official titles.
The Natural Resources Institute is indebted to the Food and Agriculture Organization of the United Nations for the use of the locust frequency maps used in Figs 65-88. In addition, NRI is especially grateful to Dr P. M. Symmons of the FAO for much of the information used in Chapter 7, Controlling Locusts.
NRI would also like to thank Micronair Ltd. Micron and CDA and Lockinge Farm Enterprises for providing material on their sprayers which are described in the Appendix.
The contributions and suggestions of the following NRI staff offered during the production of this edition of the handbook are acknowledged:
R. Allsopp, E. M. Ambridge, R. A. Cheke, 1. F. Grant, V. P. Howe, N. D. Jago, D. R. Johnstone, J. 1. Magor, G. R. Manners and D. R. Reynolds.
The Overseas Development Administration provided funds to cover the costs of production for this handbook.
<section>Introduction</section>
The first edition of the Locust Handbook was published in 1966, shortly after the end of the last major plague of the Desert Locust. The second edition was produced in response to another upsurge in the numbers of Desert Locusts and, for the first time for 50 years, the simultaneous appearance of swarms of African Migratory Locust, the Red Locust and the Brown Locust. In addition, significant populations of grasshoppers have been damaging crops in West Africa. Demand for the second edition was such as to warrant this third edition. It is timely as recently there have been grasshopper outbreaks on an unprecedented scale in the Sahel and Oman.
Locusts and grasshoppers remain a potent factor in the agricultural environment of many countries and plant protection departments need to be vigilant because of the locusts' potential to reduce crop yields. This edition of the handbook is designed to help those involved with locusts and grasshoppers to understand them and to assist in their control. The opportunity has been taken to update many of the maps and to include more information on grasshoppers. The emphasis is on those species which do most damage. Readers who require information on the complete range of locust and grasshopper pests should consult the Locust and Grasshopper Agricultural Manual (COPR 1982) and those who are responsible for detailed forecasting of the Desert Locust should consult the Desert Locust Forecasting Manual (COPR 1981). Relevant information from both manuals has been included here.
This handbook deals more with the Desert Locust than other species but much of what is said about this locust is applicable to all species. Information on biology, distribution and movement is included with particular reference to those matters which directly affect control or survey practice.
Locusts have the capacity to live in a wide range of habitats, and consequently exhibit great variation in the details of their life cycle and behaviour; this plasticity of behaviour adds to their interest but in this book it is neither possible nor desirable to describe all this variation in detail. Here we are concerned with what usually happens, telling the story of the majority of cases and giving an account which is sufficiently accurate for most practical purposes.
A handbook cannot be a substitute for practical training and experience in the field, but it can provide a source of basic data for those who are just beginning to work on locusts and those whose duties concern anti-locust work only occasionally. It is hoped that the handbook will also serve to indicate to the more experienced officer the detailed observations on particular aspects of the problem that are needed to supplement existing knowledge. It is expected that many of the readers of the handbook will be people who not only actually do anti-locust work, but also have the opportunity to observe locusts in areas seldom if ever visited by research teams. Such people are in a position to make careful observations on locusts which will help anti-locust organisations to learn more about the pests, and so help to develop new and better methods of killing them and ultimately preventing new locust plagues arising.
<section>1. What are locusts?</section>
Locusts belong to a large group of insects commonly called grasshoppers, which are recognisable by the big hind legs used for jumping. All grasshoppers (except the so-called long-horned grasshoppers) belong to the super family Acridoidea, and the most important locusts are all in the family Acrididae. Locusts are special grasshoppers, usually large ones, which have a capacity for changing their habits and behaviour when they occur in large numbers. When their numbers rise they become gregarious in habit and stay together in dense groups. These groups are called swarms when they are composed of adults and bands when they consist of the wingless young stages, commonly called 'hoppers'. The swarms of several species of locust can migrate over great distances, and this and gregarious behaviour are the outstanding characteristics that distinguish typical locusts from other grasshoppers. When locusts are in small numbers they live their individual lives like ordinary grasshoppers.
There are several different kinds of typical locusts capable of forming large migrating swarms, but there are also several other species of grasshoppers which do not normally live in vast crowds but which have the capacity to multiply rapidly and produce groups or swarms in special circumstances. These sudden population rises may be started by unusual weather conditions or changes in land-use; they may not actually result in large migrating swarms but the groups formed will stay together in a typical locust way. Such species may be regarded as being intermediate between grasshoppers that live alone and typical locusts that often do not.
<section>Locust and grasshopper distribution</section>
The green areas in Fig. 1 indicate the parts of the world where crops may suffer damage caused by locusts and grasshoppers. The area covered by this handbook is enclosed by a black line. Some areas may be liable to attack by only one species while in other areas several kinds may be serious pests. The species covered in this handbook are listed below; an individual distribution map accompanies the description of each species.
Desert Locust
Schistocerca gregaria
African Migratory Locust
Locusta migratoria migratorioides
Oriental Migratory Locust
Locusta migratoria manilensis
Red Locust
Nomadacris septemfasciata
Brown Locust
Locustana pardalina
Tree Locusts
Anacridium spp.
Bombay Locust
Nomadacris succincta (formerly Patanga succincta)
Sudan Plague Locust
Aiolopus simulatrix
Senegalese Grasshopper
Oedaleus senegalensis
Rice Grasshopper
Hieroglyphus daganensis
Variegated Grasshopper
Zonocerus variegatus
Javanese Grasshopper
Valanga nigricornis
Other Sahelian grasshoppers
Diabolocatantops axillaris (formerly axillaris) Catantops
Kraussaria angulifera
Cataloipus cymbiferus
Cataloipus fuscocoeruleipes
<section>Damage and losses caused by locusts</section>
Locusts have probably been an enemy of man ever since he began to grow crops. The Desert Locust is mentioned in ancient writings such as the Old Testament of the Bible and the Koran. Carved images of locusts have been found on Sixth Dynasty (2420-2270 BC) tombs at Saqqara in Egypt. Locusts are still a great enemy of the farmer and in some countries they are the determining factor between sufficient food for the people and starvation. Damage is sometimes diffuse and not very obvious, but it can be very severe in many more restricted areas. This depends on whether the swarms are moving about quickly or whether they stay for several days in one area.
The number of people in the world is increasing by about 220,000 every day, so that more and more crops must be grown to feed them. No one wants to grow more crops to feed locusts. Table 1 gives examples of crop losses caused by locusts.
TABLE 1
Year
Country locusts (in £ sterling)
Value of crops destroyed by(in £ sterling)
1986 value
India
400,000 per year
6 million
1928 and 1929
Kenya
300,000 per year
4.5 million
Somalia (Southern Region)
Morocco
4,500,000 in a single season
40 million
FAO estimate for only 12 out of 40 affected countries
1,500,000 per year; in 1955 over 5,000,000
45 million
It is even more significant to reckon the losses in terms of quantities of actual food or other crops and the examples in Table 2 show how serious these can be.
TABLE 2
Year
Country
Amount of crops eaten by the Desert Locust
Libya
7,000,000 grapevines; 19% of total vine cultivation
Sudan
55,000 tonnes of grain
Senegal
16,000 tonnes of millet, 2000 tonnes of other crops
Guinea
6000 tonnes of oranges
Ethiopia
167,000 tonnes of grain, which is enough to feed 1,000,000 people for a year
India
4000 hectares of cotton (value £300,000)
These are not all the figures available for damage by locusts; they are only a few examples and much more information is required about exact crop losses. There are several reasons why locusts are able to do so much damage.
1. They eat a wide range of food.
2. Each one eats its own weight of food every day. This increases gradually from the small hoppers to the adults and reaches a maximum of about 2 9, two or three weeks after fledging. Young swarms of this age cause the most severe damage.
3. There are often so many of them together. We know that there can be at least 40 million and sometimes as many as 80 million in each square kilometre of swarm. Figure 2 shows a small part of a swarm which measured over 1000 km², and therefore contained about 40,000 million locusts weighing about 80,000 tonnes. (Half a million locusts weigh approximately 1 tonne.)
One tonne of locusts (a very small part of an average swarm) eats as much food in one day as about 10 elephants or 25 camels, or 2500 people.
Locusts do damage by eating the leaves, flowers, fruits, seeds, bark and growing-points, and also by breaking down trees because of their weight when they settle in masses, and sometimes even by spoiling plants with their excrete. They do not, as far as we know, carry any disease but some laboratory workers have developed an allergy to them.
An analysis of 2000 records of Desert Locust damage shows that: 8% of the damage is done by hoppers, 69% by immature and maturing swarms and 23% by mature swarms. The figure for hoppers is low because the breeding areas are mostly outside the main crop areas.
Damage by the Desert Locust
The following illustrates the nature and degree of damage to both food and cash crops that can occur in the Desert Locust invasion area.
Bulrush millet
Bulrush millet (Pennisetum americanum) is a staple grain crop along the southern edge of the Sahara and in the Indo-Pakistan desert. It is much liked by the Desert Locust as a food plant and since it is grown extensively in areas which are highly frequented by this locust for breeding, considerable damage is caused; both leaves and ripe grain are destroyed (Fig. 3).
Sorghum
The staple food crop in areas receiving a slightly higher rainfall than the Pennisetum zones. Many varieties of this crop are not greatly liked by the Desert Locust during their main period of growth but the ripening grain of most varieties is readily attacked. Heavy damage is caused, mainly by newly fledged swarms of the summer generation.
Maize
Usually grown as a staple food crop in areas which are either too cool or too wet for the Desert Locust, but in parts of eastern Africa where it is grown under hotter and drier conditions than is customary for this crop, heavy damage does occur, the plants often being entirely defoliated and the cobs eaten away (Fig. 4).
Wheat and barley
Staple food crops in the spring breeding areas of the Desert Locust where they can be severely damaged, especially when they are approaching harvest. At this stage locusts bite through the last remaining moist part of the plant, the section of stem just below the ear, causing complete loss of grain, often without attacking the ear itself.
Rice
Comparatively little damage is done to irrigated rice even in areas highly frequented by the Desert Locust, probably because the artificially wet conditions in which it is grown are not liked by this locust.
Sugarcane
The effect of damage varies according to the stage of cane growth and the variety; for example, in Pakistan damage is greatest during the first four months of cane growth.
Cotton
This crop can be severely attacked. The effect of damage on yield is great if it occurs just before flowering but less if it occurs afterwards. In the Desert Locust summer-breeding zone the start of cotton flowering generally coincides with the fledging of adult locusts and as the young adult is the stage at which most feeding takes place, this increases the danger to cotton in these areas.
Coffee
Coffee is rarely attacked. Occasionally defoliation of bushes occurs, but locusts do most damage at the flowering stage or when they settle on bushes in such large numbers that their weight breaks the branches.
Fruit trees
These are particularly vulnerable to attack by immature swarms which have a preference for roosting in trees. Serious damage has occurred on oranges, lemons, pawpaw, dates and grapevines. Once damaged by locusts, the trees are liable to have their fruit yield affected for more than one year. Orange trees were very severely attacked in 1954-1955 in Morocco (Fig. 5).
Grasslands and rangelands
It is not always fully realised how much damage can be done by locusts to natural grassland because it is generally less noticeable than damage to crops. We know that in the
United States grasshoppers, at a density of only 2/m² can eat approximately 2 kg of grass/ ha/day. Locust densities can be as much as 15 times higher than this, so that 30 kg/ha of grassland may be lost each day. This is particularly important in areas where rangeland is not very good anyway and there can be widespread losses of cattle.
Damage by other species
African Migratory Locust
During plagues, the last of which ended in 1941, swarms of the African Migratory Locust invaded most of Africa south of the Sahara. Throughout this invasion area there are extensive areas of cereal crops which are liable to severe damage. For example, in West Africa in the late 1920s and early 1930s, when there were also plagues of Desert Locusts and Tree Locusts, the Migratory Locust was the most voracious crop pest and in East Africa, where there were plagues of the Desert Locust and the Red Locust, it had a similar reputation.
The Migratory Locust feeds mainly on grasses. During recessions it feeds on wild grasses and cereal crops in and around the flood plains of the Middle Niger that form its main outbreak area.
Millet crops are particularly liable to damage, especially the bulrush or fox-tail millet and sorghum. Rice can also be severely attacked and it was reported that in 1930 40% of the rice crop in Guinea was destroyed. Non-cereal crops are damaged much less frequently, usually when wild grasses are dry or where cereal crops are not available. They include: sugarcane, palms, pineapple and less frequently pigeon pea, cabbage, carrot, cassava, coffee, cotton, groundnut, kidney bean, hyacinth bean, lettuce, Lima bean, pawpaw, pea, potato, turnip and yam. In Guinea 30-40% of the banana crop was lost annually from 1931 to 1934 because locusts ate the foliage, thus removing shade from the fruit. In general there is little damage to trees, either from feeding or breakage by the weight of settled locusts, because, unlike the
Desert Locust, swarms of the African Migratory Locust normally settle on low vegetation or bare ground.
Red Locust
The last major Red Locust plague lasted from 1930 to 1944 and during two of those years South Africa reported spending some £933,000 on crop and grassland protection measures.
The Red Locust feeds principally on grasses and damages cereals and sugarcane. Other crops attacked include: citrus, fruit trees, cotton, legumes, palms, root crops, tobacco and vegetables. Damage has been reported throughout the invasion area and the branches of trees can be broken by the weight of roosting locusts.
Brown Locust
The Brown Locust is a pest in southern Africa. It feeds on grasses and is an important pest of pasture. When these are not available it feeds on cotton, lucerne, potato, legumes and citrus, particularly lemon which can have both leaves and bark attacked. In 1964 the mean annual expenditure on control in South Africa was reported to be between £100,000 and £200,000.
Tree Locusts
Tree Locusts are not major pests. Anacridium melanorhodon has been recorded as being a significant but occasional pest of millet from Senegal to Sudan. In the Sudan it damages Acacia Senegal (the tree from which gum arabic is obtained), cotton, fruit trees, dates and shade and ornamental trees. Anacridium aegyptium is a minor pest throughout the Middle East and North Africa where it has been recorded damaging tobacco, grapevines, dates, vegetables, fruit and other trees.
Grasshoppers
The group of West African grasshoppers (see Chapter 4) are nearly all grass feeders and are pests of cereal crops, especially millet and sorghum. The exception is the Variegated Grasshopper, which can be a major pest of cassava.
Oriental Migratory Locust
This locust causes damage throughout its distribution area in the low-lying grasslands of Southeast Asia. Unfortunately there are few estimates of the value of crops destroyed. In the Philippines damage estimated at 10 million pesos was reported for 1911-1912 with damage of varying intensity every year from 1925 to 1934.
While grasses are the preferred food it will feed on a wide range of crops including bamboo, banana, barley, beans, citrus, coconut, groundnut, lettuce, maize, millet, pea, pineapple, rice, sago, palms, sisal, sorghum, soyabean, sugarcane, sweet potato, tobacco and wheat.
Bombay Locust
The Bombay Locust was reported doing most damage at the turn of the century in India. At that time crop damage was almost as great as that caused by the Desert Locust. In 1903 locusts were reported to be dispersed over an area of 350,000 km². There have, however, been no reports of swarms of Bombay Locust since 1908. Elsewhere it damages coconut leaves in the Laccadive Islands and in Thailand is a pest of maize. Significantly, although the maize lost due to locust action is only a small proportion of the crop, estimated to be 1%, this amount has a substantial cash value as the maize is exported. It is therefore worthwhile for the Ministry of Agriculture to carry out control.
Both hoppers and adults feed on a wide variety of plants including bamboo, banana, betel nut, millet, cashew, cassava, castor, chinese cabbage, citrus, coconut, cowpea, cucumber, durian, fig, ginger, groundnut, guava, lime, maize, mango, mulberry, mung bean, mustard, oil palm, orange, rambutan, rice, rubber, sorghum, soyabean, sugarcane, sweet potato, talipot palm, tea and tobacco.
Javanese Grasshopper
This grasshopper is not a major pest but does considerable local damage to a wide range of cash and tree crops mainly in Java and West Malaysia. Most records of damage concern leaves, growing shoots and young plants. Teak is attacked and the largest infestation recorded was some 5000 km² in the teak forests of central Java in 1915. Almost all the plants damaged by the Bombay Locust are attacked by the Javanese Grasshopper which also damages cover crops.
Conclusion
While locusts have been reported doing damage to a wide variety of crops it is difficult to obtain detailed information. It would appear that this is an area for future study so that only populations which really need to be controlled are sprayed. It is now widely recognised that the careless and liberal use of pesticides can lead to the poisoning of the environment both locally and in areas far from the original site of application. Therefore it is vital that all those responsible for locust control are fully informed of the ecology of the pest in their area so that spraying is only done when necessary and at the time when the minimum amount will have the maximum effect.
<section>2. Desert Locust-Schistocerca gregaria</section>
The Desert locust is probably the most important locust species because it has a vast invasion area of some 29 million km², affecting 57 countries (Fig. 6). This is more than 20% of the total land surface of the world. During plagues the Desert Locust has the potential to damage the livelihood of a tenth of the world's population. Fortunately there are not plagues every year; they occur intermittently (Fig. 7). Recent plague years are 1926-1934,1940-1948, 1949-1963,1967-1969 and 1986-1989. Between plagues the Desert Locust occupies a smaller area known as the recession area, where it lives in small scattered populations. However, with suitable weather conditions, which include sufficient rainfall, these scattered populations are concentrated where they can breed successfully, leading to a vast increase in numbers of insects so highly mobile they may travel up to 1000 km in a week.
<section>Anatomy of a locust</section>
The basic anatomy of an adult locust is described here and illustrated by photographs of the Desert Locust (see also Plate 2).
External anatomy
The body can be divided into three main parts: head, thorax and abdomen (Fig. 8).
Head
On the head can be seen:
1 A pair of jointed antennae or feelers which the locust uses to recognise things by touch or smell.
2 A pair of large compound eyes which give the locust a wide field of vision and enable it to detect movement easily. It is not known how many colours locusts can recognise but it has been shown that they react to green.
3 The mouth which has several parts that can easily be separated and identified. These are the upper lip, a pair of hard, black, serrated jaws which move sideways to cut through plant food, a pair of secondary jaws which help in holding the food, and a lower lip.
4 Jointed appendages which are called palps. These are used for tasting food.
Thorax
This is the part of the body which contains the muscles for walking, jumping and flying, and to which the wings and legs are attached.
On the thorax can be seen:
5 A sheath covering the top and sides of the front part of the thorax. It is called the pronotum.
6,7, 8 Three pairs of legs, the hind legs being large and used for jumping. Each leg has three main parts, the femur (9), the tibia (10) and the tarsus or foot (11).
12 Two pairs of wings. At rest the harder front wings cover and protect the softer hind wings which are folded fan-wise.
Abdomen
On the abdomen can be seen:
13 The ear, on the first section of the abdomen just behind the first joint of the large back legs. This is where the locust receives sounds. Locusts can hear one another up to about 2 m apart.
14 The ovipositor valves in the female. These are two pairs of short, curved, black hooks which form the tool with which the female locust digs a hole in the soil when the eggs are laid. This is how the female can be distinguished from the male, as the male does not have these hooks (Figs 9 and 10).
Along the sides of the thorax and abdomen are small openings called spiracles (15). These are holes through which the locust breathes. The largest spiracle on each side is just above where the middle pair of legs joins the body and can be seen opening and closing if a live locust is examined. The spiracles lead to very fine tubes which carry air directly to all parts of the locust's body; these tubes, which appear as slender silvery threads when you examine the internal organs, constitute the tracheal system.
Cuticle
The locust is covered with a special kind of skin which is referred to as the cuticle. It has three different layers. The layer nearest the inside of the body is soft and flexible; then comes a harder layer and on the outside is a thin layer of wax. This wax makes the skin waterproof and its presence also means that insecticides required to kill locusts by contact action should be dissolved in oils which will penetrate the wax. The hard part of the skin serves as the skeleton of the locust and is thinner at the joints so that movement can take place.
On many parts of the skin very fine short hairs can be seen. These are connected to nerves inside the body and serve in many ways to make the locust aware of the conditions in which it has to live. Those on the face detect air movement so that the locust can take off and land into the wind.
Inside the locust's body can be seen a dark-coloured tube running from the mouth at the front to the anus at the hind end (Fig. 11). This is the food tube or gut. There is always some material in the gut and it is not possible by merely looking at the gut to decide whether the locust has been feeding or not. The more feeding a locust does the quicker the food passes through the gut. The usual length of time is from 0.5 h to 2 h, but it can take 3-4 days if there is little food available. This allows the locust to withstand long periods of starvation.
The front part of the gut is wider than the remainder; it is concerned with grinding and storing food. The middle part is where digestion takes place and the hind part is where the water is absorbed. The waste is passed out at the anus as small dark-coloured faecal pellets, 4-5 mm long and 2 mm thick. These can often be seen in vast numbers beneath bushes and trees where locusts have been feeding and can provide evidence of the recent presence of locusts even though the locusts themselves have departed. After digestion some of the food material is stored in the body as fat, which can be seen inside the abdomen when a locust is cut open. It is a soft, yellow, shapeless mass, and it provides fuel for activities such as marching and flying. Flying locusts use up this fat at the rate of about 14 mg (0.8% of the body weight) per hour. A complete lack of fat inside the body of a fully grown (not a fledgling) locust means that it has probably been flying a long time without
feeding. A large amount of fat means that it has probably been feeding considerably without much flying.
Many very fine silver-coloured tubes can also be seen inside the body. These are the air tubes or tracheae already mentioned.
In a mature female locust the yellow eggs are conspicuous and arranged in rows in the ovary. When fully grown the eggs of the Desert Locust are each about 7 mm long.
If red spots can be seen in the ovary this usually means that the female locust has already laid at least one egg pod. It can, however, mean that some eggs started to develop and then stopped because of unfavourable conditions.
<section>Life cycle</section>
The life cycle comprises three stages: egg, hopper, adult (Fig. 12). The time spent in each stage varies considerably depending on the weather. This is discussed in more detail in the the section on seasonal movements and breeding areas (page 36).
Immature adults
Immature adults are usually pink, lighter or darker according to whether the locusts have been bred under high or low temperatures. The bright pink may change to a brownish red if the locusts have spent more than two months in this immature stage.
Maturation
Desert Locusts may become sexually mature in a few weeks or a few months, according to environmental circumstances. In unfavourable weather and food conditions, as for instance when they are subjected to low temperatures and drought, maturation may take as long as six months. If they have the right kind of food and weather, maturation can take place rapidly in 2-4 weeks. The exact conditions that cause locusts to mature are not known but the process is usually associated with the start of the rainy season. Male locusts start to mature first and then give off from their skin a chemical substance the odour of which causes maturation to start in females, and also in any males in which it has not already begun. The beginning of maturation can be recognised by the disappearance of the pink colour from the hind tibia. At this stage yolk is deposited in the eggs. It is at this stage that the eggs present in the female locusts begin to accumulate yolk and as they grow to full size
over the next week the abdomens of the females become distended.
Mature adults
The mature adult is yellow, the males being a brighter yellow than the females. The ovaries of the female locusts contain eggs which can easily be seen if the abdomen is pulled away from the thorax. At this stage large swarms break up into smaller ones, as those locusts that mature first settle on the ground for breeding, while those not yet quite mature fly on.
Copulation
This is the mating act. The male jumps on the back of the female and holds on to her with the front pair of legs (Fig. 13). The tips of their abdomens come into contact and the male sex cells (spermatozoa) are passed into the body of the female where they fertilise the eggs. The time spent in copulation varies from 3 to 14 h. Several females can be fertilised by one male and the spermatozoa can be stored inside the female's body and used to fertilise more than one set of eggs. Sometimes there are many more males than females in a mature swarm and then fighting occurs amongst the males for possession of females.
Laying and eggs
When copulation ends the males usually remain for some time on the backs of the females. The females become restless and walk about carrying the males. They begin to select a suitable place to lay their eggs by probing and testing the soil with the tip of the abdomen. During this probing they can detect warmth, hardness, moisture and salinity (salt content) of the soil. They are also attracted to each other at this time, assembling together in groups. Selection of laying places then depends partly on the soil conditions and vegetation and partly on the presence of other locusts. Laying can occur at all times of day and night provided that the soil surface does not become too hot or too cold, and that the soil is moist, at least below the surface. Laying can also occur in a wide range of soil types varying from quite coarse sand to silty clays, but the female must be able to dig into the soil with the extremity of her abdomen. Generally the top layer, about 6 cm deep, is dry,
and there is a layer of damp soil below. This must be sufficiently deep to take all the eggs, that is, about 4 cm.
When a suitable place is found the female pushes the ovipositor into the soil and makes a hole. The abdomen stretches to about twice its normal length and the eggs are laid (Fig. 14). The whole process of probing, digging and laying takes 1.5-2 h. A copulating and laying swarm usually stays in the same area for 1-2 days. Sometimes copulation occurs with females which appear not to be fully mature, that is, females in which the eggs are not fully developed. Mature female locusts often dig holes without laying eggs in them, even though the soil conditions appear to be suitable. The reasons for this behaviour are not known. On occasions females have been seen to lay eggs on the surface of the ground or on trees. This is usually because the soil is too hard and dry. Once eggs are fully developed inside the female she can only keep them for about 3 days; then they must be laid whether suitable soil is available or not. Eggs laid on the soil surface or on trees do not hatch.
Abnormal laying of this kind, especially when on a large scale, constitutes important information and should be either mentioned in the routine locust reports, or reported separately.
Female locusts lay many eggs at a time and these are bound together by a frothy secretion which forms them into an egg pod (Fig. 15). The egg pod is 3-4 cm long, the bottom being usually about 10 cm down in the soil. On top of the eggs the frothy substance hardens to form a plug which extends almost to the surface of the soil. The plug helps to prevent the eggs drying and it also provides a medium through which the young hoppers can easily reach the surface when they hatch. Egg pods are nearly always laid in groups, which may be either large or small. It is useful to record the maximum density in one square foot. The area over which egg pods are laid, which varies from a few square metres to a square kilometre or more, is called an egg field.
The number of eggs in a pod can vary from about 20 to over 100 but the number for swarming locusts is usually between 70 and 80 for the first laying, between 60 and 70 for the second laying and less than 50 for the third laying, if it occurs. It is noteworthy that the egg pods of locusts not in swarms usually contain many more eggs than pods laid by swarming locusts. Three is probably the maximum number of egg pods laid by swarming locusts in the field, but those kept in laboratories can lay many more. There is some evidence that in the field non-swarming locusts lay more pods than swarming ones, about five on average.
When the eggs are laid they are yellow in colour but in the soil they turn brown. They absorb water from the soil, about their own weight of water in the first five days if it is available at the time, and this is enough to allow them to develop successfully. Research has shown that 20 mm of water is sufficient. If they do not get this quantity of water they will not hatch. If, however, there is not sufficient water in the soil during the first few days, they can absorb as much as the supply permits and then wait for several days before taking in the remainder, after more rain has fallen. It is not possible for Desert Locust eggs to stay dry in the ground from one rainy season to the next and then hatch when the rain comes.
Incubation period and hatching
The period of egg development, between laying and hatching, is called the incubation period. The rate at which eggs develop varies according to the soil temperature. For example, in the summer breeding areas of West Africa, the Red Sea coast and lowland India the incubation period takes 10-14 days but this is extended to 25-30 days in the cooler spring breeding areas of central Arabia, southern Iran and Pakistan while in North Africa it can take as long as 70 days in exceptionally cold weather. More detailed information can be found in the section on seasonal movements and breeding areas (Page 36).
When they are fully developed in the eggs, the young hoppers burst their way out of the egg shells, wriggle up the froth tube to the surface, and immediately shed a thin white skin. These white skins are easily visible on the surface of the soil and are an indication that hatching has recently taken place. They are, however, soon blown away by the wind. Hatching takes place either shortly before or within 3 h of sunrise, and all the hoppers from one egg pod normally hatch on the same morning. It usually takes three days for the complete hatching of a whole egg field but longer periods have been recorded. Only a few hoppers hatch on the first of these days, most on the second and a few more on the third.
Hoppers
When hatching is complete, some small and some larger groups of hoppers will be noticed all over the egg field. Sometimes there is very little movement of hoppers on the first day of hatching but after a day or two the groups of hoppers will have joined together to form larger groups which move about; these are called bands.
Moulting
By the time they are a day old the hoppers have started to feed. Their skin is hard and tough by now and will only stretch a little. They therefore have to grow by casting off their skins from time to time. This process is called moulting. When the hopper sheds its old skin it has a new, soft skin underneath. This stretches for a short time, allowing the hopper to grow, before it hardens. Moulting usually occurs five times during the development of the Desert Locust (apart from the skin-shedding that occurs at hatching).
Instars
The hopper stage of the life cycle is thus divided into five instars. (Hoppers are sometimes called nymphs and the hopper instars are then called nymphal instars. The word 'stage' is occasionally used instead of 'instar' in locust reports, e.g. 'fifth-stage hoppers'; it should, however, be restricted to the three main stages of the life cycle, egg, hopper and adult.) Figures 16-22 show the distinctions between the different instars of the Desert Locust.
The first instar is whitish in colour when newly hatched but in 1-2 h turns mainly black. As it grows bigger and becomes ready for moulting a pale colour pattern becomes more obvious.
It is not always easy to distinguish the second instar from the first but with experience one recognises that the pale colour pattern is more obvious and that the head is much larger. It is easily distinguished from the third instar because there is no sign yet of wing growth.
The third instar is easily recognised by the two pairs of wing 'buds' which can be seen projecting from underneath the pronotum on each side of the thorax.
The colour now is conspicuously black and yellow, more black in cold conditions and less black in hot. The wing buds are larger and more obvious but they are still shorter than the length of the pronotum measured along the middle line.
The colour of the fifth instar is bright yellow with a black pattern, again varying with temperature. Wing buds are now longer than the pronotum, but still cannot be used for flight.
Fledging
The final moult is from the fifth-instar hopper to the adult stage. This change is called fledging and the young adult is called a fledgling. After this there is no further moulting and the adult locust cannot grow in size but gradually increases in weight.
Notice the thin bent wings hanging down; later they will be pumped full of blood and take up their final shape.
The fledgling is pink and the wings, head and body are relatively soft. Activity is limited to walking and short descending flights. Fledglings gradually become hard and able to fly strongly. Locusts in this condition are called immature adults.
Duration of life cycle
The length of life of individual adults varies. Some have been kept alive in cages for over a year, but in the field they probably live between 2.5 and 5 months. Apart from accidental death the life span depends on how long they take to become sexually mature. The quicker they mature the shorter the total length of life.
Phase
Desert Locusts can exist as scattered individuals within the recession area or, when numerous, as swarms throughout the invasion area. This is because the locust exists in different phases. When breeding conditions lead to an increase in the numbers of locusts crowded together the insects have the ability to change their colour, behaviour, shape and physiology. Not all these characteristics change at once; behaviour and colour being the characteristics to change first.
Colour
An adult in the solitary phase is likely to be pale grey or beige when immature, with the males becoming pale yellow on maturation. In contrast, an adult from the swarming (gregarious) phase will be bright pink when immature and bright yellow when mature.
Behaviour
Solitary locusts live separately, the hoppers do not move together and the adults usually fly individually at night. They are often difficult to see and their colours blend with their surroundings. Gregarious hoppers move in marching bands and have distinctive black markings. The brightly coloured adults move together in cohesive day-flying swarms. In between the two extremes are locusts exhibiting some characteristics of solitary locusts and some gregarious ones; such locusts are referred to as transient locusts.
Shape
Scientists have tried to describe the changes in shape which occur by measuring parts of the locust (Fig. 8). For example, if the length of the front wing or elytron (E) is divided by the length of the femur (F) of the hind leg the resulting ratio is greater in the case of locusts taken from a swarm than for those locusts living alone. These measurements are called morphometrics. Changes in the shape of the pronotum and sternum of the Desert Locust are shown in Fig. 24.
Unfortunately it is necessary to introduce a note of warning at this point. Morphometric studies do not always give a completely reliable indication of the behaviour phase. One reason is that changes in behaviour and appearance do not always occur at the same rate. In the Desert Locust for example, some swarms comprise locusts whose morphometrics are the same as those of solitary-living ones. The environmental conditions during the development of the hopper can affect the morphometrics of the adults. Nevertheless, it is safe to state, as a general rule, that locusts taken from swarms will have a certain appearance (and certain morphometrics), whilst those of the same species taken from an area where there have been no swarms for several months will have a different appearance (and different morphometrics).
Physiology
Solitary locusts lay pods containing 95-158 eggs each. In the laboratory they have been known to lay more than three pods; gregarious females lay pods usually containing less than 80 eggs and laying occurs twice, rarely three times.
Hoppers in the solitary phase usually develop through six instars before fledging, each moult is indicated by a marked stripe on the eye (total 7). Gregarious hoppers invariably fledge after five instars and have a total of six eye stripes although sometimes the eye can be a uniform dark brown.
<section>Behaviour in relation to habitat</section>
Different species of locusts and grasshoppers behave in different ways. The behaviour of a single species also changes with age and the size and density of the population. Behaviour is affected by a wide range of external factors which characterise the habitat and among which the weather is dominant.
Knowledge of locust behaviour and of the factors which determine it are essential for an efficient locust officer.
In this section the behaviour of the Desert Locust in the hopper and adult stages is described. Particular reference is made to those features of behaviour which are of direct importance in locust control operations.
Hoppers
When hoppers occur in large numbers-as the result of breeding by a swarm or dense concentrations of locusts-they gather together in bands, which move about as distinct units (Fig. 25). The behaviour of the individual hoppers in bands and that of the bands themselves are best considered separately.
Behaviour of individual hoppers in bands
Hoppers have several kinds of general activity. There is a more or less regular pattern of daily behaviour which is summarised in Table 3.
Roosting. At night and in the early morning hoppers will be found roosting {Fig. 26). This means that the hoppers are off the ground, resting on plants, bushes or stones. Roosting also occurs during the middle of the day when the temperature exceeds about 36ºC.
Ground grouping. When hoppers are concentrated in dense groups on the ground and are mainly stationary they are said to be in ground groups (Fig. 27). These are seen in the morning when the hoppers come down from the bushes and again in the evening before they roost for the night. Ground grouping may occasionally occur at other times of day.
Marching. Hoppers usually spend the greater part of the day marching (Fig. 28). This means that they are moving together by either jumping or walking in a definite direction. They stimulate each other and the whole band moves from one place to another. The net distance covered is called the displacement.
Feeding. The main feeding period occurs when the hoppers go up into the bushes for their evening roost, but they also feed when marching, stopping briefly to eat low vegetation in their line of march. These two types of feeding are important to note because the success of certain control operations depends upon them.
Many kinds of plants are eaten by hoppers but they seem to have preferences when there is a choice of plants. Note in your own area those which seem to be specially liked by hoppers, and also record any common plants which you never see them eating. This will be useful information when applying insecticide. On some plants hoppers prefer the flowers or fruit while on others they prefer the leaves and on some they eat both. They have on occasions also been seen eating bark.
The readiness with which a plant is eaten depends on the presence of other food plants at the site. A plant which is readily eaten in one area may be avoided in another because of the presence there of other plants which are even more palatable. Furthermore, the appetite of hoppers for a given species of plant may change with their age and their physiological condition. For instance, as hoppers grow older they may eat tough plants which they avoid when young, and thirsty hoppers may eat fleshy, watery plants which would normally not attract them.
Feeding mainly occurs in the evening but sometimes it is seen on a considerable scale in the middle part of the morning. Very little occurs during the hotter, middle part of the day. Hoppers eat more during the middle part of each instar than at the beginning or the end. Just before moulting they feed either very little or not at all. For the first three moults this non-feeding period lasts about one day, but for the two later moults it may be 2-4 days. This is important when bait or vegetation poisoned by insecticides is being relied on to kill them when they eat it, since it means that at certain times the hoppers will not eat enough to pick up a lethal dose of insecticide. It is therefore necessary to learn to recognise these periods by observation and postpone control work until the appetite of the hoppers returns, or to use persistent insecticides which will remain effective for several days at least.
Daily pattern of behaviour of hoppers in bands. Table 3 shows the daily pattern of behaviour usually seen in the East African region. If you find differences in your own area make notes of them and think about why they are different. Organise your control work to fit in with the hopper behaviour.
Behaviour of hopper bands
Hopper bands vary in size from a few square metres to several square kilometres, depending upon the number and density of the hoppers they contain (Fig. 29). The number of bands and the proportion of large and small ones depend on the amount and pattern of egg laying in the egg field from which they have arisen. Bands often join together to form larger ones, but sometimes they divide into smaller bands.
Hopper bands tend to occur in groups and this is important from a control point of view. It means that if one band is discovered it is probable that there will be others nearby.
Change in size. The joining and splitting of bands clearly causes changes in the numbers of hoppers in them, and sometimes bands of different ages join together so that hoppers of very different instars occur in the same band.
Hopper bands also change size without any alteration in the number of hoppers that they contain, partly through growth of the hoppers and partly because they are sometimes close together while at other times they are further apart.
Daily change in size due to changing behaviour. Hopper bands change their size during the day according to the behaviour of the hoppers. The size of a band when it is marching can be up to eight times that of the same band when it is roosting or in ground groups. The reduction in size at roosting time tends to be greater in an area with shrubby vegetation.
Change in size due to change in instar. Hopper bands get bigger as the hoppers in them grow older. A band of fifth-instar hoppers may be 20-30 times as large as a first-instar band containing the same number of hoppers. This means that bands are more concentrated in the early instars and are therefore easier and more economical to destroy.
Change in size due to fusion or splitting. Hopper bands may get larger owing to the fusion of two or more bands. Sometimes, however, bands split up and this is most likely to occur after moulting.
Movement of hopper bands
Speed and distance. Large bands move quicker and farther than small bands of the same instar. The thicker the vegetation the more slowly will bands move through it, and where there is much bare ground bands will move more quickly and therefore farther each day. Sunny weather, provided it does not get too hot, i.e. above 36ºC air temperature, favours marching, so that bands move quicker and farther each day under sunny conditions than in cloudy overcast weather.
Marching speeds of individual young hoppers may be as high or higher than those of old hoppers, but in general, older bands move faster and further. This is because by the time the fifth instar is reached in the same area the weather is usually sunnier and the vegetation has become drier and thinned out, and these conditions induce more rapid marching.
The following observations on the daily movement of hopper bands of various sizes and instars were made in East Africa.
TABLE 4
Size of band (night roosting area in square yards)
Instar single day (yards)
Displacement in a
150,000 (12.5 ha)
96 (88 m)
160,000 (13 ha}
579 1530 m)
165,000 (14 ha}
916 (838 ml
One mile wide (260 ha)
1 mile (1610 m)
Direction of movement. Hopper bands often move in the same direction for several days at a time and sometimes in roughly the same direction throughout hopper life. They have often been observed moving generally downwind.
Very often all the bands in a quite large area move in the same general direction. This kind of behaviour is more likely in flat unbroken country. In hilly country and country broken up with many water channels there is likely to be less constant direction of movement. Sometimes hoppers move in long narrow bands along dry water channels and sunken roads. This is known as canalisation.
Shape of hopper bands
There is no regular shape. The shape depends on the behaviour of the hoppers and the type of country. Nearly always when the band is marching the hoppers are most dense at the leading edge, which is known as the front (Fig. 30). In large bands there is a high density for about 150 m back from the front; the density then decreases until at the trailing edge of the band the hoppers are very scattered.
It is useful to find the fronts of hopper bands when controlling them. If the leaders of each band stop to feed on bait or sprayed vegetation the ones following behind them are likely to do the same. To find the front, drive or walk in the direction in which most of the hoppers appear to be marching.
It is possible with experience to recognise the tracks of hoppers where they have recently marched over loose sand. This is quite useful in finding bands which might otherwise be missed. Hoppers may not be sighted but the tracks will indicate that they have passed by on that day or on the previous day.
Adults
Flying capabilities
One of the most striking features of the Desert Locust is its great mobility. Experiments have shown that locusts suspended in a windtunnel can go on flapping their wings non-stop for 6-17 h and there is evidence that the maximum period may be as long as 20 h. By gliding some of the time, locusts can stay in the air very much longer. This allows them to make long journeys over the sea. For example, locusts regularly cross the Red Sea, a distance of 300 km, and in October 1988, a large number crossed the Atlantic Ocean to the West Indies-a distance of 5000 km!
The muscles of a flying locust perform 10-20 times as much work in proportion to body size as those of a human being working at top speed. The energy needed is derived from fat stored inside the body, so that the flight endurance depends on the amount of fat present. The fat content of Desert Locusts, according to age, is shown in Table 5.
TABLE 5
Age
Total weight (g)
Weight of fat (mg)
Newly fledged
Second week after fledging
4-5 weeks after fledging
Very old locust about to die
Same as fledglings
The longest flights are made by immature locusts which do most feeding and store most fat. The Desert Locust has a flying speed of 16-19 km/in but the rate at which it moves relative to the ground depends on the wind as well as its own flying speed.
Locusts are cold-blooded animals and their activity is affected by air temperature and sunshine. If there is no sunshine long continuous flight usually takes place only above 23ºC. Rain and cloud generally decrease the amount of flying. In sunshine long continuous flight is possible when air temperatures are above 14ºC but flight decreases when the air becomes hotter than 40ºC. Locusts take off and land into the wind, but in very strong winds they do not take off at all but shelter behind rocks or vegetation.
Adults in swarms
The size of Desert Locust swarms ranges from less than one square kilometre to several hundred square kilometres. There are about 50 million locusts in each square kilometre of a medium-density swarm. The total number of locusts in a swarm varies from a few hundred millions to tens of thousands of millions. The volume density varies from one locust per thousand cubic metres to 10 locusts per cubic metre.
The shape of flying swarms can be stratiform or cumuliform (Fig. 31).
Large swarms generally fly higher than small ones. According to the weather conditions it experiences, a swarm may appear at one time as a very large single swarm, and perhaps a few days later as several small swarms with scattered locusts between them.
Daily pattern of behaviour of swarming locusts
Desert Locusts in swarms fly by day and settle on vegetation at night, although they have sometimes been seen flying after dark. It is not easy to say exactly what their behaviour will be at any time as this depends on the weather, the type of country they are in and the state of the locusts themselves, but the general daily behaviour pattern for immature (pink) swarms in tropical Africa or Asia is as shown in Table 6.
TABLE 6
Time
Behaviour
Night (1 h after sunset to sunrise)
Settled on vegetation; the place where the swarm settles is called the roosting site.
Sunrise +0.5 h
Crawling about slowly on vegetation. A few locusts may jump or fly about on disturbance. During the next hour or so more locusts make short flights and many come to the ground.
Sunrise +2 h (about 0800 h)
Dense carpets of locusts now on the ground lying broadside on to the sun's rays to absorb radiant heat. Some still basking on trees and some flying between groups. During the next hour the amount of local flying increases with streams of locusts flying in different directions while still remaining within the area of the roosting site.
0900 to 1000 h
Majority of locusts become airborne. As the air temperature rises and convection starts the locusts rise higher into the air and the swarm begins to leave the roosting site; this is usually referred to as mass departure.
1000 h to sunset or just
Swarm in flight as a whole, but there are nearly always many locusts temporarily settling beneath after it the swarm, particularly at the leading edge; near the rear edge these take off again and follow the rest of the swarm. Thus the swarm moves in a rolling manner.
Sunset or soon afterwards
Settling in progress, very often followed by heavy feeding.
A locust swarm consists of streams of locusts. Figure 32 shows that all the locusts in a stream face in the same direction (or orientation). There will be other streams in the same swarm facing in many other directions.
Any locusts which fly or are blown outside the edge of the swarm turn back into it and as this is taking place all round the edge of a swarm it means that the different directions of flight by the various streams do not affect the direction of movement of the swarm as a whole. The net distance traversed by the swarm as a whole is called the displacement. The presence of streams of locusts flying in all directions within a swarm means the direction of movement of the swarm as a whole cannot be judged merely by watching which way individual locusts are facing or moving, or noting which way the stream which happens to be above at the time appears to be moving.
The direction of movement of a whole swarm can be judged roughly from the ground by seeing where it is centred at different times in relation to landmarks. A much better way of finding out which way it is moving is by obtaining successive fixes of its position from an aircraft.
Non-swarming populations
Solitarious Desert Locusts living at low densities start flying after dusk on warm evenings and continue flying during the early part of the night. Large-scale night flight normally requires a temperature of 23ºC or more at take-off. Some solitarious locusts seem to fly frequently, but others hardly at all. There is evidence that long-distance migrations, rather similar to those of swarms, can occur but as they occur at night, unlike those of swarms, they are not seen, and thus can lead to surprise infestations of locusts in areas previously clear. During the daytime solitarious locusts will fly only when they are disturbed, or flushed. When they are flushed they usually fly low and settle quickly. Sometimes they rise almost vertically, and then drop quickly like a stone, or they may be carried out of sight by the wind.
Direction of swarm displacement in relation to meteorology
By using aircraft to find the direction of displacement of individual swarms and knowing the wind direction at the same time it has been shown that swarms move in a downwind direction (Figs 33 and 34). The steadier the wind direction the steadier will be the direction taken by the swarms. In unsettled weather or in mountainous country with frequent changes of wind, swarm movement will be more irregular. Steady flight against very light winds has often been seen in mature swarmlets flying only a few feet above the ground. These observations have led people to believe that in these cases whole swarms are moving upwind, for short distances at least. No satisfactory proof of swarm displacement upwind has so far been provided. The speed at which swarms move varies greatly. Rates of movement measured have ranged from 1.5-16 km/in and swarms have been known to travel a few kilometres to over 100 km in a day and as much as 3500 km in a month. The speed at which swarms travel is
often considerably less than the wind speed because all the locusts in a swarm are not flying all the time.
Since swarms travel consistently downwind they eventually arrive at a place where winds meet or, in other words, where masses of air converge. These places of convergence can be recognised on synoptic weather maps which can be used to find out where swarms are likely to be found. A synoptic map shows the weather over a given area at a given time, it can be used to estimate the wind and temperature at any place in the area for that time. Winds and temperature vary with height above the ground but information of most use to a local control organisation can be found on synoptic maps prepared for the surface (nominally 10 m above ground) 850 and 700 millibars (about 1.5 and 3 km above sea level). Cooperation between a country's locust control organisation and its meteorological service is therefore highly desirable. Converging air masses produce certain recognisable weather systems in different parts of the world at different times of the year and it has been possible to relate
the position of locust swarms to these. For instance in the summer months Desert Locust swarms tend to congregate along the belt where the northerly and southerly winds meet.
This is called the inter-Tropical Convergence Zone (ITCZ) or sometimes the Inter-Tropical Front. From late September onwards swarms generally move northwards out of the ITCZ into North Africa and the Middle East, often during periods of warm southerly winds associated with the passage eastwards of depressions through the Mediterranean and the Near East. In East Africa swarms follow the seasonal displacement of the ITCZ southwards across the Somali peninsula between late September and February.
Areas of convergence of air masses are the areas where rain is most likely to fall. An important result of the way swarms move is that they eventually come into places where there is rain. This is essential to the survival of the Desert Locust, for it can only breed successfully where there is moist sold in which eggs can be laid and develop and suitable plants will grow and provide food for the young hoppers. Sometimes the rainfall is too little and this leads to heavy mortality in locust populations.
At present weather stations in the Desert Locust area are few and far between, so there is room for much improvement in the weather maps for some parts of the area. As weather information improves, it should be possible to have a fuller understanding of swarm movements and to forecast them better. This improvement will occur with the widespread use of information from weather satellites. The cloud pictures now available from geostationary satellites show the growth of rainstorms throughout the day and can be used with surface observations of clouds and rain to determine the time and extent of rainfall. Pictures from both geostationary and polar-orbiting satellites are particularly useful in helping to estimate possible rains in remote places.
Anti-locust organisations themselves should do all they can to provide weather information from places where there are no permanent weather stations.
<section>Seasonal movements and breeding areas of desert locust during plagues and recessions</section>
This section uses maps to show the seasonal changes in distribution, frequency of infestation and breeding of the Desert Locust in plagues and recessions. The Desert Locust lives in a generally dry, arid environment where rainfall is sporadic and seasonal. As this locust needs moist soil for egg laying and egg development and the hoppers need fresh vegetation on which to feed, they are only able to breed after periods of rainfall.
Desert Locust plagues do not start within permanent outbreak areas. Both solitary and gregarious adult locusts move downwind between seasonal breeding areas. The areas within countries where breeding occurs differs during plagues and recessions.
During plagues, the breeding areas associated with the spring rainfall are found in North Africa, the Middle East, southern Iran and Pakistan (Fig. 35). The resulting swarms move southwards as the area dries (thus no longer providing food or suitable laying sites) to the belt where summer rains occur. This is in Mauritania, Mali, Niger, Chad, Sudan, Ethiopia and southern Arabia. Swarms also move eastwards to the monsoon rainfall areas of Pakistan and India (Fig. 36). There is also a winter breeding season around the Red Sea coasts. In East Africa (Somalia, southeast Ethiopia, Kenya and northeastern Tanzania) breeding occurs between October and December on the short rains and from February to June on the long rains (Fig. 37). In contrast to plague populations, recession populations are restricted to the central, drier parts of the distribution area where the average rainfall is less than 200 mm. As a consequence breeding does not occur in North Africa, the Middle East and
East Africa. The area of recorded seasonal breeding during all recessions since 1920 is shown in Figs 38-40. In any one season, however, rainfall and consequently, breeding will be more limited.
All areas are not equally liable to infestation and Figs 41-64 show for each month, the number of years between 1939 and 1963 in which swarms and hopper bands were recorded. This was a period of almost continual plagues. The maps show the frequency but not the severity of infestation in the degree squares. A comparison of the maps for each month shows the area most likely to have swarms or hopper bands and their changing distribution throughout the year.
In contrast, Figs 65-88 represent frequencies during a period of almost continuous recession. All reports of gregarious or non-gregarious adults or hoppers, were summarised for the period 1964-1985; 1968, which was a plague year, was omitted.
Figs 41-64. The following maps show, for each calender month, the number of years in which swarms and hopper bands were recorded during the 25-year period 1939-1963, i.e., they represent frequencies during a period of almost continual plagues. Records are grouped into 'squares' of one degree latitude and one degree longitude.
Figs 65-88. The following maps show, for each calender month, the number of years for which adults or hoppers were recorded during the period 1964-1985 (excluding 1968 which was a plague year), i.e., a period of almost continual recession.
Incubation period and hopper development
Research has shown that about 20 mm of rainfall in a short period is sufficient moisture to allow eggs to complete their development. Rain does not need to fall over the breeding site as areas can become sufficiently moist from run-off from nearby hills and mountains. If eggs do not absorb enough moisture in the first few days, however, they can remain dormant and continue their development when rewetted. Dormant periods of up to 60 days have been recorded in the field.
The speed of egg development varies with the soil temperature; the warmer the soil the faster the eggs develop. Tables 7-10 show the ranges recorded in the field for each breeding area.
Hoppers, like the eggs, develop faster in warmer temperatures. Laboratory experiments showed that at 24ºC hoppers developed at 1.5% per day but at 38ºC this rose to 5%. Thus the total time taken for hopper development would be 66 and 20 days respectively. Table 11 shows the range of hopper development periods recorded in the field. Hoppers generally spend a similar period in each of the first four instars, e.g. 6-7 days, and a longer period in the fifth instar before fledging, e.g. 10 days.
To estimate the total time from laying to fledging the egg incubation period should be added to the hopper development period, e.g. laying in Niger on 1 June
egg incubation hopper development total
+ = ..fledging from 14 July
12 days 32 days 44
When fledging is complete the immature adults will begin to fly together in swarms. If the area in which they bred is beginning to dry out they will move to another area of green vegetation. If rain falls the swarms may mature and lay to produce a second generation. At the end of the breeding season when further rain is unlikely, swarms will move away on the prevailing winds to new breeding areas. These movements are summarised in Figs 35-37. It is at these periods that the spectacular long distance migrations of the Desert Locust take place. For example, swarms produced from summer breeding in Sudan can move westwards and northwards to invade Morocco in October; swarms from monsoon breeding in India can migrate via Oman and southern Arabia across the Gulf of Aden and invade northern Somalia in November; and spring swarms from Iraq can fly eastwards to India in June. Field officers should be aware of the likely sources of swarms which could invade their country. The Desert
Locust Forecasting Manual (D. Pedgley, ed. COPR 1981) discusses these in far more detail than is possible here.
TABLE 7. Egg incubation periods (days) in the summer and monsoon breeding areas
June
July
August
September
October
India
Pakistan
Arabia
Ethiopia, Somalia, Kenya (below 1500 m)
Ethiopia, Somalia, Kenya (above 1500 m)
Sudan, Chad, Mali, Niger, Mauritania, Senegal
TABLE 8. Egg incubation periods (days) in the long rains and short rains breeding areas in East Africa
February
March
April
May
October
November December
Somalia, south-east Ethiopia
0-900 m
Kenya 900-1500 m
TABLE 9. Egg incubation periods (days) recorded in the coastal areas around the Red Sea and Gulf of Aden
January February March April May June July August September October November December
TABLE 11. Hopper development periods (days) recorded in the field
Summer and monsoon breeding areas
Short rains
31-45 (63% of records 35-391
Long rains
Red Sea coast/Gulf of Aden
24-48 (71% of records 30-39)
Winter-spring
<section>Recession periods, outbreaks and the origin of plagues</section>
During recession periods, swarms and hopper bands are rare and the Desert Locust inhabits the central, drier part of its distribution area (Fig. 89). This dry desert and semi-desert area of some 16 x 10^6 km² receives less than 200 mm of rain annually. Sufficient water must be available in the soil when females lay to ensure both the development of the eggs and the growth of vegetation to sustain the resulting hoppers and adults. Consequently, the Desert Locust survives best, and its numbers are highest, where adequate falls of rain are most frequent and reliable, where direct rain is enhanced by run-off and flooding and the soils and vegetation create especially suitable habitats. While less is known about the movements of solitary locusts, they appear to migrate between seasonal breeding areas in a similar way to swarms but not to travel so far. Figure 90 shows the resultant seasonal distribution of solitary hoppers during recessions.
Outbreaks, marked increases in population leading to the appearance of gregarious populations, may follow successful breeding. Three processes are involved in their formation, concentration, multiplication and gregarisation.
Concentration occurs on two scales. On the larger scale, solitarious locusts moving into a seasonal breeding area may be concentrated by wind convergence and selectively settle in certain areas which are especially favourable, notably areas where it has rained recently and where there is green vegetation which provides food and shelter, and moist soil for egg laying. Within these generally favourable habitats conditions are not uniform and locusts further concentrate on the smaller scale when they are sheltering, roosting, basking and, very importantly, when they are laying (which is normally at night).
Egg-laying sites are often very restricted in extent so quite dense groups of egg pods can be laid by non-swarming adults (densities of up to 700 pods/m² having been recorded). Solitarious females lay up to about 160 eggs in each egg pod and probably at least two egg pods each. There is thus a potential multiplication rate of about 200 times but this is rarely, if ever, achieved, principally because there always seems to be very high mortality amongst the young hoppers, for reasons which are not yet clear. When the hoppers emerge from the egg pods they sooner or later encounter other hoppers. As a result of these repeated encounters they start to gregarise; i.e. they become conditioned to the presence of their fellows and start to form small basking groups, then small marching groups which later become larger. If there are sufficiently large numbers of hoppers present the marching groups can join up and form small bands and subsequently swarms. The above is a very simplified
account of the early stages of an outbreak. Occasionally, these processes occur sequentially in a succession of the geographically distinct but interrelated seasonal breeding areas and should the build-up continue long enough, a plague results. While outbreaks are frequent, however, upsurges marking the start of a plague are rare. More frequently, potentially dangerous, partially gregarised populations die down without producing hopper bands and swarms. High hopper mortality is often caused because rains fail, or sometimes because of parasites and predators. In most seasons, initially low density populations do not achieve the multiplication rates needed to produce a major upsurge. In others, however, gregarisation occurs after several successive generations so it is essential to search for and report any populations during recessions, particularly if they are located in areas and under conditions suitable for successful breeding.
Figure 91 shows areas of observed or deduced gregarisation between 1926 and 1976. Although these are widely distributed throughout the recession area, the distribution does suggest that the following factors are important in producing outbreaks:
- the borders of highland areas where run-off can provide favourable breeding sites, e.g. central Sahara, interior of Oman and the valleys of Mekran of Iran and Pakistan;
- the Indo-Pakistan summer breeding areas with complex mesoscale convergence systems which concentrate the locust populations;
- the Red Sea and Gulf of Aden coasts with a rainfall regime that can provide suitable conditions for breeding all year round.
Should an outbreak occur in one or more of these regions and the resulting adults move to a complementary breeding area, e.g. locusts from the Red Sea coast moving inland to the summer breeding belt of the Sudan, and there find favourable conditions for breeding then it is likely that the resulting populations could lead to a plague. A plague is in progress when there are many bands and swarms over large areas.
Habitats of solitarious Desert Locusts
Solitarious locusts live in fewer varieties of habitat than swarms. In general they occur in open sandy steppes with few or no trees. They are not usually present in places where the trees are on average less than 10 m apart. The vegetation generally consists of perennial
bushes and herbs less than a metre high and annual plants which come up after rain. The actual distribution of locusts within this general type of area is affected by the pattern of the vegetation. The capacity to concentrate locusts varies considerably between different habitats. Thus if the pattern is fairly uniform the locusts will probably remain scattered, but when it is patchy groups will be formed because the locusts prefer certain plants for food and shelter.
Some of the plants with which solitarious Desert Locusts are often associated are Heliotropium spp., Dipterygium glaucum, Tribulus spp., Schouwia purpurea, S. thebaica, Aerva persica, Hyoscyamus muticus and, among cultivated plants, the bulrush millet. In the absence of these, however, they may show preference for other species. These should be noted. Such preferences, the patchiness of vegetation and the presence of only occasional patches of bare moist soil suitable for laying help to bring locusts together. This is an important step leading to gregarisation and outbreaks; the number of locusts in the resulting concentration must be large, otherwise it may not be able to maintain itself against the disrupting influences of weather and predators. The habitats of solitarious locusts in the different regions are described below.
Pakistan, Iran and India
During the summer locusts tend to concentrate in the open steppe vegetation where there are patches of bare ground on dune crests and slopes (Fig. 92). The main food is Tribulus alatus, and perhaps Aerva persica or Crotalaria burhia. These sites become particularly important when the period of monsoon rain is long enough to allow two generations. There is good vegetation cover after the first rain and patches of bare ground suitable for laying are restricted to sites such as bare dune crests and cultivations, resulting in concentration of locusts at later layings.
In the winter and spring, in the lowlands of Pakistan, Baluchistan and southeastern Iran, Desert Locusts are to be found principally in the so-called 'rek' or 'rig' sand-dune country which occurs at intervals along the coast between Karachi and Bandar Abbas. The vegetation is of the open steppe type containing such species as Heliotropium undulatum, Sericostoma pauciflorum, Sphaerocoma hooker), Aerva persica and Panicum turgidum (Fig. 93). The locusts breed regularly in these areas in the winter and spring, but no important concentration seems to occur. Concentration usually happens later when the locusts move into inland valleys such as Turbat, Panjgur and Kharan, where suitable habitats are restricted to small cultivated areas and deposits of sand similar to the 'reks' (Fig. 94).
Arabia
When it rains the sand-dune areas of southern Arabia become suitable habitats for Desert Locusts and small concentrations can be found in patches of such plants as Chrozophora oblongifolia, Tribulus sp., Dipterygium glaucum and Aerve persica. More important concentrations can be formed if rain and breeding take place in the plains of the interior where the only suitable habitats are sandy wadis draining from the mountains towards the sands (Fig. 95). Good vegetation develops in the wadis and concentrates the locusts. There are many of these wadis between Oman and Yemen. Those in southwest Arabia receive more regular floods and have some cultivation which also provides suitable habitats for locusts (Fig. 96).
Red Sea coastal areas
The southern Red Sea area is one of semi-permanent convergence of winds in winter and comes under the influence of the ITCZ in summer. Most rain in the Red Sea area occurs in winter, but there is also some rainfall in the summer and the south then has more than the north. The prolonged rainfall in the south is probably one reason why the Red Sea coastal areas frequently have locust populations.
The important locust habitats are almost all man made, created by the practice of shifting cultivation, principally within the wadi areas. When the original vegetation is cleared and Pennisetum millet (dukhn) is planted this provides a suitable locust habitat, especially when left unweeded (Fig. 97). Abandoned fields are invaded by weeds such as Heliotropium pterocarpum, Dipterygium glaucum and Aerva persica which are favoured by locusts. The vegetation cover is usually patchy and therefore leads to locust concentrations (Fig. 98).
The south coast of Arabia and the north coast of the Somali peninsula have similar habitats to those along the Red Sea coasts.
West Africa
Here the locust habitats are associated with the Saharan highlands and with the open steppes surrounding the Sahara. In the drainage areas around the highlands there are restricted areas of Schouwia thebaica, Tribulus alatus and Hyoscyamus muticus. These plants grow on silty and clay soils which are often covered by sand and hold moisture well (Fig. 99). Here Schouwia may stay green for 3-4 months after heavy rain. These slow-drying soils allow laying for a long period, and their patchiness causes concentration of locusts during laying.
<section>3. Other African locusts</section>
There are three other plague species of locust in Africa south of the Sahara: the African Migratory Locust, the Red Locust and the Brown Locust. All these species have one important difference in common from the Desert Locust. Whereas the Desert Locust can form large populations leading to plagues in several geographically separate parts of its distribution, the other African species have been shown to have more restricted outbreak areas. This knowledge has been used to prevent further plagues in the first two species by siting control organisations in or near the outbreak areas. These organisations have kept populations small and restricted. Figure 7 compares the annual fluctuations in the number of countries infested from 18871 970.
While chemical control of the Brown Locust has not prevented swarming or reduced the frequency of outbreaks it has, nevertheless, reduced the area subject to invasion and breeding and the intensity of swarming and crop damage.
<section>African migratory locust-Locusts migratoria migratorioides</section>
The distribution of this locust is shown in Fig. 100. Its main breeding area is in the flood plains of the Middle Niger in West Africa and it is the escape of swarms from this outbreak area that are known to lead to plagues. Elsewhere throughout the distribution area populations of solitarious phase Migratory Locust may be found. These solitary populations are generally regarded as residual populations left over after the dying out of plagues. There are other areas, such as parts of Sudan, Ethiopia and Angola, where swarms of Locusta have formed, but none are as meteorologically favourable as the Niger flood plains and none has yet led to the start of a plague. It is considered that Locusta is actually indigenous in the Lake Chad basin.
Life cycle
The life cycle of the Migratory Locust is similar to that of the Desert Locust. The time spent in each stage varies from season to season in different parts of the distribution area according to environmental factors; temperature appears to be the most important factor. There can be 4-5 generations per year in the Niger delta but this falls to three in the Chad basin area and to between one and two generations in South Africa. Elsewhere in the invasion area there are two generations per year.
Immature adults
Solitarious adults differ from gregarious ones in shape and colour. They have an arched pronotum and are green or brown in colour. The gregarious adults have a much flatter pronotum and are a yellowish-brown or greyish colour with darker markings.
Maturation. The first signs of egg development in solitaries usually appear a few days after fledging and males can copulate four days after fledging. In the laboratory solitarious females start to lay about 10 days, and gregarious females 2.5-3 weeks, after fledging. Egg laying may start as soon as this in the field, but much more information is needed. It is known, however, that there is no delay in maturation even though laying does not always occur as the eggs can be resorbed. Resorption means that yolk and other material already deposited in the developing eggs are removed from these eggs which, therefore, get smaller and typically become red and are known as corps rouges. In the African Migratory Locust the presence of a double row of red spots in the ovaries (they are very small and it is best to use a hand lens when looking for them) usually means that the female has resorbed her eggs.
Resorption can be caused by low temperatures and also occasionally by a lack of food or prolonged flight. Corpora lutea are also found in the ovaries. They are another kind of resorption body and their presence indicates that the female has laid. They too are sometimes reddish but they are usually colourless or yellow; they are more conspicuous in the Desert Locust than in the African Migratory Locust.
Mature adults
As solitarious locusts mature they become somewhat darker, particularly the abdomen of the females. In swarms the general colour becomes duller and the males become partly yellow (Plate 4).
Egg laying. Egg laying usually takes place in soil which has been moistened either by rain or by recent flooding but can also occur in dry soil. Reports during the last plague indicate that typical laying sites for swarms include burnt patches of grassland with a flush of new green grass, open areas near swamps, cleared farmland or bare areas near villages where cattle have been herded.
After the eggs are laid and the female has withdrawn her abdomen she scrapes the soil over the hole and pats the surface. The Desert Locust has never been seen to do this.
The eggs are roughly similar to those of the Desert Locust but their arrangement within the pod is different in that they are arranged in rows (compare Fig. 15 with Fig. 101). The eggs are surrounded by a thin but tough film of froth to which sand particles adhere.
The number of pods laid by a single female varies considerably as does the interval between successive layings. During the winter months in the outbreak area locusts have been found to lay 1-3 pods each at intervals of about 20 days, while in the summer they may lay 2-4 egg pods each at intervals of only 3-4 days. Solitary locusts lay about 65 eggs (range 551101 per pod which falls to 39 eggs per pod for gregarious females.
Incubation period
For successful development Locusta eggs must absorb water from the soil in the early stages, and provided there is adequate water in the soil the incubation period is dependent upon temperature. In the outbreak area of the Niger flood plains the incubation period is generally 10-20 days in the summer and 20-40 days in the winter. Eggs laid in dry soil, or in soil that dried out rapidly after laying, may take over 100 days to hatch.
Hoppers
An important aspect in which the life cycle of the African Migratory Locust differs from that of the Desert Locust is in the number of instars. In both phases of the African Migratory Locust there are normally five instars but in the solitarious phase under very dry conditions there may be six or even seven instars.
The duration of each instar is, as in the Desert Locust, dependent on environmental factors of which the most important is temperature. In the laboratory hoppers developed faster in humid conditions (38 days) than in dry conditions (57 days). Records from the last plague in both East and West Africa show that the whole hopper period lasts 30-60 days for the five instars. In the Middle Niger area hopper development lasts only 24-35 days.
Gregarious Migratory Locust hoppers have a very distinct brown and black colour pattern. Solitarious hoppers are grey in the first instar but the colour may vary in later instars: there can then be a very wide range of colours including green, grey, buff, brown, red and black, often resembling the background colour of their habitat, e.g. black hoppers on burnt grassland, and various shades of green according to the tone of the vegetation.
Table 12 gives an indication of the size and weight of the various hopper instars.
TABLE 12
Average length
Average weight
(mm)
(mg)
First instar
Second instar
Third instar
Fourth instar
Fifth instar
Behaviour
The behaviour of the African Migratory Locust is similar in many respects to that of the Desert Locust, described above. In order to avoid undue repetition the emphasis in this chapter is therefore upon the differences in behaviour between the two species.
As the last plague of the African Migratory Locust ended in 1941 most present-day observers have seen neither swarms nor hopper bands produced by swarm laying. The only hopper bands seen since the last plague have been produced as a result of local concentration, multiplication and gregarisation, but not on a scale sufficient to initiate a new plague.
Hoppers
Migratory Locust hopper bands are sometimes much denser than those of the Desert Locust. For example, there are records of hopper bands piling up to 15 cm deep. During the first instar the hoppers tend to form very dense ground groups or small bands on bare patches of soil or on stones. In the later instars bands march and the daily pattern of behaviour is similar to that of the Desert Locust.
Hopper bands tend to march downwind, but canalisation along gullies, tracks etc. is common. Bands, particularly large ones, often cross obstacles such as stones and the same direction of march may be maintained for long periods. There are records of bands marching 24 km during the whole period of hopper life.
Hopper bands can maintain cohesion in the very dense grasslands of the flood plain but move less under these circumstances, or when there is no sun or temperatures are low.
Adults
Swarms of the African Migratory Locust can be very dense and the densest locust swarm ever recorded photographically was a Migratory Locust swarm in West Africa during the last plague (Fig. 102).
The daily pattern of behaviour is similar to that of the Desert Locust as described on page 31 except that swarms often settle at mid-day but feed little. The main feeding occurs in the evening when they settle for the night roost. Migratory Locusts mainly eat grasses or low vegetation and cereal crops are often damaged.
Swarms generally fly in stratiform shape and tend to avoid obstacles such as prominent topographic features or lines of trees. Swarms of African Migratory Locusts seem to be somewhat less active than those of the Desert Locust, perhaps because they tend to fly close to the ground, but large-scale migrations do occur and are described below.
Seasonal migrations of swarms
While the details of swarm movement during the last plague have not yet been fully worked out it appears that, in general, swarms of the African Migratory Locust, like those of the Desert Locust, move downwind.
The spread of the last Migratory Locust plague, which is illustrated in Fig. 103 shows the progressive increase in the area invaded by swarms and Fig. 104 shows the number of months of infestation by swarms and hopper bands in each affected country expressed as a percentage of the 157-month period (June 1928-June 1941) for which the plague lasted. By mid-1932 swarms occurred nearly 7000 km from the outbreak area in Mali. This plague clearly spread in an easterly and then southerly direction over a number of generations. This progressive type of spread can be contrasted with the seasonal north-south migration of the Desert Locust in West Africa, northward after the summer breeding and southward after the following spring breeding, and similarly back and forth between the summer and spring breeding areas of the Eastern Region (Figs 35-37).
In addition to the eastward and southward spread in Africa there are regular small-scale migrations in areas such as Nigeria where swarms were reported every month between December 1929 and January 1940. During the winter months from December to February and March swarms and hopper bands were found in the southern part of the country whilst in the summer months from July to September they occurred in the north. It would appear that swarms move northwards on southwesterly winds with the northward extent of the ITCZ from February to September. There is then a corresponding southward displacement of swarms as northerly winds spread over Nigeria following the ITCZ as it retreats towards the south from October to December.
In the event of another plague arising in Mali it is not possible to say whether it would develop in exactly the same way.
Seasonal breeding and migrations of solitarious African Migratory Locusts
Solitarious African Migratory Locusts are found over many widely separated areas of Africa but this account is limited to the regular seasonal breeding and migrations in and around the main outbreak area in Mali. The map of the outbreak area (Fig. 105) shows the flood plains which are subject to seasonal inundation and in which the vegetation consists of various types of grassland, and the surrounding semi-arid areas of Sahelian vegetation consisting of wooded steppe and woodland.
Rain in this outbreak area normally falls between June and September but the rains may begin as early as May and continue into October. The River Niger begins to rise because of the rain which falls in the highlands to the southwest, and each year it floods the surrounding country. The first areas to flood are in the south in August and the floods generally do not reach the north until November. The floods begin to recede in October at the southern end of the plains but last until February or even March in the north.
This flooding of the outbreak area enables locusts to breed there not only in the rainy season from June to September but also during the period October to March or April as the floods recede, whereas in the surrounding areas of the Sahel breeding is limited to the rainy season alone. In the flood plains (Fig. 106), where breeding can occur throughout the year, there is some overlap between generations but it is possible to recognise a general seasonal pattern of breeding and migration (Fig. 107).
At the time of the high floods in August and September most of the adults leave the flood plain for the surrounding Sahelian areas where they breed and fledging of the Sahelian generation begins in September. (It should be noted that the Sahelian generation can also be referred to as the last rains generation.)
Adults of the new generation then begin to migrate from the Sahel back to the flood plains and to concentrate in those areas most recently exposed by the retreating flood waters. The locusts move gradually northwards behind the retreating floods and lay in their wake, thus producing the first retreating flood generation during the period October to January. During January to April some locusts breed again and a last retreating flood generation is produced in the northern plains by locusts resulting from earlier breeding in the south.
By the time the rains begin in May or June the floods have disappeared and a first rains generation results. Fledging occurs in July and August. A second rains generation occurs in the northern plains, laying begins in June and there is fledging from August onwards. Adults from both generations disperse widely in the surrounding Sahel to breed and produce the last rains generation-a total of five generations during the year (Fig. 107). If the annual rains are late or finish early there will probably be only two generations (annual total four).
Habitats of solitarious African Migratory Locusts
Solitarious African Migratory Locusts have been found in widely separated parts of Africa but the following account of their habitats is restricted to the main outbreak areas in and around the flood plains of the Middle Niger in Mali. The vegetation of the flood plains is mainly controlled by the rise and fall of the floods, the lower areas being under water the longest. Over the whole flood plain the soil generally consists of a silty clay with more clay in the lower areas, but sandy patches are widespread.
The higher levels (those flooded for the shortest time)
In this zone the vegetation consists of very dense grasses, up to 2 m high. The main species are Andropogon gayanus and Hyperthelia dissolute. Owing to the extremely dense nature of the vegetation this zone is not a good habitat for the African Migratory Locust and it is seldom found there.
In addition to the natural vegetation there are also cultivations of rice, and no locusts are found in these, but in the northern part of the flood plain, where the last flooding occurs, crops of sorghum can be grown and harvested on the rains before the area is flooded. These cultivations form a good habitat for locusts breeding during the rains.
The intermediate levels
The vegetation in this zone also consists of grasses, forming a mixture of dense stands and areas of more open patchy vegetation. The main grasses in the dense stands are Andropogon africanus and Vetiveria nigritana. In the more open areas the principal species are Eragrostis atrovirens, and Panicum anabaptistum and P. fluviicola.
The more open areas constitute the better locust habitat and the scattered patches of bare soil provide good laying sites. These sites are particularly suitable for laying when locusts which bred in the drier Sahel return in November and give rise to the first retreating flood generation. Parts of this intermediate zone are also used for the growing of rice but locusts are not found in the rice-growing areas.
The lower levels (those flooded for the longest time)
This zone is dominated by Echinochloa stagnina and Oryza barthli. These grasses give rise to a mass of floating vegetation which forms a mat on the ground when the floods recede. This zone is a poor habitat for the African Migratory Locust, both when the vegetation is floating and when it is dry. All three zones described above are burnt in the dry season between February and May, after which time the locusts concentrate in the fresh regrowth, particularly in the middle zone.
The Sahel
Part of the locust population flies out of the flood plain in August and September and breeds in the surrounding more arid areas of the Sahel.
The areas of principal interest lie to the east and west of the flood plain between 15ºN and 17ºN. Further north there are semi-desert areas and to the south there is woodland vegetation. The vegetation of the Sahel, which consists of wooded steppe, is dependent upon rainfall, unlike that of the flood plains where the rise and fall of the flood is more important. In the Sahel the soils vary from silty sands on fixed dunes to clay-filled depressions, and in some areas the underlying laterite layer is exposed.
The southern Sahel. The natural vegetation in this area consists of quite dense low woodland in which Pterocarpus spp. and Acacia spp. are the main trees. Seasonally waterlogged clay-filled depressions support dense Pterocarpus woodland to a height of 5-8 m. Clearings have been made in the natural woodland for the cultivation of bulrush millet (Pennisetum americanum). From time to time the fields are abandoned and annual grasses soon begin to colonise the bare ground. The main species are Cenchrus biflorus and Eragrostis tremula and when the fields are left longer Diheteropogon hagrupae becomes important together with the perennial, Andropogon gayanus.
The northern Sahel. The northern part of the Sahelian zone is drier and the tree cover is lower and less dense. There are numerous fixed dunes and undulating country is common. in this region there are natural open areas as well as man-made clearings for growing millet. The commonest trees in this area are Acacia spp., Balanites aegyptiaca and Euphorbia balsamifera. When the fields are abandoned annual grasses soon appear; as in the south these include Cenchrus biflorus and Eragrostis tremula, but in this drier zone Aristida spp. are also important.
When locusts invade the Sahel from the flood plains the most important habitats are the man-made clearings. Judging from the numbers of locusts found in both the northern and southern Sahel, the fields in current use and those which have been recently abandoned seem to provide equally good habitats and in the northern Sahel man-made clearings are more favoured than those which occur naturally.
Recession periods and outbreaks
The African Migratory Locust has been in recession since 1941 when the last swarms of the plague died out in Chad, Sudan and Zambia. This recession period has been far longer than any known for either the Desert Locust or the Red Locust and historical evidence suggests that there have only been two plagues this century (1891-1903 and 1928-1941). Hopper bands have, however, formed in 16 years between 1941 and 1971 in the Middle Niger outbreak area since the end of the plague. The most important upsurge in the main outbreak area occurred in 1951 when over 17,000 bands were chemically controlled and a possible new plague was prevented.
The last plague was traced to the flood plains of the Middle Niger in Mali from where swarms left at the start of the rainy season in 1928. Hopper bands occur more frequently during the cool dry season as the floods recede and are rarely found during the rainy season. They appear to result from the breeding of concentrations of parent locusts which have appeared in larger numbers than usual due to increased multiplication during the preceding rainy season. Such high rates of multiplication are associated with above-average rainfall and were a feature of both 1927 (the year preceding the plague) and 1950 (the year preceding the upsurge). Hopper bands produced on the retreating floods rarely give rise to swarms because they are too widely scattered and the fledglings disperse. Such fledgling populations are, however, sometimes larger than usual due to increased survival of gregarious individuals compared with solitarious ones. If they persist until the following rainy season,
and if breeding is successful and multiplication occurs on a large scale, they may be potentially dangerous. This is again associated with above-average rainfall during the early part of the rainy season and occurred in 1928 (the start of the plague), 1951 (the year of the upsurge) and 1968 (a recent upsurge).
The distribution of the egg pods appears to be the primary factor involved in the formation of hopper bands because gregarisation mainly occurs during the first instar. The concentration of solitary hoppers or adults does not normally lead to any gregarious interaction, as it does in some other species of locust, unless densities are extremely high.
The processes involved in an outbreak are basically similar to those already described for the Desert Locust. However, because the African Migratory Locust has a relatively fixed annual cycle where a period of dispersal and population increase (rainy season) alternates with a period of concentration and decrease (retreating floods season), the processes of concentration, multiplication and gregarisation are not so much concurrent as consecutive events and are restricted to certain breeding seasons. They tend to lead to outbreaks at the start of the rainy season when conditions are most favourable for the spread of swarms.
Although the origin of the last plague has been traced to the main outbreak area in Mali there have been reports from other widely separated parts of Africa of Migratory Locusts multiplying to form large hopper bands and small swarms.
These small outbreaks have been reported from around Lake Chad, on the Red Sea coastal plains of Ethiopia and Sudan, the Gash, Gedar and Gezira areas of Sudan, the northern coastal plains of Somalia, Hippo Valley in Zimbabwe, northwest Angola and the Lake Ngami region of Botswana. Some of these populations have been large enough to require control measures; whether or not outbreaks in such areas could start a plague is not known. If resident solitarious populations of the African Migratory Locust become established in areas of new agricultural development in countries such as Sudan and Zimbabwe, there is always the possibility that under suitable conditions they may concentrate, multiply and gregarise and so create migratory swarms. It must be borne in mind that areas which have so far only produced unimportant outbreaks may be capable of initiating a new plague. It is therefore essential that a watch be kept on the establishment and growth of solitarious populations of the
African Migratory Locust in these suspect areas.
<section>Other subspecies of Locusta migratoria</section>
Figure 108 shows the widespread distribution of this species. Nine subspecies have so far been distinguished. The Locusta found in Arabia and India is very similar to the African subspecies but forms swarms much less frequently and except for India is a relatively minor pest. In Madagascar Locusta is regarded as the most important agricultural pest, especially of rice and sugarcane. There can be four generations in a year: one in the dry season and three in the rainy season. Development is continuous so that all stages can be found at the same time. Plagues last 1-3 years (last plague was 1960-1961) and follow periods of above average rainfall. Recession periods are much longer. The main outbreak area is in the southwest corner of the island where Locusta is concentrated by the North East Trade Winds into marshy areas suitable for breeding.
Locusta m. manilensis is described in Chapter 5 and the other subspecies are outside the area covered by the handbook.
<section>Red locust-Nomadacris septemfasciata</section>
Figure 109 shows the outbreak areas and invasion area of the Red Locust. Small numbers of this locust are present in grasslands up to 2000 m over much of Africa south of the Sahara. It is also found in the Cape Verde Islands, Madagascar, Mauritius and Reunion.
The Red Locust is similar to Cyrtacanthacris tatarica tatarica and Ornithacris spp. Plate 2 shows how they can be distinguished.
Life cycle
The immature adult is a brown colour which gets deeper and redder with age with a distinctive yellow band running along the head, pronotum and central edges of the folded wings. There is only one generation per year unlike the Desert Locust and Migratory Locust. Adults mature and lay at the beginning of the rainy season which for most of the distribution area is in November-December. Females of both phases usually lay twice but the egg pods of solitarious ones contain 20-195 eggs while those of gregarious females contain 20-100 eggs.
The average time taken for the eggs to hatch is 30 days, ranging from 18 days in Mozambique to 54 days in cool areas in South Africa. Hoppers go through 6-8 instars and the average development period is 60 days, ranging from 37 days in Madagascar to 78 days in Natal in South Africa. Solitarious hoppers have more instars than the gregarious ones. Fledging begins in February and continues until May. The adults then remain immature for some six months until the start of the next rainy season. This is true even in areas like southwest Uganda where there is no marked dry season. Breeding can occur all year round but there is still a six-month period as an immature so that there is one generation per year.
North of the equator, Red Locust populations in the Lake Chad Basin and Niger delta flood plains mature and lay between April and August depending on the beginning of the rains. Hoppers are found from mid-July to October.
Behaviour
Hoppers
The most detailed studies have been carried out in the Rukwa Valley where the locust populations are never wholly solitarious. The habitat is a mixture of tall and short grasses, sedges and patches of open ground. At the end of the dry season most of the grasses are burned so that when the rains start and laying begins large areas are available for oviposition. Laying is restricted in areas of unburned grassland.
Newly hatched hoppers disperse but regroup in the second and third instars. Bands are most likely to be found in the tall grass and mixed short and tall grass habitats. The hoppers like to roost in the tall grass but feed on soft leaves with a high moisture content. Bands do not move very far in dense vegetation but in open country can march up to 700 m/day.
Adults
Adults fledging from hoppers in bands remain in cohesive groups. Swarms are formed by locusts congregating in tall vegetation to roost and by restriction of the habitat during the dry season usually caused by burning of the grassland. Gregarious behaviour is most fully developed in dry years when the vegetation becomes patchy earlier. Flight behaviour changes from wandering to a migratory pattern and swarms leave the outbreak area.
Flight occurs during the day at temperatures above 26ºC; swarms do not travel far, rarely more than 20-30 km/day. The direction of displacement is generally downwind.
The mechanisms leading to migration from the outbreak areas are not fully known. It is unlikely that food availability is important. The level of gregarisation of adults is important as are numbers in excess of 5-10 million locusts in a swarm. The escape of a few swarms from the outbreak areas does not always lead to a plague; these swarms frequently seem to disappear.
Red Locusts in West Africa behave in a broadly similar way. In Madagascar breeding occurs in the extreme southwest of the island from November to March and the ensuing adults disperse northward and northeastward during the dry season from April to October. Swarms form at the end of the dry season during outbreaks; the last one occurred in 19601961.
Plagues, outbreak areas and seasonal movements
There have been three plagues documented since the middle of the nineteenth century, i.e. 1847-1854, 1891-1920 and 1930-1944. Detailed studies of the last plague suggested strongly that they start from restricted outbreak areas. The most important of these are the Mweru wa Ntipa marshes in Zambia and the Rukwa valley in Tanzania. The ideal environment for an outbreak area is either a treeless grassland with poor drainage or seasonally flooded plains and valleys.
Other likely outbreak areas are: the Malagarasi and Wembere plains in central Tanzania; the plains north of Lake Chilwa in southern Malawi; the Busi-Gorongosa plains in Mozambique; the Kafue marshes in Zambia; the Niger flood plain and the Lake Chad basin in West Africa. Swarms have formed in some of these areas but so far none has led to the formation of a widespread plague.
The last plague began after populations increased over three generations from 1927-1930 probably in the Mweru wa Ntipa and also the Rukwa valley. When swarms left these initial sites they eventually reached two areas: the first, the Shire and lower Zambesi valleys in Malawi and Zimbabwe and the second, an area to the west of Lake Victoria in northwest Tanzania and southwest Uganda (Fig. 1 10). In both these areas swarms successfully survived as immature adults until the next breeding season. Elsewhere either dry season survival or breeding success was low. Figure 1 10 shows the retention areas and the seasonal movements out of them.
The general movements of young immature swarms are westwards from April to June towards Angola and Zaire with some swarms moving northwest towards the retention area in Uganda and others northward along the coastal area of Tanzania to Kenya. By August, when swarms are beginning to mature and the dry season coming to an end, movements are more likely to be southward. Swarms move through Botswana and Zimbabwe to the Transvaal and Natal provinces of South Africa and into Swaziland. These movements continue until October-November and the beginning of the rains.
It is important to note that the movements are general shifts of population and included swarms which had bred in areas along the displacement routes. Further, although there is an apparent 'return' movement of swarms it is thought unlikely that the swarms reach those areas where they or their forbears were produced. Thus many of the swarms died without breeding or reached areas where breeding was not very successful. Similar movements the following year were dependent on new swarms from the retention areas where breeding was successful. Once the main area in Zimbabwe, Mozambique and Malawi became clear of Red Locust in 1943 the plague rapidly declined.
Figure 111 shows the frequency of breeding during the 1927-1945 plague. Although Natal is shown as a high frequency area and was regularly invaded by swarms from the north, the breeding success was very low. There was a virtual absence of any swarms leaving this area. Other movements took swarms to places where breeding was unsuccessful such as the striking northward movements along the Nile to about 20ºN and along the Somali Peninsula to the Gulf of Aden coast. Elsewhere there were occasional invasions of Gabon from northwest Angola and of Namibia from Zimbabwe.
The maximum spread of swarms occurred in 1934-1935 when 7,000,000 km² was infested. Thereafter the plague became more restricted until it declined in 1945. A total of 16 generations was estimated to have been involved.
<section>Brown locust-Locustana pardalina</section>
The Brown Locust is illustrated in Plate 4. Figure 112 shows the outbreak area and the invasion area for this locust. Between plagues solitary Brown Locusts can be found throughout the Karoo area of South Africa, some 250,000 km². This is a dry region where the rainfall is erratic and generally less than 300 mm annually. The vegetation is made up of dwarf grasses which are the main food of the locust, and many bare patches which become suitable basking and egg-laying sites.
Life cycle
As its name suggests, the immature adult is brown, but there are occasionally green forms. In seasons with good rainfall there can be three generations but in droughts the eggs can remain dormant for up to 15 months. Laying takes place in well-drained, loose soil and the eggs hatch after 10-20 days; sometimes there is a 1-3 month diapause. Both diapause and non-diapause eggs can be laid in the same pod but generally solitarious locusts are more likely to lay eggs which diapause than gregarious locusts. Females can lay up to five pods but the average is nearer two; each pod contains 10-82 eggs.
Solitary hoppers can be brown, green or grey; gregarious hoppers are black with orange markings in the later instars. There are 4-5 hopper instars for males and five for females. Development periods vary from 21-38 days for solitarious hoppers. Gregarious hoppers take longer, at least 42 days, because they are significantly larger than the other phase. The size difference between the two phases is greater than in any other species.
In the Karoo Brown Locusts overwinter in the egg stage (1-3 months). Hatching begins in September after the first rains and continues in October with fledging in December. In years of good rainfall further laying can occur in January to produce a second generation in March; this generation in turn can lay again, probably in April.
Behaviour
Hoppers
Solitary hoppers move away from other hoppers while gregarious hoppers group as with other locust species. Research has suggested, however, that within a single egg pod hoppers hatch with different behavioural characteristics. For example, reared under similar conditions, hoppers from the top of the egg pod are more likely to have gregarious characteristics than those at the bottom of the egg pod.
Bands of Brown Locust hoppers usually march in very long narrow columns and can cover up to 2 km during their life time, although up to 40 km has been recorded. Densities of hoppers in bands sometimes reach 3000-4000 hoppers/km².
Adults
Gregarious behaviour takes at least two generations to develop. Adults fledging from grouping hoppers fly in loose swarm formation and lay together some 15-50 km from the hatching site. The following generation produces more strongly gregarious characteristics and swarms (up to 43 km²) can fly hundreds of kilometres in a generally downwind direction.
Gregarisation occurs when the numbers of solitary locusts begin to rise over a wide area. As more contacts are made between insects phase transformation begins. This is further encouraged by good rainfall especially if it follows an early summer drought the previous season.
Plagues, upsurges and seasonal movements
Studies of the Brown Locust this century have suggested that periods of 7-11 years of great swarming activity are separated by recessions of similar length.
Neither chemical control nor exceptional periods of rainfall have effectively disrupted this periodicity of swarming and recession. This has led to the suggestion that there are other factors influencing changes in numbers and behaviour, such as an individual locust's response to density. Locusts not sensitive to density will not be active movers and if numbers increase locally these locusts will not move away but become part of a transient population. Such a population will provide a target for predators and will be reduced. Locusts sensitive to density will initially move away from contact with other insects. They will thus be difficult to find and will not be a prey to natural predators. Eventually the proportion of these adults in the total population will predominate. When overall numbers reach a critical level phase transformation takes place quickly and highly mobile swarms are formed. At the end of an active swarming phase it is thought that the density-sensitive
adults are killed or die out leaving the remaining solitary population with a majority of locusts not sensitive to density. There must then be a period of build-up of locusts which are density sensitive before there will be another swarming period.
When swarms do form they can be displaced by the winds northward, northwestward and northeastward away from the recession area in the Karoo. This movement takes place in February (early summer) but later in the summer movements have an easterly trend both north and south. Control has been relatively effective so there have been few invasions outside South Africa in the last 50 years. Botswana and Namibia are the countries most likely to be invaded. Invasions of Mozambique, Angola and Zambia are not well documented. In 1924 swarms from northern Botswana invaded Zimbabwe. Breeding occurred in the extreme west in January and the resulting swarms moved eastward throughout the country from late April. By June maturation had occurred and laying took place. The eggs hatched in September when the rains came and the ensuing adults flew westward into Botswana in December.
<section>Tree locusts-Anacridium melanorhodon melanorhodon, Anacridium melanorhodon arabafrum, Anacridium wernerellum, Anacridium aegyptium</section>
Figure 113 shows the areas where Anacridium species are found in Africa and Arabia. Anacridium melanorhodon melanorhodon, A. melanorhodon arabafrum and A. wernerellum are illustrated in Plate 3.
Anacridium melanorhodon
This species is generally found in areas of short grass and scattered trees, the most common trees in such habitats being various species of Acacia, but it may also be found in treeless areas. Of the two subspecies, A. melanorhodon melanorhodon occurs in the western part of the distribution area and A. melanorhodon arabafrum in the eastern, but the two subspecies meet in Ethiopia and Sudan.
Anacridium wernerellum
This species is mainly found to the south of A. melanorhodon in areas where there are more trees, but the two species overlap considerably.
Anacridium aegyptium
This is the Egyptian Tree Locust. It occurs in countries around the Mediterranean and in the Middle East.
Life cycle
The life cycle of Tree Locusts is similar to that of the Desert Locust except that the males generally have six instars and the females usually 7-8 or, more rarely, 6 or 9. The extra instars are quite common in both swarming and non-swarming Tree Locusts.
There is little definite information on the duration of the various stages of the life cycle but Table 13 gives some idea of the time taken for egg and hopper development. Breeding occurs during the rainy season and there is usually only one generation each year, but as in other locusts development is influenced by the weather and food supply.
TABLE 13
A. melanorhodon melanorhodon
A. melanorhodon arabafrum
Incubation
15-65 days
27-48 days
Hopper development
48-69 days
63-141 days
Immature adults
Immature adults are grey, sometimes a little brownish, brighter in younger, and duller in older individuals; in the latter a pinkish tinge is present at the base of the hind wings. In swarming adults this pinkish colour appears quite early, a month or so after fledging. Non-swarming adults are somewhat browner than swarming ones and the pink coloration may not develop until maturation.
Mature adults
Maturation takes place at the start of the rainy season. Mature adults cannot be distinguished from immature adults by colour alone. The only certain way to recognise a mature female is by dissection. The presence of yellow yolk in the eggs, whatever their size, is a sure sign of maturity.
Copulation and laying are much the same as in the Desert Locust except that the copulating pairs are mostly in trees or bushes until egg laying begins. Laying usually takes place at night. The eggs are laid in moist, usually sandy, soil in the form of an egg pod similar to that of the Desert Locust. The average number of eggs in a pod is high, about 150. Whether swarming Tree Locusts, like Desert Locusts, lay fewer eggs than non-swarming ones is not known. Females can lay up to three pods.
Hoppers
Gregarious hoppers of Tree Locusts are yellow with black markings, the pattern being different in detail for each species; solitarious hoppers are green with white and black dots. It is not unusual to find solitariously coloured hoppers amongst gregariously coloured ones, and vice versa. Various intermediate colour forms are also quite common.
Behaviour
Hoppers
After hatching hoppers climb the nearest bush or tree and may form groups. Where laying has been concentrated groups may be found over many hectares. Feeding occurs at night.
Adults
Adults form swarms which are usually both smaller and less mobile than those of the Desert Locust. During the day adults normally roost in trees, and if disturbed they will fly from branch to branch or to a nearby tree. Such disturbance, however, does not cause mass departure. Swarms usually fly at night, a habit which has earned them their Arabic name, Sari el leil, which means the 'night wanderer'. Flight usually starts soon after sunset when mass departure from the roosting sites occurs. It is in stratiform formation.
There is little information on how far Tree Locust swarms can fly because none has ever been followed. They probably settle fairly soon and begin to feed. If trees and bushes are available they settle on them, but they settle on herbaceous crops if there are no taller plants, and eat them. Feeding on crops is most likely during the dry season.
Seasonal movements and breeding
Little is known about the seasonal movements of the various species of Tree Locust. There are records which suggest that swarms of A. melanorhodon melanorhodon can move considerable distances during the dry season. For instance, it seems likely that swarms gradually moved eastward from northern Nigeria into eastern Chad between February and June 1956 and individual locusts have been caught at sea up to 100 km off the West African coast.
Anacridium melanorhodon melanorhodon breeds in an area where there is a well-defined wet season between June and October, during which breeding occurs, and a long dry season lasting from November until May. Records show that this breeding area is smaller than the overall distribution area. There is only one generation per year. Anacridium melanorhodon arabafrum occurs in areas of East Africa and Arabia where the rainy seasons are much more complex and variable. Hoppers of this subspecies have been recorded for every month between October and May. It is probable that there is only one generation a year, but it is possible that in some areas there may occasionally be two. Anacridium wernerellum has one generation per year in Nigeria where it survives the dry season as an immature adult, but two generations have been observed in Tanzania.
Further studies in the field are needed to increase the sparse knowledge of the seasonal breeding and movements of Tree Locusts.
<section>4. Sahelian grasshoppers</section>
The Sahel covers an area from about 9ºN to 20ºN from the west coast of Africa across to Sudan in the east (in East Africa, Sahelian-type habitat extends north to about 24º and south into Tanzania, about 5ºS). It covers sub-Saharan semi-desert, dry savannah and dry woodland, especially Acacia. It is characterised by regular, but unpredictable rainfall and frequent drought.
The region includes the recession areas for the Desert Locust and is prone to invasion by both the African Migratory Locust and the Red Locust but it is the grasshoppers that are the major agricultural problem in the region. Many grasshopper species cause varying amounts of damage, but only a few have locust-like gregarious behaviour. The Sahelian grasshopper problem is a chronic one with damage caused in one area or another on an annual basis by different species acting together or on their own.
Only the eight most important species from over 200 species of grasshoppers that occur in West Africa and the Sahel are discussed here.
Little is known about the hopper instars of the various species and their reproductive behaviour, however, a general idea can be given of adult appearance, distribution and an outline of each life cycle.
<section>Senegalese grasshopper-Oedaleus senegalensis</section>
The Senegalese Grasshopper is widely distributed in dry Savannah areas from West and North Africa to India (Fig. 114) and is a major pest of millets (Pennisetum spp.) in the Sahel. Adults are some 19-47 mm long and generally straw-coloured with black markings (Fig. 115), females generally being larger than males. It is differentiated from other acridids by the X-mark on the pronotum, the rounded back edge of the pronotum and the black crescent on the yellow-based hindwings (Plate 4). It is distinguished from O. nigeriensis (found in Africa from the Sahel southwards) by the latter having red inner surfaces to the hind femora and an angled back edge to the pronotum.
Life cycle
Mating is preceded by the female flicking her hind legs in a closed position when the male is in close range. The male then makes a rapid approach and mounts the female. Copulation can be maintained either with the male on the female's back or with the pair pointing in opposite directions.
Egg laying occurs at any time of the day. The egg pods, each containing 8-37 eggs, are laid in patches of open or bare, well-drained sandy or sandy-silty soil; each female lays 2-3 egg pods. Egg pods are often clustered in the more favoured positions.
Hatching is dependent upon soil moisture, thus eggs laid towards the end of the rains are unable to hatch and diapause until the next or up to at least three seasons later. Eggs laid early in the season hatch in 12-15 days, whilst eggs which have been through a period of diapause hatch within 9-10 days of the onset of the rains.
There are five hopper instars (Table 14) and development from hatching to fledging takes about 17-25 days.
TABLE 14
Hopper instar
Length (mm}
The fifth-instar hopper has a green head, pronotum, upper thorax and hind femora; the rest of the body is brown above and creamy (very pale brown) below, with a broad green stripe down each side of the abdomen. The wings are yellow with dark (grey) veins and brown edges. In the gregarious form, the basic colour is creamy on the abdomen and legs and pale brown on the head, pronotum and thorax, the whole cuticle is densely covered with dark brown spots; the wings are yellow with black veins and there is a black mark on each of the hind femora.
Polymorphism
The Senegalese Grasshopper has two main colour types. At the beginning and end of the season, a brown form predominates, whilst mid-season a green form is more common. The controlling factor is believed to be the vegetation eaten by the grasshoppers, which varies through the season. There is also a seasonal variation in size, the grasshoppers being generally larger from mid-season onwards (males) or peaking mid-season (females).
Gregarisation
Differences in colour are also noticeable between solitary individuals and gregarious individuals. In the solitary form, the markings give a somewhat mottled effect on the green or brown basic colour. In the gregarious form, however, there is a marked contrast with dark markings on a basic fawn (pale brown) colour. It is also likely that morphology varies between solitary and gregarious individuals, much as it does in the Desert Locust.
There are several factors which lead to gregarisation. The most important of these is the tendency for egg pods to be laid on patches of bare soil. In seasons of good rains, these patches will be less common and so, gregarisation will be more likely. If the vegetation dries out at different rates, adult and hopper concentration can occur in the course of seeking fresh food or shelter. On migration, adults can be concentrated by converging winds.
In Sudan, hoppers of the Senegalese Grasshopper have been seen marching among hopper bands of Desert Locust in July.
Migration
In West Africa, the migratory flights of the Senegalese Grasshopper take place at night, on the wind at heights of several hundred metres above the ground. Populations often migrate between fledging and egg laying. The first eggs to hatch are those in the south of the region where the rainy season occurs first. Once fledged, this first generation follows the ITCZ some 50 km or so north. Here they breed and lay their eggs just as the rains arrive. The rains signal the development of both the local diapaused eggs (first generation) and the newly laid eggs. The second generation eggs hatch about 10 days after those of the first generation. The mixed population continues to move northward into the north of the breeding range. Here eggs are laid again in the moist conditions. Thus hatching in this northern region is of three generations: diapaused eggs (relatively few) from the previous season and eggs from first- and second-generation grasshoppers from the intermediate zone. By
the time these have all fledged, the ITCZ is retreating rapidly southward. Pairing begins quite quickly and pairs leave the southward migration at various stages to lay their eggs: a few in the northern region, rather more in the intermediate zone and the majority in the southern breeding area (Fig. 116). The location of the highest density egg fields depends on the speed of retreat of the ITCZ and the maturation rate of the adults; usually adults lay at greatest density in the intermediate and/or southern zones. These eggs all diapause during one or more dry seasons and the cycle begins again in the following March-April. A fourth generation in one season has been recorded once in Niger-Mali in 1978. In Sudan, it is likely that there are two generations per year, but little work has been done on the Senegalese Grasshoper life cycle outside West Africa.
The dry season (winter) diapause of the eggs is a product of both physical (environmental) and physiological factors. The physical factors involved are the lower temperatures and the lack of rain, causing the soil around the egg pods to dry, thus inhibiting development which requires moisture. The physiological trigger for diapause is within the female grasshopper. It is probable that it is related to day-length and begins towards the end of the rainy season when a shorter day-length initiates diapause. The role of physical factors starts to predominate at the end of the rains and beginning of the dry season.
The 1985-1986 grasshopper upsurge in northwestern Mali
Upsurges in grasshopper populations have been recorded in the Sahel on several occasions when several years of drought have been followed by good rains (e.g. 1974, 1977). An account is given here of the events as they occurred in northwestern Mali in 1985 and 1986.
Late but extensive rains in October and November 1984 marked the end of the Sahelian drought. Local hatching of eggs gave rise to remarkable hopper densities. Hoppers were seen marching at a density of up to 30-40/m² over areas of 4 km² and 20 km². Eggs had accumulated in the soil over four years (it is known that Senegalese Grasshopper eggs can remain in the soil for up to five years before hatching) and 1984 was a prelude to the hatching of a massive store in 1985 and 1986.
In 1985 the rains were less than average in total amount, but were well distributed in time and space. By the end of August there was a massive upsurge with 7000 km² of northern Mali infested with hoppers. Many of these populations were at high densities of 7-10/m². In September, OICMA were requested to control these grasshoppers and used two aircraft for aerial spraying and also ground teams. They laid down barriers in the wild vegetation against the hoppers and the return flight of the adults. At this time the millet was at the 'milky grain' stage favoured by the Senegalese Grasshopper. An infested area of 900 km² was successfully sprayed, but crop loss across northwestern Mali was estimated at 20-30%. At the end of the 1985 season the ITCZ moved south rapidly, taking young adult grasshoppers further south than usual, to about 13ºN.
At the onset of the 1986 rains (June), early first generation hatching in the south covered about 1000 km². At the end of June and the beginning of July (mostly in the first week), the new adults from this first hatching moved northward into the centre of the millet-farming belt and laid at the onset of the rains in this area. Diapaused eggs (laid 1985) hatched, followed 7-10 days later by the recently laid (1986) eggs. By the end of July, these had all fledged (fledging time was remarkably short at about 15 days and the resultant adults were considerably smaller than average) and had caused major crop loss to young millet plants, forcing the farmers to resow up to four times. Many farmers became discouraged at this stage and either abandoned their fields or started to sow sorghum instead of millet (sorghum is resistant to grasshopper attack). It is estimated that 25% of the potential area for cultivation was lost.
The area then cleared of grasshoppers as they migrated northward to lay their eggs. The belt of hatching reached 15ºN and possibly even farther. By the end of August-beginning of September, infestations were detected covering 2800 km² (conservative estimate) in the sector 7-9ºW and 14º30'N into Mauritania. By mid-September, the infested zone from about 14º30'N to 15º30'N had suffered approximately 20% crop loss. Despite spraying, some populations escaped and moved south so that there was a residual population of grasshoppers in the southern part of the infested zone in 1986.
In 1986, the ITCZ moved south rather slower than it did in 1985. The result of this was that the egg fields were at their most dense at about 15ºN.
Throughout most of the region the Senegalese Grasshopper was the predominant species, but in the south, i.e. about 14º30'N to 15º30'N, the August-September populations were 50% Senegalese Grasshopper and 50% Kraussaria angulifera.
Oedaleus johnstoni
Oedaleus johnstoni is a larger, stockier species, 26-45.5 mm in length, found in the semidesert and northern Sahel which corresponds to the northern breeding zone of the Senegalese Grasshopper. It is distinguished from the Senegalese Grasshopper by its size and shape, orange inner surfaces of the hind tibiae and parts of the hind femora and an incomplete black crescent on each of the hindwings.
In West Africa, it has one generation per year. Localised concentrations of hoppers and low-flying swarmlets have been noted; these can be a serious pest of grazing pasture in wadis.
In Sudan, there are possibly two generations per year with dry-season diapause in the egg stage. It has been recorded damaging millet and cotton.
<section>Sudan plague locust-Aiolopus simulatrix</section>
The Sudan Plague Locust is found from east of The Gambia in a narrow belt across West Africa to Sudan and then southward to northern Tanzania and east as far as western Burma; it also occurs in the Seychelles. A second subspecies occurs in a narrow L-shaped belt from central Tanzania to southeast Zambia and South Africa (northern Transvaal) (Fig. 117). It is found in a wide range of grassland habitats, including moist grasslands, irrigated lands and cultivated fields, often by streams. In Africa, it is restricted to areas of clay soil which cracks in the dry season; this is mainly in the valleys of the major rivers. It is a serious pest of sorghum and millets in Sudan and a 'moderate' pest of these crops elsewhere. It is some 2122 mm long and generally a dirty sand colour with darker markings (Fig. 118). It is distinguished from other acridids by its forewing pattern, i.e. two dark irregular patches on a sandy background, the colour of the hind tibiae is banded
yellow/black/yellow/orange-red to the tarsus, the hind femora is deep (length=3 x depth) and the hind tibiae are shorter than the hind femora (Plate 4); the last two characteristics distinguish the species from A. thalassinus (distributed throughout Africa, southern and southeast Asia to eastern Australia).
Life cycle
The Sudan Plague Locust passes the dry season in the adult stage in the Sahel and Sudan and in the egg stage in Egypt and India. Information is given for Sudan.
Females lay 2-3 egg pods, each containing 15-40 eggs, in bare soil at the edges of often dense, patches of wild or cultivated sorghum. Egg pods may be straight, slightly curved (most common) or distinctly bent; the pod is 16-29 mm long with a froth tube 20-37 mm long. There are two generations, the first in the south in June and the second in the north in September. Incubation lasts about 18 days (first generation, June) and 23-28 days (second generation, August-September); there are five hopper instars and hatching to fledging takes 35-50 days.
The fifth-instar hopper is generally a pale brown colour with no markings, except for dark brown spots on the upper abdomen and a green stripe down each side of the abdomen. In the gregarious form, the whole cuticle is covered with darker brown spots and there are no green abdominal stripes. It is approximately 13 mm in length.
Migration
It is possible that the Sudan Plague Locust follows an annual migration cycle related to the ITCZ, similar to that of the Senegalese Grasshopper.
It is likely that the minor importance of the Sudan Plague Locust in West Africa is due to the lack of continuity of suitable habitat in a north-south plane. The cracking clay soils occur mostly within the major river valleys and whereas the Nile runs south-north, the major rivers in West Africa (the Senegal, the Niger and the Chad) do not. Thus, whereas first generation Sudan Plague Locusts in Sudan follow a strip of habitat in their migration following the ITCZ, any northerly migration in West Africa would take the locusts away from any suitable habitat. Hence, the build-up of populations which occurs in Sudan cannot occur in West Africa.
Behaviour
The cracks in the mud provide a constant environment, in terms of temperature and humidity, for the over-wintering adults. At the beginning of the dry season, the clay soil begins to crack (October) and these cracks are used as shelter from the sun by the Sudan Plague Locust and Acorypha clara. This habit continues with the cool nights spent in the cracks and activity in the warmth of the morning and afternoon; the heat of the day is spent in shade, either in the cracks or in vegetation. From mid-November through the winter, the Sudan Plague Locust only leaves the cracks during the heat of the day.
As the surface of the cracks dries out, the locusts move further down to more favourable (moist and warm) areas, finally retreating into horizontal cracks 70-90 cm below the surface.
<section>Rice grasshopper-Hieroglyphus daganensis</section>
The Rice Grasshopper is a strictly Sahelian species ranging from Senegal to western Somalia, between about 1§N and 22ºN (Fig. 1 1 9). It inhabits clay-based grassland subject to seasonal flooding. It is some 38-59 mm long and is generally a green colour with black markings. Its distinguishing features are the black markings on the front end of the sides of the thorax and on the sides of the pronotum (Plate 1, Fig. 1 20), and the rounded back edge of the pronotum. The shape of the pronotum and the thoracic marks distinguish this species from H. africanus also a Sahelian species, and Hieroglyphodes occidentalis, which is also smaller.
As might be expected, the Rice Grasshopper is principally a minor pest of rice, its natural habitat being wild rice grasslands, but it has also been recorded damaging millet.
Life cycle
Mating takes place from October to early December. Copulation may last for many hours. The egg pods are laid in the soil and diapause until the next rainy season.
The rains start in July and hatching occurs in that month. The hoppers are rather elongated and are bright green in colour. Gregarisation occurs at high population densities.
The gregarious form of the fifth- and sixth-instar hoppers are generally an orange-brown colour with a black facial mask and antennae and black spots all over the thorax, pronotum and legs; the abdomen is yellow on top with a black stripe each side, brown on the sides with darker brown spots and black underneath. There is also a black stripe on the hind femora.
Fledging occurs from August to November, the adults dying after mating and egg laying.
Polymorphism
The adult Rice Grasshoppers occur in two forms: a fully winged form and a flightless form with much reduced wings. Under normal circumstances, the flightless form is more common, but in upsurges more flying types occur-flight being an aid to dispersal.
Behaviour
Both adults and hoppers can swim and this is the normal route by which a rice field is infested. In fact, the swimming capabilities are such that individuals can remain under water for several minutes. Even mating pairs can swim, the female being the active swimming partner.
An unrelated species, Orthochtha venosa, is very similar in its biology and behaviour and the two species often form mixed populations.
<section>Diabolocatantops axillabis</section>
Diabolocatantops axillaris is distributed throughout the Sahel, the southern two-thirds of the Arabian peninsula and south to Zimbabwe and Mozambique (Fig. 121). It is a pest of a wide variety of crops. It is a pest of millet at the 'milky grain' stage, but does not eat the green parts of the plant, and can also be a problem on sunflower and sesame. In Sudan it is a pest of cotton. It is some 27-44 mm long and generally pale brown in colour. Its distinguishing feature is the red markings on the lower part of the inside of the hind femora and the bottom half of the hind tibiae (Plate 3), coupled with a small black, hyphen-like mark on the outer side of each hind femur (Fig. 122).
Life cycle
In Sudan and northern Nigeria, mating takes place in June and the females lay their eggs at the beginning of the rains, in late June and July. The eggs hatch in July and fledging occurs from August to September. In Tanzania, egg laying occurs from November to January, hatching from January to March and fledging from April to June. In these areas and in West Africa in general, D. axillaris has only one generation per year and the dry season is passed in the immature adult stage.
Generally, 18-81 pale brown eggs are laid in a mass some 11-30 mm long which is plugged with 7-25 mm of soft, off-white 'froth'; the eggs take on a green tinge just before hatching. There appear to be six hopper instars.
The hoppers of D. axillaris are green. The sixth-instar hoppers have white streaks on the abdomen and a few small black spots on the pronotum, thorax and legs; the eyes and antennae are brown. Their length is approximately 24 mm.
At the onset of the rains, the adults undergo a dramatic colour change, becoming considerably darker.
Behaviour
At the end of the rains, nocturnal migrations of solitary D. axillaris occur on the winds at heights of several hundred metres above the ground. After passing the dry season in adult diapause, the grasshoppers may migrate again at the beginning of the next rainy season.
The species has been observed to form 'small swarmlets' and 'incipient swarms' in Tanzania in December and May.
First- and second-instar hoppers normally feed by rasping away the top layer of leaf cells on either surface without producing a hole. Other hopper instars and adults bite holes through the leaf and continue to feed from the initial hole, making larger holes in the leaf, or they eat the leaf from the edges. Unlike other grasshopper species, D. axillaris does not normally straddle the leaf edge when feeding.
<section>Kraussaria angulifera</section>
Kraussaria angulifera is distributed throughout the Sahel from the west coast of Africa to the Red Sea coast of the Arabian Peninsula (Fig. 123). Its favoured habitat is savannah with trees, but it is also found in tall-grass savannah in the south of its range. It is often associated with other species, e.g. Senegalese Grasshopper and Rice Grasshopper, and can be an important pest of millet and cowpea.
It is some 40.5-64 mm long and generally brown in colour (Plate 1, Fig. 124). Its distinguishing features are its pronotum, which is very angular viewed from above, and has four yellow spots on each side and its hind tibiae, which are dull maroon not blue (as in Cataloipus fuscocoeruleipes).
Life cycle
The dry season is passed in the egg stage and the hoppers hatch at the onset of the rains in June and July. Fledging occurs from July to October and breeding and egg laying occur from September onwards. Egg pods, each containing 80-100 eggs, are laid in shade, often on abandoned agricultural ground. Most are found in the shade of a shrub known as Guiera senegalensis and may be at a density of 15-20/m² or more. In dry areas, where eggs could die due to excessive drying, an egg capsule is made of earth, which prevents loss of moisture from the eggs, and this is topped by a spongy substance mixed with sand grains. It is only these dry areas that are normally prone to K angulifera upsurges.
The hoppers are bright green with red spines on the hind tibiae and a white stripe low on each side of the abdomen. There are white spots on the pronotum and the eyes are striped blue and yellow-green. The third-instar hoppers have black spots all over their pronotum, thorax, legs, abdomen and behind the eye on the head. They are about 12.5 mm long. The sixth-instar hoppers have black markings only on their pronotum, thorax and legs and are about 25.5 mm.
Kraussaria angulifera has only one generation per year. The hoppers are more susceptible to insecticides than the adults.
Behaviour
Kraussaria angulifera is not normally gregarious and feeds alone. Morning and late afternoon flights are made to farmland from the surrounding habitat for feeding; the heat of the day is spent roosting in shade in the natural habitat.
The preferred food is millet, of which both the leaves and the immature seed-head are eaten, at the 'milky grain' stage only the grains are taken from the head. It is the 'milky grain' stage which is primarily attacked because it is ideal food and tends to occur at about the time when K. angulifera is in the late hopper or fledgling stages. Once a millet crop is destroyed or too mature, K angulifera populations move on to cowpea, where the flowers and green pods are eaten; both crops may be attacked at the same time if the cowpea begins to fruit before the millet is uneatable. Sorghum and groundnut may also be attacked in areas with no millet or cowpea. Economic loss to these crops is common and in 1972 in northern Nigeria was estimed at £120,000.
<section>Cataloipus cymbiferus and cataloipus fuscocoeruleipes</section>
Cataloipus oberthuri and C. cymbiferus are at present under review. For the purposes of this handbook both are described as C. cymbiferus.
Cataloipus cymbiferus is found throughout much of Africa from eastern Senegal to the south-western Arabian Peninsula and south to the northern border of Lesotho. Cataloipus fuscocoeruleipes is a strictly Sahelian species found from northeast Senegal to Ethiopia (Eritrea) and northern Kenya (Fig. 125). Both species occur in grassland habitats, favouring moist grasslands, but C. fuscocoeruleipes is confined to areas subject to periodic flooding. In a classification by habitat, the species are grouped with the Senegalese Grasshopper, the Rice Grasshopper and Diabolocatantops axillaris. Cataloipus fuscocoeruleipes occurs on cultivated and abandoned cultivated ground.
Identification
Cataloipus cymbiferus is some 34-63 mm long and generally brown in colour (Fig. 126). There is a pale stripe near the top of each side of the pronotum (Plate 1) which extends along the body. The hind legs are relatively large with a dark mark along the femora and the tibiae are blue with a pale ring near the base. There are faint pale spots on the sides of the pronotum.
Cataloipus fuscocoeruleipes is of a similar size and colour, but the pronotal spots are clear (not faint) and the femur mark is more ven along its length (Plate 1).
Females of each of these species are easily confused with Heteracris leant and Jagoa gwynni which are often abundant in the same areas.
Life cycle
In areas with a distinct dry season, Cataloipus cymbiferus has one generation per year, with egg diapause. The six hopper instars develop through the rainy season and adults mature soon after fledging and lay. In Tanzania, 22-103 brown eggs are laid in a mass some 11-32 mm long, they are enclosed in a strong case and are surrounded by a hard brown 'froth', this is extended into a plug some 11-53 mm long with occasional air-spaces in it. The adults die at the onset of the dry season.
In areas with a more moist and warmer climate, C. cymbiferus adults occur all year. It is possible that in these areas the adults can survive the dry season, but it may be that there is semi-continuous breeding with two generations each year.
The life cycle of C. fuscocoeruleipes is little known, but it undergoes a long egg diapause in the dry season in northern Nigeria. It is possible that there is sexual dimorphism in the number of hopper instars: five in males and six in females, but no detailed research has been carried out into this.
The fourth-instar hopper of C. fuscocoeruleipes is generally black and brown. The head is dark brown at the 'apex', pale brown-yellow in front of the eye-upper jaw line and the remainder is a golden brown colour. The pronotum is similarly golden brown with three black stripes towards the top. The wing-buds are black with yellow veins. The front two thoracic segments are pale brown with darker brown spots, whilst the third thoracic segment and the front of the abdomen are black. The abdomen becomes grey with black spots for most of its length; there are yellow stripes along the top and bottom. The hind legs are golden brown to two-thirds of the length of the tibiae, the remainder of the tibiae and tarsi being black. The other legs are pale brown with dark brown tibiae. The antennae are black for most of their length. The length of the hopper is approximately 19 mm.
Behaviour
Gregarious behaviour by Cataloipus cymbiferus in its long-winged, probably gregarious form, has been observed in West Africa and a large loose swarm occurred in Malawi in 1974. Gregarious behaviour has not been noted in C. fuscocoeruleipes.
In its short-winged, probably solitary form, C. cymbiferus is a poor flier. It has been noted to 'spend a considerable time flexing' the 'hind legs back and forth, often placing the femora in front of the head'.
Cataloipus cymbiferus feeds and roosts on the lower parts of plants, preferring areas of dense vegetation. Feeding is carried out with the head orientated upwards. In Tanzania, adults of this species have been observed to seek out preferred food plants (drying Cyperus spp.) among dense vegetation.
Cataloipus cymbiferus is recorded as damaging various crops, but only maize and rice seedlings to any great significance. Cataloipus fuscocoeruleipes is a pest of pasture in Kenya. Cataloipus, with other grasshopper species, can be an important pest of bulrush millet in Chad and Sudan.
<section>Variegated grasshopper-Zonocerus variegates</section>
The Variegated Grasshopper belongs to the group known as pyrgomorphid grasshoppers, some of which are brightly coloured as an advertisement to predators that they are poisonous. It secretes its poison from a gland in the first abdominal segment.
Zonocerus is found throughout West Africa to western Chad and the western border of the Central African Republic. It also occurs in a band from northeast Angola to central Kenya and Ethiopia (south of Eritrea) (Fig. 127). Its habitat is cultivated areas of the 'forest zone'. and riverine forest and swamp areas of the 'savannah zone'.
In appearance, Zonocerus is unmistakable (Plate 4). The basic body colour is yellow with extensive black markings on the head, thorax, legs and tip of the abdomen; there are also red markings on the head, legs and tip of the abdomen and white markings on the head. The pronotum is green-yellow. The forewings are green. The antennae are black except for the last segment and one or two other segments which are orange-yellow. The eyes are red. The adult is 28-49 mm long (Fig. 128).
Life cycle
The life cycle of Zonocerus is perhaps the best studied of all Sahelian grasshoppers. There is one generation per year, but in many damper areas there are two distinct periods of hopper and adult abundance suggesting two physiological races.
Zonocerus seeks out areas of continuous shade in which to lay its eggs. Any plant offering such shade is used as an egg-laying site, particularly isolated shrubs and shrubs on the edge of denser vegetation; these sites are very often within 30 m of fields of cassava (Manihot esculenta). The Variegated Grasshopper is active during daylight only and the females are attracted to the egg-laying sites from mid-morning onwards usually from a downwind direction. Before this they may be found roosting either in the tops of the same shrubs or in adjacent fields of food crops. As a female approaches the egg-laying site (at 2-5 m ranger, it is mounted by a male. Whilst mounted, the males buzz, continually move their legs and display rapid bursts of body twitching. Once under the shade, the females probe the soil with their abdomens (ovipositor}; 'test probe holes' are made as near to the base of the bush as the presence of other females and egg pods already laid will allow until a
suitable laying position is located. Once egg laying has occurred, the females depart, males dismounting at 2-6 m from the site. Egg-laying activity tends to peak around midday and trails off rapidly in the afternoon. Females arriving at the site in the afternoon climb the bush to roost overnight, many with partners. Marking experiments have shown that most females only visit the egg-laying sites once and for one day only, whereas many males return to the site and/or are present for more than one day. In the course of her lifetime, a female may lay up to six egg pods, however, normally only one or two are laid as females are killed by the parasite Blaesoxipha filipjevi.
The dark brown eggs, numbering 17-98, are laid in a mass some 15-28 mm long with a 15-40 mm plug of 'soft, pale brown froth'. The egg pod is cushioned by a 1 mm layer of froth. The egg pod itself is usually straight, but may be slightly curved. As they develop, the eggs become reddish-brown. The eggs develop in four months or 6-7 months if there is a diapause.
There are generally six hopper instars (Table 15), but 4-9 have been recorded (mostly in the laboratory}. In the field, fledging takes place 100-120 days after hatching.
TABLE 15
Instar
Length (mm)
Duration {days}
At hatching, first instar hoppers are uncoloured, but within about 2 h they are fully coloured. The first-instar hopper is predominantly black and yellow. The body has alternate black and yellow stripes running along its length. The head is a mozaic of black and yellow, the eyes red-brown and the antennae black with one yellow segment half way along their length and a yellow tip. The legs are black, banded in yellow; there is only one broad yellow band on each of the hind femora.
The second and subsequent instars are similar but the body stripes are black and white, the white becoming yellow at the rear of each segment.
Seasonality of life cycle
Much work has been carried out on the two populations of Zonocerus which appear in the same regions of Nigeria. In the savannah regions of Nigeria, there is only one generation and one population of Zonocerus. Further south, however, there appears to be two populations separated each year by climate. These are known as the dry-season and wet-season populations. In the north, the wet-season population is restricted to river valleys and this corresponds to the wet-season population in the south.
The precise timing of the life cycle in each population seems to vary a little from area to area. Generally, dry-season population eggs hatch from mid-September to November, adults emerge from January to March and breeding occurs from March to June. Wet-season population eggs hatch from December to April, adults emerge from July to August and breeding occurs from August to October. In these cases, the two populations will not overlap and therefore not interbreed. In other areas, the actual periods concerned are longer, overlap occurs and individuals from the two populations are capable of interbreeding.
In one area of southwestern Nigeria, it has been suggested that there is only one population and that oviposition occurs continuously from February to December, peaking in April. Hatching occurs from October to March, peaking from October to November. Eggs laid early in the season take six months to hatch and include a period of diapause. In June there is a switch to non-diapause and eggs hatch within four months and this is true for the rest of the season, although hatching in as short a period as two months has been recorded. The second peak of adult abundance (June-July) is thought to be due to heavy mortality of early-season adults caused by parasitism by the fly Blaesoxipha filipjevi (Chapter 8), leaving no live adults during April.
It is the dry-season population which produces the greatest numbers and is a pest in many areas. In wet situations, the population is limited by the fungal disease caused by Entomophaga grylli (see Chapter 8).
Behaviour
The more abundant dry-season population tends to become concentrated at egg laying. Thus, the first- and second-instar hoppers of this population occur at high concentrations and have a tendency to become aggregated. This habit is reduced in the third, and generally lost in later instars and only partially returns in the aggregated egg-laying habits of mature adults.
The first- to third-instar hoppers show a preference for Siam weed (Chromolaena odoratum) and do not thrive on cassava. All other stages feed on cassava leaves, especially the lower, partially wilted ones.
In general, adults have short wings and cannot fly for long distances, but the more successful dry-season population does produce a proportion of fully winged adults capable of flight.
The Variegated Grasshopper is only active in the daytime, spending the night and cooler hours of daylight resting near the top of the broad-leaved herbs on which it feeds or under which it lays its eggs.
Pest status
Cassava is the main crop attacked, but the yield of tubers is only affected if damage is prolonged. In cases where cassava plants are only stripped of their leaves once in a season the plants regrow and no yield loss occurs. Where cassava leaves are eaten throughout the season, however, damage is considerable. If no natural vegetation is available throughout the hopper and adult life of the dry-season population, bark, buds and growing points may be damaged and natural regeneration may be delayed sufficiently to reduce tuber yield at harvest (up to 65% loss).
Other crops which may suffer significant damage include citrus, coffee, oil palm seedlings and yams.
Control
The Variegated Grasshopper's specific requirements for egg laying means control measures can be carried out by the farmers themselves. To reduce the following year's dry-season population, all that needs to be done is to locate as many egg-laying sites as possible. Once egg laying is complete, the eggs can be destroyed by hoeing or digging up the pods in all of the sites discovered.
Elegant Grasshopper-Zonocerus elegans
The Elegant Grasshopper effectively replaces the Variegated Grasshopper in southern, east-central and East Africa, although there is some overlap of ranges in northeast Angola, central Congo, southern Uganda and western Kenya; the Elegant Grasshopper also occurs in Madagascar. It is a savannah species and feeds on a wide range of plants causing particular damage to fruit trees and young cotton.
Zonocerus elegans is mainly yellow, but heavily marked in black, white and green. The eyes are orange-red and the antennae are alternately banded black and orange. The wing bases are red. The hoppers are yellow and black with white markings, but Z. elegans is rather more variable in colour than Z. variegatus. The adult is 32-51 mm long.
The adult is normally slow-moving, short-winged and flightless, although fully winged adults occur but are poor flyers. The life cycle is similar to that of the Variegated Grasshopper.
<section>Kraussella amabile and ornithacris cavroisi</section>
Kraussella amabile and Ornithacris cavroisi were virtually unknown as pest species until 1989, when both occurred in grasshopper outbreaks in Mali.
Kraussella occurs from Mauritania and Senegal in the west to northeast Ethiopia in the east. It is a grassland species. Ornithacris cavroisi has a much wider distribution, extending from Cape Verde to southern Libya and central Sudan, and south to northeast Angola and northwest Kenya. It also is a grassland species found in most grass types.
Identification
Kraussella amabile is some 21-35 mm long, principally yellow in colour with dark markings. The combination of dark blotches on the hind femora and dark lines on the pronotum (much like the Rice Grasshopper) is diagnostic.
Ornithacris cavroisi is about 63 mm long and is similar to the Red Locust in appearance. The markings on the pronotum form an arrowhead like marking, unlike the Red Locust which has parallel marks on the pronotum (See Plate 2).
Life cycle
Both species have only one generation per year.
Kraussella lays its eggs at the beginning of the dry season and these lie dormant until the onset of fresh rains. Hoppers develop from June to August and adults emerge August to November. The hoppers are distinctive, being typically green with a brown dorsal stripe (along the back), which is edged by darker brown and cream.
Ornithacris cavroisi survives the dry season in the immature adult stage. Adults mature and disperse to lay in May and June. The eggs hatch in July and August and 6-7 hopper instars precede the adults which appear in October. Green hoppers have a distinctive pronotal crest which is white broadly edged with pinkish brown; the hind tibia is also pinkish brown. Brown hoppers have pink markings on the legs and mouthparts.
<section>5. Southeast Asian locusts</section>
<section>Oriental migratory locust-Locusta migratoria manilensis</section>
The Oriental Migratory Locust is a subspecies of Locusta migratoria and its distribution in southeast Asia is shown in Fig. 129. It has long been an important agricultural pest in the region; the earliest records of plagues come from China in 707 BC and the Philippines in the sixteenth century. Locusta is widely distributed in low-lying grasslands, and gregarious populations have been produced in several outbreak areas.
Life cycle
There can be as many as five generations a year depending on the temperature; there are more generations in warmer areas. For example, 3-5 generations in the Philippines; 4 in Thailand; 3-4 in China at latitude 25ºN and 1-2 at latitude 40ºN.
This species is very similar to the African Migratory Locust but is smaller. Solitary adults can be green or brown but as the population becomes more gregarious there are more brown varieties. Maturation takes 3-4 weeks.
Eggs are laid in a variety of soils including volcanic ash, alluvium and sand. There is a wide range in the number of egg pods laid and the number of eggs that they contain. Females can lay 2-7 pods (a maximum of 12 has been recorded) at intervals of 4-15 days. The number of eggs per pod can be as low as 15 and as high as 100. Solitary locusts lay more eggs than the gregarious phase.
Hoppers hatch from 10 to 24 days after the eggs were laid. At this time they are grey-brown in colour but this changes with age according to population density and habitat. Solitarious hoppers are green or brown, brown being more likely in dry conditions. At high densities the colour becomes a reddish-brown or brownish-orange with distinctive black markings. There are usually five instars but some females go through an extra moult. The hopper development periods recorded in four countries are given in Table 16.
TABLE 16
Malaysia
26-32 days
China (Taiwan)
37-52 days
Philippines
43-61 days (Males)
42-58 days (Females) Mean 48
Thailand
27-39 days (Males five instars)
30-40 days (Females six instars)
Behaviour
There is little information on solitary hoppers but gregarious hoppers form basking groups and bands which can march up to 4 km/day. Solitary adults fly at night but gregarious adults fly during the day.
Outbreaks, plagues and seasonal movements
Outbreaks of the Oriental Migratory Locust have occurred in several parts of the distribution area, in western Malaysia, Sabah, eastern China and the Philippines (Fig. 129). There seem to be two types of habitat likely to provide ideal conditions for population increase. The first is a region of low-lying intermittently flooded ground such as that in the delta areas of the Hwang-Ho in China. The second is an area of tropical grassland where forests have been cleared. Local people practice shifting cultivation for a few years then abandon their plots for a new area. The abandoned plots are invaded by grasses such as Imperata species which provide food and shelter for locusts. Nearby open spaces used for cultivation are ideal laying sites.
China
Recent studies suggest that outbreaks occurred every 9-11 years between 1906 and 1956 but 75% only lasted for one year. They were more likely to occur after dry summers and autumns followed by warm winters. It is thought that the dryness provided more laying sites; warm winters put less stress on the eggs so that a higher proportion survived. Limited flooding also lead to population increase but continued flooding had the opposite effect. Unlike Locusta in Africa swarms produced in China do not migrate very far. They expand outwards from the breeding areas but are restricted to the river valleys by the surrounding uplands. Movement northwards takes them to areas that are too cold and to the south it is too wet for successful breeding. By 1962 the outbreak areas had been changed by flood control, drainage, cultivation and afforestation. Since then there does not appear to have been another outbreak.
Philippines
There have been four plagues this century: 1919-1929, 1932-1939, 1941-1949 and 19581960. Analysis of the plagues has suggested that they originated from an area in southern Mindanao around Sarangani Bay. This was an area of natural grassland. The soil is porous containing a high proportion of volcanic ash so that in any month when the rainfall is less than 50 mm there can be local droughts.
The last outbreak occurred in 1958 and is one of the few where the early stages were recorded. In Mindanao the rainy season lasts from April to November when the island is under the influence of the summer monsoon. The wettest months are July and August with average falls of 106 mm. Although rain falls every month there is a drier season between December and March when the winds are mainly from the northeast. March is the driest month with an average rainfall of 45 mm.
Analysis of the previous plagues suggested that they occurred after a period of drought or below average rainfall. In 1957 concentrations of adults were controlled in June around Sarangani Bay. In November and December increasing densities of locusts were observed. By early February 1958 hoppers and clouds of locusts were reported some 100 km north of Sarangani Bay in the central plain of Cotabato. The rainfall from October 1957 to March 1958 was 133 mm, about 22 mm per month. This was the driest period on record since January 1949.
In mid-March there were reports of mature locusts in groups and at the end of the month small swarms were seen in the outbreak area. Laying occurred in April and hoppers and adults appeared in May and June. By July and August there were many reports of concentrations of solitariform locusts in central Cotabato as well as the outbreak area. In September there was a northward shift in the infested area and several reports of day-flying swarms which covered a reported area of 5478 ha.
There were fewer reports during the following dry season but locust activity increased again in 1959 and terminated in late 1960. There were no reports of swarms outside southern Mindanao.
The movement of swarms within the Philippines is not clear. Analysis of earlier plagues suggested that there was a gradual northward extension across the archipelago over seven years (some 1500 km). There have also been outbreaks on other islands, for example, Palawan, Bohol, Panay in 1952, Luzon 1959, Masbate 1962-1964 and Negros 1965.
Outside the Philippines it is thought that locusts invaded Taiwan from Luzon and invaded Sulawesi, probably from Mindanao, but movements from Sabah and Sulawesi to the Philippines have also been considered.
The origin of swarms in Peninsula Malaysia and Sabah is also unclear but both have areas of cleared forest with extensive grasslands providing ideal local breeding conditions which could lead to outbreaks.
Recent outbreak in Japan
There was an outbreak of Locusta migratoria manilensis in southern Japan in September 1986. Swarms were reported on Mageshima Island which is located to the south of Kyushu and west of Tanegashima. The island has been uninhabited since 1980 and the vegetation is a mixture of wild grasses and abandoned rice fields.
There are two generations of Locusta a year and population increase is thought to have begun in 1984 following dry conditions. On 20 November 1985 a grass fire burnt a quarter of the island. The effect of this appears to have been to concentrate the locusts in the remaining areas of vegetation for feeding, this was followed by further concentration as the females preferred to lay in the open sites of burnt land. This laying took place in spring 1986.
The grass fire also allowed good growth of a favoured food plant, Eulalia (Miscanthus sinensis) so that there was plenty of young green vegetation for the emerging hoppers.
By September, when the island was visited after fishermen had reported seeing swarms, locusts were found widely over the whole island. There were marching bands of hoppers reported at densities up to 1000/m² and flying swarms were seen between 26 September and 21 October. Spraying was carried out in December and the locusts were said to have been controlled.
Decline in Locusta plagues
Since 1960 Locusta has become a minor local pest in the Philippines. This would appear to be the result of increasing land use in those areas where grassland had predominated. Not only rice and maize are cultivated but cassava, pineapple and citrus; other areas have been planted with imported grasses and used for ranching. There are thus few sites where locusts can live and breed undetected. In both the Philippines and China Locusta has declined as an agricultural problem, however, its wide distribution and ability to live successfully in newly created grassy habitats make it a potential danger elsewhere in the region where land use change is in progress. It is also important to remember that if a pest has not been a problem for several years it does not mean that it will not be again.
<section>Javanese grasshopper-Valanga nigricornis</section>
The genus Valanga is probably undergoing a period of rapid evolution and shows considerable adaptability. Some 18 subspecies of Valanga nigricornis have been recognised occurring throughout the area shown on Fig. 130. It is found in forest clearings and shrub thickets, feeding mainly on trees.
Life cycle
The adult is a yellowish-brown, or yellowish or greenish with blue-black markings; the hindwings are a deep rose colour. Males are 45-55 mm long and females 15-75 mm (Fig. 131). There is one generation a year. Eggs are laid in clearings in moist soil. Females can lay 2-4 pods at intervals of 12-41 days, with a mean of 158 eggs per female. Hoppers hatch after 60-75 days in Malaysia but in Java it takes 6-8 months. Males usually develop through six instars, sometimes seven, and females through seven, occasionally eight. In Malaysia, hopper development in cages was 92-125 days for males and 111-136 days for females. Hoppers are light green with black markings.
In areas with marked wet and dry seasons it appears that hoppers hatch and develop through the wet season so that Valanga survives the dry season as an immature adult, e.g. Thailand. In Java however, the dry period is survived in the egg stage. Elsewhere where there are no pronounced seasons all stages can be found together. For example, in west Malaysia there are two peak periods of laying, December-January and June-July. Hoppers hatch about 60 days later and fledging occurs in June and December. The adult stage from fledging to final laying lasts about six months which suggests that there are two physiological races each having one generation a year.
Behaviour
Young hoppers remain close to laying sites. Both hoppers and adults like sunlight and seek out sunny areas for basking, either at the top of vegetation or on open sites. Most feeding is done during the daytime. There are no records of gregarious behaviour.
Outbreaks
Most damage done by Valanga occurs when numbers increase locally. There are no records of invading swarms and large-scale movements. Local population increases appear to be related to rainfall. Heavy rainfall causes high mortality in both eggs and hoppers but dry periods allow numbers to increase. In Malaysia it is thought that several consecutive dry seasons are unlikely and as Valanga has only one generation a year then widespread population increase also seems unlikely.
In Java, where eggs remain dormant for 6-8 months, dry weather delays hatching but the arrival of the west monsoon results in the simultaneous hatching of many hoppers. Fledging occurs towards the end of the west monsoon and the young adults soon mature and lay. If the succeeding east monsoon is particularly dry, however, the adults concentrate and if these conditions are repeated the following year then there will be many locusts. The outbreak in 1915 in the teak forests of Java was thought to have followed three seasons of below average rainfall and synchronisation of breeding.
Elsewhere outbreaks are thought to have arisen when forest areas were cleared for cultivation.
<section>Bombay locust-Nomadacris succincta (formerly Patanga succincta)</section>
The Bombay Locust is widespread in southwest and southeast Asia (Fig. 132). It was a major locust pest in India in the eighteenth and nineteenth centuries but has rarely swarmed since the end of the last plague in 1908. However, in other countries it has become an important local pest as forest areas are cleared for cultivation. It inhabits grassy plains up to 1500 m and secondary habitats of coarse tussock grasses (mainly Imperata spp.), shrubs and trees.
Life cycle
There is one generation a year and the adults usually spend the dry cool season as an immature adult. With the arrival of the rains they mature and lay. In Thailand females lay 1-3 egg pods containing 96-152 eggs; in India up to four pods have been recorded with a maximum of 606 eggs from all four pods. The adults die soon after laying. There is considerable variation in the timing and extent of egg and hopper development. Table 17 compares the laboratory data from Thailand, India and Malaysia.
TABLE 17
India
Thailand
Malaysia
Laying
June-July
March-April
August-September
Egg incubation
Hopper development
Number of instars
6,7,8 males
(7 most common)
7,8 females
Fledging
September
July-August
January-February
The hoppers are green with black spots in the early instars. In the later stages the colours are more variable: there is a green form and a dark orange-brown form both with two black spots at the base of the wing pads and a green form without the black spots.
Immature adults are generally pale brown with a pale yellow stripe along the back of the body. There is also a pale band on the sides of the pronotum with darker bands above and below. This colour darkens after 6-8 weeks to a rosy red colour especially on the lower part of the hindwings. On maturation the locusts become dark brown.
Behaviour (data from Thailand)
Hatching occurs at night or in the early morning and the young hoppers move into stands of various small wild grasses. From about the third instar they begin to feed on other plants, especially maize. During the early part of the day hoppers are found near the tops of plants on the east-facing side, at midday they are likely to descend to shaded parts returning in the early evening to enjoy the last of the sun on west-facing plant stems. Bands of the Bombay Locust have not been reported but this could be because they would be difficult to see in dense vegetation.
Adults
Immature adults in non-swarming populations are remarkably static. In Thailand up to 50 immature adults have been recorded from a single maize plant. Their movements are restricted to a few adjacent plants. If disturbed they will fly up to about 100 m and then settle. After the maize is harvested the locusts move back to the local grasses to feed. Populations become concentrated during the dry season especially when burning of the old maize stalks occurs. However, there is still only local movement between stands of grasses and shrubs and banana groves. With the onset of the rainy season the adults rapidly mature, copulate and lay.
Plagues, seasonal movements and outbreaks
It is only in India that the Bombay Locust has formed swarms, and the following account is based on the last plague in 1901-1908.
From November to March swarms spend the cool months in the western Ghats. In March and April the swarms become more active during the day leaving the forest areas where they have spent the preceding four months. By May the southwest monsoon has become established, and the swarms are displaced northwards, northeastwards and eastwards towards Gujarat, Indore, Nagpur, Hyderabad and the eastern Ghats. By this time the infested area can be some 500,000 km²; the vegetation is typically dry woodland, perennial grasses and millet cultivations. If rain falls in June the swarms split up and lay, but if the rains fail then the swarms continue to move northeastwards possibly as far as Orissa, Bihar and Bengal.
Females prefer to lay in the black cotton soils avoiding dry or waterlogged soil. They choose areas of open grassland, burned millet fields or the bunds (raised areas) between fields. Fledging begins about September and the young adults remain amongst the ripening millet or wild grasses. As numbers increase loose swarms are formed which gradually increase in size and density. In October and November northeasterly winds become predominant over the area and swarms begin to displace in a south and westerly direction until they once again reach the western Ghats. Interestingly, although some movement occurs by day most of the return movement apparently occurs during the night.
The last swarm was reported from India in 1927 since then the Bombay Locust has only been found in small numbers. It is thought this is a result of cultivation spreading into areas of natural grassland and reducing the area available to the locust.
There have been outbreaks in Thailand and Malaysia where forest clearance has been the major ecological factor. In Malaysia the outbreaks appeared to be restricted to one year after clearance. This would seem to be a result of growing upland rice and regularly weeding the plots. Outbreaks in Sabah, the Philippines and Vietnam occurred in areas where natural grasslands were cultivated and a wide range of crops were damaged.
<section>6. Locust reporting</section>
It is absolutely essential to have accurate and timely information about the occurrence, numbers and movements of locusts, from the whole of the area in which they can be found so modern control methods can be properly used and so that even better ones can be discovered and organised (Fig. 133). Information is required about both swarming and non-swarming locusts, the latter being particularly important when swarms are not present.
This means that there must be an improvement in the present extent and standards of reporting and this guide is presented with that object in view. It aims to be simple and practical because any reporting system which is complicated will never be used by those who have large areas to cover and many different jobs to do, especially during anti-locust campaigns when time must be mainly devoted to killing locusts.
<section>The aims of a reporting system</section>
The aim in preparing locust reports is for the person who sees the locusts to give an accurate and useful picture of what is happening in the infested area to someone else who needs to know about them. This guide shows how that can be done, but it must not be forgotten that the usefulness of any report depends not only on the skill, accuracy and energy of those who prepare it, but also on the experience and efficiency of those who interpret and use it. All must be trained and disciplined to use the chosen system correctly.
In preparing a system of reporting there are five things to consider.
1. Who needs the reports?
2. Why do they need reports and how can they be used?
3. What information is required?
4. How can the information be collected and recorded?
5. How can the information best be transmitted to those who need it?
Who needs the reports?
(a) Those in charge of anti-locust campaigns.
(b) Emergency Centre for Locust Operations, Food and Agriculture Organization of the United Nations, (FAO), Rome.
(c) Research workers.
Why do they need reports and how can they use them?
(a)Those in charge of anti-locust campaigns need them so that they can arrange for supplies of personnel, vehicles and stores, determine priorities and generally carry out the campaign efficiently.
(b) FAO needs them to provide information on the general locust situation to all interested countries, and to give timely warnings to those countries in danger of invasion.
(c) Research workers need them so that they can learn more about locusts: their life and behaviour, their distribution and intensity of breeding, their movements, and the factors affecting all these. Forecasting could then be improved and new and better ways of killing locusts devised, and the ultimate object of such research is to discover how locust plagues may be prevented, rather than merely suppressed after they have occurred.
What information is required?
(a) Those in charge of anti-locust campaigns need to know:
date and time of observation;
where the infestations are;
what the infestations are, i.e. eggs, hopper instars, or adults; swarming or non-swarming; extent and scale of infestation;
estimated consumption of materials used in control operations;
amount and state of available transport and equipment.
(b) FAO needs to know:
date and time of observation;
where and what the infestations are;
extent and scale of infestation;
what sort of weather prevails;
what control measures are being carried out; whether the infestations are being completely destroyed or whether locusts are surviving in large numbers to form new swarms.
(c) The information required by research workers depends upon the kind of research being carried out, and special requests will have to be made as occasion demands.
How can the information be collected and recorded?
Much of the information required by various people is the same, which means that the number of observations to be made at any one time can be limited. The people who make observations in locust habitats are the local inhabitants, locust scouts and locust officers.
The basis of all reporting must be the record obtained by the locust officer. This is composed of what he sees himself and the knowledge gained by careful questioning of his scouts and the local people, particularly in places where people gather together and talk, e.g. market places and watering places. The most convenient way for the locust officer to keep his record is in the form of a daily diary printed in book form and consisting of a basic questionaire (Fig. 134).
Daily diary
The locust officer who fills in a daily diary regularly will have a complete record of both what he has seen and the work he has done. It is suggested that the pages to be filled in should be printed, and should be bound, with instructions on how to complete the diary printed on the inside of the cover. Accuracy is vital. Only report those facts that you believe to be correct. Do not rely on your memory; fill in the diary at the time when you make the observations or receive reports from other people.
Locusts observed or reported
1. Date and time
Record the time of your observation, or the date and time that refers to a report you have received. In the case of a report, which may be some days old, you must be careful to give the date correctly, since it will not be the same as that for your daily diary, which must be entered at the top of the page. Use local time and the 24-hour system, e.g. for 3.30 p.m. write 1530 hours.
2. Place and/or latitude and longitude
Give a place name which is marked on the maps that are in general use, or fix the position from such a place, e.g. 23 miles southeast of such a place. Remember that the people who will study your reports may never have heard of the small villages in your country; therefore latitude and longitude should also be given, when possible to the nearest minute, e.g. 27º12'N, 73º16'E.
3. Type of population
Write 'swarming' or 'non-swarming', whichever applies.
4. Stage
Write eggs, hoppers or adults, whichever are seen.
5. Age, colour, maturity
Write down, if known, for:
eggs; the number of days since they were laid
hoppers; the instar and colour, if the instar is not known, give colour only
adults; maturity of eggs in dissected female or colour of body and wings.
6. Size of infestation
Give any dimensions that can be measured.
Flying swarms. It is difficult to measure their size without aircraft, but useful observations can be made. For example:
(a) took 1 h to pass overhead (give wind speed and direction)
(b) extended from point A to point B, if there are two suitable recognisable points, such as villages or hills
Settled swarms. For example:
(a) took 1 h to walk through
(b) 2 km across by vehicle
Hoppers. Give the number of bands seen in an area or distance traversed on foot or by vehicle, and the size of the largest band seen.
Egg fields. Give the size of the area if you can, but this will only be possible if you or someone else actually saw the swarm laying. Otherwise state the points at which egg pods were found.
Non-swarming adults and hoppers. Give the number of adults and/or hoppers seen, together with the distance or area in which they were seen.
7. Density
This is one of the most difficult things to record. The best way is to count the numbers in sample areas.
Swarms. Record the time of day when the swarm was seen, and whether its shape was stratiform or cumuliform, because the time and shape are sometimes related to density. The terms thin, dense and medium are used for describing the density of swarms, in the following way:
thin -the swarm is visible only when near enough for separate locusts to be distinguished
dense -the swarm blots out parts of nearby features, such as trees
medium-the density is between 'thin' and 'dense', most swarms seem to be in this category
Hoppers in bands. Density is very closely related to behaviour, so state whether the hoppers are roosting, marching or in ground groups.
Eggs. Carry with you a wire square of a standard size, e.g. 1 m². When groups of egg pods are discovered, count the maximum number which can be included in the wire square. Detailed fir-scale surveys of egg fields require several well-trained observers and are very time-consuming. Such an elaborate survey must be considered as being a research responsibility rather than the duty of a control officer.
Non-swarming populations. Adults. Use a hand tally counter to count the number disturbed (flushed) in a stated distance, and whether done on foot, by animal or by car. Adult locusts can only be counted in this way at a time when they are active. When walking and doing counts of this kind you should move upwind. Also record whether the locusts were seen singly or in groups. State if possible the highest number seen in a group. The proportion of the locusts present that are flushed during a traverse will depend upon the air temperature. Record the air temperature at the time of the traverse and include it in your report.
Hoppers. Walk through the infested area and count the hoppers seen in a 1-m wide strip. If there is insufficient time for this, state whether the hoppers are seen singly or in groups. Estimate and state the number in the largest group and the area covered by the group.
8. Direction of displacement
Swarm. Wait until you can determine the direction in which the whole swarm has moved.
Hoppers. Again it is the direction of displacement of the whole band which is wanted. Mention if there is a general direction of movement of bands over a wide area.
9. Remarks
Write down the name of the person who saw the locusts (even if it is yourself). Express an opinion about the reliability of the report if it is not your own. State here any other observation which you consider interesting, for example, locusts dying, showing signs of maturation or being eaten by predators.
Control measures
1. Area over which control has been carried out
Give the area between certain reference points which can be recognised on a map; if possible, give size of area in square kilometres or say within a certain radius of a known place.
2. Action against swarms
State whether a whole swarm or part of a swarm was destroyed and by what means. Also state whether you think there was a good kill (because dead locusts were obvious) or a bad one (because you had to look carefully to find any dead).
3. Number and size of hopper bands destroyed
Write down the figures which you know to be correct; add a suitable note if you believe these to be underestimates. Make sure that any bands that escape destruction are recorded in a separate note; failure to destroy all bands in a heavy infestation must not be regarded as bringing discredit on the control teams, and must always be mentioned when it occurs.
4. Material used
It may not always be possible to record this daily, but do it whenever you can. The total number of bags of bait, litres of liquid insecticide or tons of dust used should be recorded.
5. Transport
Information can often be obtained from log books which should be kept by the drivers of the vehicles. The information particularly needed is the number of vehicles of each type used, the number of miles they travelled and the amount of fuel used. Criticisms of performance should also be noted, including the number of days during which any vehicles were broken-down.
From this diary the locust officer can prepare his daily report which should be sent by radio to operational headquarters. It should contain the following information: locusts observed or reported; requests for supplies based on the stocks position recorded in the diary. Locust reports should be forwarded to country headquarters every 2-3 days where weekly summaries should be prepared on FAO forms and despatched without delay. Requests for supplies should be sent to headquarters according to an agreed schedule. At the end of the campaign the officer should use his diary to prepare a Campaign Report. Guidelines on preparing such a report are given in the Appendix. Filling forms can be a tedious activity but it is the only way to collect and store important information. Much of what is now known about locusts was discovered by the careful analysis of reports; without them little progress would have been made. It is still vital to have accurate reports quickly so that control
organisations can operate efficiently. It is cheaper and safer to control small, gregarising populations of limited area than to cope with many mobile swarms over hundreds of kilometres.
How can the information be best transmitted to those who need it?
Information which is needed for immediate action must be sent quickly either by radio or telephone and can be confirmed in writing later. Figure 135 indicates the channels along which the locust informaton should flow so that it can travel quickly from the people who collect it to those who need it elsewhere.
<section>7. Controlling locusts</section>
<section>Control strategies</section>
The principal aim of strategies designed for locust control is to reduce the size of the total population of insects and not only to attack insects in or near crops. This is the only way to achieve crop protection with such mobile pests, and to prevent plagues.
1. Red and Migratory Locusts. These species have outbreak areas where the first sequences of a plague develop (pages 102 and 106). The aim of control is to prevent the plagues by controlling any bands and swarms which form in these areas. Swarms of the African Migratory Locust have formed outside the recognised outbreak areas and caused local upsurges on a significant scale, notably in the Republic of South Africa during 1982 and in Sudan during 1985, but these have not led to a plague. Similarly swarms have escaped from the Red Locust outbreak areas many times but no plagues have resulted.
2. Tree Locust. This species does not migrate long distances and is, therefore, a localised problem.
3. Desert Locust. Upsurges leading to plagues are believed to occur through sequences of successful breeding by initially solitary-behaving populations. The sequences start in the arid central belt recession area. Each upsurge then requires a series of above average rains in areas to which the locust migrate in successive generations (page 36). Some experts claim that control of gregariously behaving locust populations at the start of the upsurge sequences can prevent a plague. However, while the gregarious populations are obvious targets for spraying, their destruction does not necessarily make significant inroads into the critical mass of the population. Solitary locusts, which are not targets in practical or economic terms, migrate and continue to multiply. These locusts expand into an ever increasing area for several generations and it is only when they come together into swarms and become recognisable targets that effective control can be achieved.
<section>Chemical control</section>
At present, for both locust and grasshopper outbreaks, the application of broad-spectrum insecticides, often on a large scale, is the only effective control measure, and so far resistance to these chemicals has not developed.
To kill locusts efficiently, five things are needed:
information -on the location, life stage, size and density of infestations (see Chapter 6);
insecticides -appropriately chosen and applied safely;
trained manpower;
machines;
an organisation -to fund, implement and evaluate control methods and campaigns.
Advice is given here on methods of chemical control. The control measures carried out by any particular country depend on its resources and on the co-ordinated plans for the region to which that country belongs. Good control operations require trained and experienced personnel, efficient materials and equipment. Consideration is also given to the safety of insecticide application and storage (see page 182) and the dangers of polluting the environment (see page 187).
Insecticides
In order to kill locusts with insecticide they must either swallow it or get it on the outside of their bodies. This is achieved by:
stomach action: putting the insecticide on or in the food, either natural vegetation or a specially prepared bait;
contact action: by spraying the insecticide directly on to the locusts in such a form, often dissolved in oil, that it will penetrate the cuticle.
Insecticides may be applied in dry forms as baits or dusts. Generally insecticide concentrations are low with these types of formulation. They can be used in oil-based formulations as ultra-low volume (ulv) applications. These formulations are applied in a concentrated form and are not diluted with water. Water-dispersible formulations of insecticides come in liquid or powder form.
Liquids: emulsifiable concentrate (ec), water miscible liquid (wml), water soluble liquid (wsl) or soluble concentrate (sc).
Powders: soluble powder (sp) or wettable powder (wp).
Insecticides and formulation
The insecticide formulation is designed to present the active ingredient (a.i.) in solid or liquid form as a toxic dose to the locust.
The formulation selected will depend upon the insecticide's inherent toxicity and the practical requirements of the application technique. For instance, the synthetic pyrethroids are highly toxic and the dosage of active ingredient applied per unit area need only be small. The application of a low volume, high concentration, formulation is, however, impractical since a high proportion of the relatively expensive insecticide would remain as a residue in the spray system, the number of drops produced would be insufficient to produce the required coverage and there are no spray generators capable of applying extremely low emission rates.
To achieve the required volume application rate and corresponding drop numbers the active ingredient is dissolved in a carrier which is usually a mixture of volatile light oils. This greatly increases the bulk of the insecticide and inevitably results in greater transportation and application costs. The ideal formulation is therefore a cost-effective compromise which takes account of these various factors.
The recommended application rate for locust control is between 0.5 and 1.0 1 /ha with a concentration dependent upon the insecticide's toxicity.
Microencapsulation. The persistence of an insecticide can be increased by coating each insecticide drop with a thin polymer film which reduces the rate of chemical breakdown. It also reduces the rate of evaporation between emission and impact. This process is known as microencapsulation; it produces insecticide spheres which are supplied mixed with water. Alternatively, the polymer film is produced on each drop after the drop has been emitted, this is known as in-flight encapsulation. Field trials suggest microencapsulation may approximately double the persistence of an insecticide but at the cost of a reduction in the immediate kill. However, with the short-lived insecticides now used, a doubling of persistence is not a critical advantage. This has to be compared with the greater cost of the microencapsulated product and the greater volume of liquid which must be transported and applied.
Insecticide specification
Liquid insecticides are defined by the proportion of active ingredient in a given amount of the product. The active ingredient is given as a weight, usually in grams, so the proportion is given by weight of active ingredient in a weight of formulation expressed as a percentage.
For example, 95% fenitrothion is 950 g a.i./kg of product. Occasionally a product may be defined as 'X% wt/vol', but this is incorrect and is not the same as X% a.i./kg. A percentage is a ratio and as such has no units. For example, 95% wt/vol means there is 950 g ad. in 1000 ml of formulation. One litre of water weighs 1 kg which means water has a specific gravity (sg) of 1 (i.e. 1 g/ml). Many chemicals, however, are heavier than water. For example, 1 ml of fenitrothion weighs 1.32 g. Therefore, 95% wt/vol fenitrothion is approximately 70% active ingredient as it contains only about 700 g a.i./kg.
Choice of insecticide
There are four main classes of insecticides, that is, organochlorines, organophosphates, carbamates and synthetic pyrethroids. The organochlorines are highly persistent and include dieldrin and HCH. These insecticides were the most favoured for locust control because of their efficacy, cost and persistence. Now they are considered to be such a risk to man and the environment that safer alternatives are recommended. Organophosphates and carbamates have a moderate range of persistence and speed of action. Synthetic pyrethroids have a rapid effect on the behaviour of the insect but if the dosage is too low, or if the application technique is poor, the insects may recover.
Insecticides known to control locusts and grasshoppers effectively under field conditions are shown in Table 18.
Most locusts experts agree that the early hopper stage is the most vulnerable but it must be remembered that the locusts are at this stage for only a few weeks and it is unlikely that they will all be reached quickly enough to be destroyed while they are still young. Therefore one must be ready to deal with all stages of the life cycle, even though the early instars are to be preferred.
The insecticide most appropriate for the target must be chosen. For example, a quick-acting insecticide is needed to treat individual bands as the hoppers may have marched out of the sprayed area before they have taken up enough of the chemical to kill them. Using a synthetic pyrethroid, which prevents marching by disorienting the locust, with a slower acting insecticide may prove very effective. A slow-acting, more persistent insecticide is necessary for treatment of blocks of land because it reduces the need for a uniform deposit. An insecticide that acts by direct contact is necessary for all swarm treatments. Synthetic pyrethroids should not be used as they may disorient the flying insects so that they drop out of the swarm but recover later.
Ways of using insecticides against locusts
Hoppers and adult locusts can be controlled with chemicals by a variety of application methods, both from the ground and from the air. The choice between these methods will depend upon several factors such as environment, farming system, locust population, technical capability and the availability of equipment. A brief outline is given here of some of factors that should be considered before a particular application method is chosen (see also Table 19).
These factors can be considered under the following headings: Where are the locusts? What is the locust stage? What are the economic and administrative problems?
Where are the locusts?
The infestation can occur either in cultivated and populated areas or in remote areas far from cultivation and human habitation.
In cultivated areas there are usually many people who have a direct and immediate interest in locust control for the protection of their crops. Such people can usually be persuaded to carry out control measures themselves, and for this purpose the ideal methods are usually baiting and dusting because of their simplicity and the ready distribution of supplies.
This applies during the hopper stage, but when cultivated areas are invaded by swarms there is little that the individual farmer can do. Swarm control can only be successfully undertaken with aircraft, and is the responsibility of plant protection departments. Plant protection departments usually have most of their resources in and around the crop areas and it is, therefore, in such areas that crop-spraying machines may be used if the infestation is too large to be dealt with by individual cultivators.
When locust infestations occur in desert or other remote areas their control is undertaken by specially equipped teams. If adequate labour is available hopper infestations can be controlled by baiting (page 148) and dusting (page 150) in crop areas, but often labour is not available and spraying must be used. If the infestation is large, aerial spraying is required.
Table 20 shows some different methods of control together with the necessary inputs to control a moderately sized plague infestation. The table shows the negligible impact of baiting and individual band spraying on a large infestation, and agrees with events during the 1986-1989 upsurge and plague when at least 90% of control was achieved by applying ulv sprays mainly over large blocks of country containing hopper bands.
What is the stage?
Eggs. Laying swarms will produce egg beds with tens or even hundreds of egg pods in a square metre. Attacking egg beds would seem to be a good method of control and formerly eggs have been destroyed by digging them up. However, the position of the beds is unknown unless someone has seen the swarm laying and marked the spot, and in practice, only a small fraction of the beds making up a major infestation are ever discovered. Also, hoppers usually hatch in waves over a period of days or even weeks, necessitating the use of either a persistent insecticide or treating the same bed repeatedly. There is no longer a suitable persistent insecticide, although the insect growth regulators may eventually prove suitable (see page 160). Ploughing destroys some eggs, but the few trials that have tested this technique show that control is poor. Attacking egg beds is not a practical method of control at present.
Hoppers. Small hopper bands can be destroyed by baiting or dusting by hand. When the bands are either numerous or widely spaced baiting and dusting can be done from a vehicle but the use of sprays makes less demand on supply transport. Target spraying with an appropriate sprayer is probably the ideal method and should be considered first.
Adults. The adult stage is the most difficult one to control. Adults can occur either in swarms which are very mobile, or in scattered populations which constitute a diffuse and extremely difficult control task. Owing to their mobility swarms can usually be successfully destroyed only by spraying from aircraft, and such operations require not only the appropriate aircraft and pilots but also considerable ground support teams based on suitably sited airstrips. If a swarm settles near a locust control or plant protection post, however, dusting, baiting and spraying with crop-spraying machines can all be used.
The following example illustrates the magnitude of the task of controlling a swarm of 60 km², which might contain 2000 million locusts, by baiting compared with spraying the same swarm using aircraft. While the ground team will probably have only a single night at their disposal before the swarm flies away, the aircraft will probably be able to attack it for several successive days. Such a swarm would require only eight aircraft loads of 375 1 each of 96% ulv fenitrothion to destroy it. The same swarm would require the spreading of 600 tonnes of bait, a truly fantastic task for one night. (See also Table 20.)
When swarms invade highland areas such as those of Ethiopia, Kenya and Morocco, low temperatures may cause them to stop migrating. Under such conditions a swarm may stay in the same area for several days, during which it can be attacked with crop-spraying and dusting machines.
What are the economic and administrative problems?
Factors in this category will be different for each country and it is only possible here to give the briefest indication of the questions that may be involved. Locust infestations often occur in areas of under-employment and may, owing to crop damage, make hard times worse; under such conditions it may suit governments to employ a large labour force as a means of helping the people, in which case hand baiting and dusting are appropriate methods of control.
The availability of locally manufactured insecticides may also be an important consideration. If one particular type is both locally available and cheap, it is clearly to be preferred. If, however, the hopper infestation is large an enormous labour force may be required, and it may well be that the savings achieved by using a cheap insecticide are more often offset by the vast labour costs incurred. Countries which are not often affected by locusts may have well-organised plant protection departments but only a relatively small anti-locust unit. In such instances crop-spraying equipment, including aircraft, can be pressed into use when the emergency arises, but it is emphasised that the specially designed machines and concentrated insecticides are more economical if they are available.
Application methods
The insecticide is actually applied or offered to the locusts in one of three forms: bait, dust or spray. All forms are used for killing by stomach action, and dusts and sprays are also used for killing by contact action.
Baiting
Bait is usually prepared by the anti-locust organisations themselves. The insecticide, or insecticide and a biological control agent, is mixed with a material, the carrier, which locusts eat readily.
The insecticide chosen for mixing with the carrier must be one which has a high stomach toxicity to locusts and is available in a convenient form. Carbamates such as bendiocarb and propoxur are now commonly used as baits.
Tests have shown that the following products are the best carriers for locust bait: maize meal, wheat bran, maize bran, cotton seed husk and rice bran. Others which can be used but are not as good are: sugarcane waste, millet stalk, groundnut husk, corn cob, sawdust and rice husk.
The carrier chosen will depend on what is available and also on the price; but it must be remembered that the material which is cheapest at source may not turn out cheapest in the long run if the locusts do not relish it. A large part of the cost of baiting is the cost of transporting the bait to the locusts, so that it is not worthwhile paying the transport costs unless the material gives good results when it reaches them.
The effectiveness of the bait will clearly depend upon the concentration of the insecticide it contains. The concentration is usually 1 or 2% active ingredient.
How to mix bait
When mixing bait it is important not to attempt to mix too much at once as this will result in uneven distribution of insecticide, so that some of it will be ineffective.
To prepare a bait from a selected carrier and insecticide:
for each 200 kg of wheat bran (or other selected carrier)
use 10 kg of 1 % bendiocarb dust
or 50 kg of 1% propoxur dust
or 25 kg of 2% propoxur dust
Spread the wheat bran on a hard dry surface and sprinkle the insecticide dust evenly over it. Mix the two together, preferably with shovels, for about 15 min. If large quantities of bait have to be mixed quickly it is advisable to use a mechanical mixer.
Packaging and storage of bait
Sacks are the usual containers because the carrier is normally supplied in them. It is best to have bait packed in sacks which hold about 10 kg. If it is in larger sacks, which are cumbersome or impossible for one person to handle, it has to be transferred from these to other containers, and this results in considerable wastage of time and labour as well as wastage of material due to spillage.
Stored bait must be kept dry: if it gets wet it cakes into large hard lumps and is then useless. if it is to be stored on damp ground, a dry, well-aired base should first be prepared and covered with waterproof sheeting. Bait stored for more than two years may lose its efficacy. It should be tested and re-mixed with more insecticide if necessary before it is used.
Methods of baiting
Bait can be used to kill both hoppers and settled adults but its main use is against hoppers. It can be used against all hopper instars but gives very poor results during the last 2-3 days of the fifth instar and during all moulting periods. It is particularly useful for control of marching bands when there is little annual vegetation and much bare ground.
Baiting is effective against adult locusts settled on the ground, for example, in the morning before take-off, and is one of the safest methods to use amongst crops. It has been used with considerable success against swarms in Sudan.
Research has shown that the best results are obtained if the following method is used.
1. Find and bait hoppers in the early instars if possible.
2. Spread the bait actually amongst the hoppers. They only eat bait if they find it in their path. They are not attracted to it from a distance. Disturbance caused by walking through them is only temporary.
3. Spread the bait thinly and evenly so as to allow the maximum area for the hoppers to feed. Do this by throwing it high in the air and letting it drift with the wind. Do not be afraid that the wind will separate the insecticide from the carrier; this does not seem to happen with well-mixed bait.
4. Start baiting if possible along the front edge of a band. This will stop the leading hoppers. Then spread bait over the whole area of the band, or, if it is a very large band, over the whole area of denser hoppers.
5. Spread bait from a truck whenever possible (Fig. 136).
6. Do not spread bait when the hoppers do not stop readily to feed, since this usually means that the ground is too hot or the hoppers are near to moulting.
7. Any bait left on the ground when a hopper band has been killed is wasted; too much has been used. Obviously in extensive control operations there will be some waste, but it should and can be kept to a minimum. Field research has shown that the kills obtained with bait are very variable, not only for different instars but also at different times for the same instar. If something is also known of the density of hoppers in different instars and in different states of behaviour, the amount of bait required to kill 1 ha of locusts can be approximately worked out. This is shown in Table 21.
TABLE 21. Quantity of bait required for locusts according to behaviour
Settled groups
Actively marching
(kg/ha)
(kg/ha)
Hoppers
Adults settled
A kilogram of bran bait is roughly 10-12 handfuls.
Baiting is particularly useful in controlling over wintering concentrations of the Sudan Plague Locust. This survives the dry season in the adult state hidden for much of the time in cracks in dry soil, emerging occasionally to feed. During this period baiting is more effective than either dusting or spraying.
When baiting is not successful, it is probably because:
the bait was spread in the wrong place;
the bait was old;
locusts did not stop to feed because the ground was too hot, or because they were near moulting;
locusts stopped to feed but did not eat enough bait to receive a lethal dose of insecticide; this happens particularly during the last 2-4 days of the fifth instar.
The advantages of baiting are:
it requires no machinery;
it can be carried out by relatively unskilled labour in undeveloped and remote areas;
it can be given to farmers to protect their fields from hopper bands;
it gives striking results which are quickly seen; dead and dying locusts are seen soon after feeding on bait (Fig. 137);
it provides seasonal labour which is convenient for the general economy of some countries;
it is a convenient method of control against small scattered bands.
The disadvantages of baiting are:
it requires much heavy transport and spacious storage facilities (Fig. 138);
it requires much labour which may be needed for other work;
it is time consuming;
the bait carrier, e.g. bran, might be difficult to obtain since it is used as animal feed.
Biocontrol baits
The use of baits containing pathogens such as the protozoan Nosema locustae, with or without an admixture of an insecticide, has had some success against range grasshoppers in the United States of America and Argentina. Mortalities of up to 60% are sometimes achieved, together with some loss of fecundity among the survivors. The method is still at an experimental stage in Africa. Given the high mobility of the Desert Locust and the impermanence of its habitats, it is unlikely to be successful against this species, but could perhaps be more so against the African Migratory Locust and some of the more sedentary species of grasshopper.
Dusting
Commercial preparations of insecticidal dusts consist of an insecticide mixed with some inert material like powdered chalk or talc. The most suitable insecticidal dust for killing locusts and grasshoppers is bendiocarb. Dust can be applied by a variety of methods.
1. By hand. When dust is distributed by hand it is advisable to use a dusting bag or to mix the commercial product with fine sand to give a better distribution. One handful of insecticide dust to four handfuls of dry sand or silt.
2. By hand-blower (Fig. 139). These block easily. Use the dust as supplied commercially.
3. By machine powder-duster. Care must be taken not to apply the dust in swaths more than about 10 m wide, otherwise the dust will be to diffuse to achieve an adequate kill.
Experience has shown that the best results are obtained when dusting is carried out under moist conditions, that is, high relative humidity or with dew on the vegetation. Dusting gives good results against the following targets:
first-instar hoppers in dense groups, particularly as they hatch (see Figs 29 and 39);
hoppers marching slowly through dense low vegetation on which they feed;
either adults or hoppers when they are roosting for the night (but it is difficult to apply dust to adults settled in tall trees).
The reason for failure is that the dust may not fall or remain on the hoppers or on the vegetation they eat. This can be caused by wind, or air turbulence close above hot ground.
The advantages of dusting are:
it can be applied by hand;
it is cheaper than bait because no carrier has to be bought or transported;
the possible risks to plants are less likely to occur with dusts than sprays;
it is less bulky than bait and is therefore more suitable when targets have to be reached by animal transport;
it is very convenient against small isolated hopper bands.
The disadvantages of dusting are:
dust is a poor contact poison against older hoppers and adults, because, unlike oily sprays, it cannot penetrate their cuticles readily;
it is generally less useful as a standard method for all kinds of locust target and less reliable than bait or spray;
it uses far more insecticide per unit area than spraying;
it has poor persistence on foliage;
it is unpleasant to use by hand, and there are tiresome and time-wasting difficulties due to blockages in dusting machines;
there is a safety risk due to inhalation of the small particles.
Spraying
In this method of locust control, liquid insecticide is broken up into fine drops and sprayed either on to the locusts or on to the vegetation which they eat. Spraying can be done either from the ground or from aircraft.
Spraying can be successfully carried out with many types of machine. Suspensions, emulsions or oil solutions of insecticides may be used. To obtain the best kills at the minimum cost, however, the insecticides require special formulations and an appropriate spraying machine should be chosen.
Until about 30 years ago all liquid insecticides were applied in large volumes diluted with water. For locust control this caused problems because water often had to be transported large distances. It was realised that much greater areas could be treated more quickly and cheaply, if the insecticide was applied at high concentration in a specially prepared, effectively involatile formulation. This is the basis of ultra-low volume (ulv) spraying.
It is clear, however, that such concentrated material must be spread over a large area and must collect on the locust and on vegetation and not simply allowed to 'rain down'. This means using very small drops which are spread by the wind. Although easy in theory, in practice, spray application is more complicated and variable, and the ideal technique for every situation is as yet largely undetermined. Nevertheless, the advice that is given here will give good results.
Drop production
All sprayers produce drops in a range of sizes called the drop spectrum. Drops are measured by diameter in millionths of a metre (µm).
Two terms often used in connection with spraying are:
volume median diameter (VMD) is the drop diameter where half the total spray volume is in smaller drops and half is in larger ones.
number median diameter (NMD) is the drop diameter where half the total number of spray drops are smaller and half are larger.
The volume of a drop is given by d^3/6 where d is the diameter of the drop. If you halve the diameter you get eight small drops for every large one. For example, one 200 µm drop will make eight drops of 100 µm, 64 drops of 50 µm and 1000 drops of 20 µm diameter. This means the NMD is always less than the VMD. The VMD/NMD ratio is a measure of the dispersion of the drop spectrum. This ratio does not have a consistent meaning in terms of the drop spectrum, although a small ratio indicates a narrower drop spectrum than a large ratio. There is no single parameter which represents the drop spectrum completely.
The best way to describe the spectrum is to give the proportion of the total volume of insecticide emitted within the drop size range which accounts for most of the emitted insecticide. For example, of the total volume of spray emitted by a Micronair AU5000 on an aircraft, 80% is in a drop size between 5 and 120 µm. Whereas, of the total number of drops, 80% are between 5 and 56 µm. That is, a relatively small number of large drops account for a substantial percentage of the total volume of spray (Fig. 140).
The output drop spectrum is the only technical characteristic of a ulv sprayer which influences the effectiveness of control. Output drop spectra can be determined reasonably easily and accurately with laser analysers.
Drop behaviour
The only effective drops are those which impact upon the locust or leave a residual deposit on vegetation which is subsequently eaten by the insect. Drops fall vertically under the force of gravity, drift laterally according to wind speed and can be carried upwards by thermal convection. The rate of movement in all three directions is affected by droplet size (Table 22).
TABLE 22. Terminal velocities of drops of various sizes*
Drop diameter (µm)
Velocity (cm/second)
*Specific gravity of 1 at 20ºC
Even with some lateral drift or convection large drops (VMD above 120 µm) tend to fall directly to the ground where they are largely wasted. Smaller drops remain airborne for a longer time and are consequently more likely to impact on vegetation or an insect clinging to it. At low velocities very small drops (<20 µm) may be deflected by the air flow around the surfaces of leaves and stems but the probability of impaction increases with both wind speed and the narrowness of the target, that is, the impaction efficiency of small drops is considerably greater on insect hairs than on leaves or large stems (Fig. 141). They are also most affected by convection and may be carried above the target site.
For a given volume application rate, therefore, a drop range between 50 and 100 µm is the most cost effective.
Atomisation
Atomisation is the way in which the spray drops are produced. There are four methods for producing drops in commercial use.
(a) Nozzles. There are various designs of nozzles, but in all of them insecticide is forced under pressure through a narrow bore orifice; the smaller the hole, the smaller the VMD of the emitted liquid. The volume which can pass through a single nozzle is relatively small, so a bank of nozzles is needed. The nozzles on an aircraft are mounted on a boom (Fig. 142). With aerial spraying the airstream further shatters the jet of liquid coming out of the nozzle. Placing the nozzles at different angles gives different drop spectra.
(b) Rotating cages. The Micronair is the only make of rotating cage in widespread commercial use with aircraft. A version is also available for use on vehicles. The Micronair atomiser consists of a central core with a cylindrical mesh cage. The core and the cage are rotated by a wind-driven propeller or, in the case of a vehicle sprayer, by a petrol motor. The insecticide is forced from the central core under pressure, but is also thrown out by centrifugal force. It is then partly shattered against the mesh and partly spun off from it. With aircraft-mounted atomisers and the new air-blast systems, wind-shear will also help atomise the insecticide. (See also Appendix, page 198.)
(c) Rotating discs. Rotating discs are generally used for ground application. A device has been developed for aircraft but is not often used. The insecticide is fed on to a rotating disc and drops are produced at the perimeter by centrifugal force. The disc edge is toothed and must be carefully engineered to get a narrow drop spectrum. The discs cannot handle a large volume without flooding so a vehicle-mounted sprayer needs a stack of discs. The discs must be fed evenly otherwise some will flood. Even flow is not easy to achieve.
(d) Air blast. In this system insecticide is fed from a narrow tube and shattered by a blast of air. This is similar in principle to a nozzle mounted on an aircraft. Air blast machines tend to produce a wide range of sizes of drops, although some 'foggers' produce mostly very small drops. The exhaust nozzle sprayer is essentially an air blast machine (Fig. 143). (See also Appendix, page 191.)
Different methods of atomisation tend to produce different drop spectra. Discs should produce the narrowest spectrum, followed by cages, nozzles and air blast. Much depends on the model used, however, and on its setting and on the flow rate. Spinning cages and discs will produce smaller drops at higher revolution speeds. All sprayers produce a wider drop spectrum at higher flow rates. The stacked disc atomisers are an exception, however, because at low flow rates (below 300 ml/min) and low pump pressures, the discs may be fed unevenly and so result in their being overloaded.
Swath width and track spacing
Drops are carried by the wind and deposited over a strip of ground. The area from where the deposit starts, to where it finishes, is the swath width.
The swath width increases with the spray emission height and is proportionally greater with smaller drops (Fig. 144).
The interval between spray runs is called the track spacing. When spraying large drops in light winds or still air, the track spacing used is about the same as the swath width as large drops fall straight to the ground. (Most non-locust spraying uses emulsifiable concentrates applied in still air and the terms track spacing and swath width are often wrongly interchanged and can lead to confusion.)
Spraying with emulsifiable concentrates
The aim of aerial spraying with emulsifiable concentrates (ec) is to use a large drop which falls rapidly, and to spray in still air or a light wind. The swath width is only a little more than the wingspan, and track spacing is normally only slightly less than the swath width. Accurate control of track spacing is essential for ec application so you must use flagmen or vehicles to mark the ends of the runs (see page 164). Most aerial crop spraying is carried out in this way using a bank of nozzles with the nozzles arranged to produce an even deposit. Little ec aerial spraying is carried out for Desert Locust control.
Ground spraying of ec is usually only used to treat relatively small areas (not blocks) using backpack sprayers. The technique is to spray with large drops in a light wind or in still air and with a track spacing roughly equal to the swath width. The drops fall in a reasonably uniform pattern over a 1 m wide swath.
Nozzles mounted on a tractor boom are seldom used because they are rarely available and because much of the terrain where hopper bands are found is too uneven to use this equipment.
Ultra-low-volume spraying
When using ulv material, one must accept the settings and flow rates given in the manufacturer's handbook. To be confident of actual emission rates an aircraft with a flow meter is highly desirable. A flow meter will be highly accurate, although it should still be checked, if possible.
Controlled drop application (CDA). The technique of Controlled Drop Application (CDA) applies insecticides within an overall drop size range of 40-130 µm. Although the ideal size of a drop is unknown it is known that insecticides should be applied in a narrow band of small drops. The best range for practical purposes lies between about 70 and 110 µm.
It is essential for CDA that drops arrive at the target at the same size as when they are emitted. Emulsifiable concentrates cannot be used for ulv application as water evaporates quickly reducing drop size. The surface area of a large drop is proportionally much smaller than that of a small one. They stay airborne for a shorter time than small drops, so they evaporate more slowly. Large drops, therefore, tend to arrive at the target mostly unchanged, whereas small drops are quickly reduced to concentrated non-volatile insecticide. Evaporation is especially rapid in the typical hot dry habitat of the locust.
Incremental spraying. The technique for ulv spraying uses a track spacing which is smaller than the swath width and so swaths overlap and an even deposit is produced. This is known as incremental spraying and a crosswind spreads the spray (Fig. 145). The track spacing can then be irregular without producing too uneven a deposit (Fig. 146). However, the upwind edge of the sprayed block will always be underdosed, especially close to the edge and will require a supplementary pass to windward to correct. A change in wind direction up to 30º can be accommodated without producing an unacceptably irregular deposit providing a wide swath is used; a strip at the sides will, however, be underdosed.
Aerial ulv spraying of settled swarms and blocks containing hopper bands. There is still much to be learnt about the behaviour of the spray and the techniques to obtain the best kill. The following recommendations are based on what is known at present.
Where possible all spray tracks should be made at right angles to the wind.
Always work upwind to avoid passing through spray released earlier.
In all but light winds make one (or more) additional pass(es) upwind of the final upwind demarcated track to compensate for the effect of downwind swath displacement.
Use a drop spectrum with as much as possible of the emitted insecticide in a 30 µm band within the 70-120 µm range. (This is difficult to achieve with nozzles.)
Recommended nozzle and Micronair settings at different aircraft speeds are given in Table 23.
Spray in a steady wind which is greater than 2 m/second measured at 2 m above the ground. Winds stronger than this are satisfactory up to the limit at which it is judged safe to fly, provided allowance is made for the effect of downwind swath displacement.
With a steady wind use a 100 m track spacing with a flying height of between 5 and 10 m. With a 10-15 m flying height a 150 m track spacing is allowable, but the savings are small with a light spray aircraft. It is better to standardise the track spacing at 100 m, and so standardise the emission rate for a particular insecticide.
Flying heights above 15 m are not recommended since there is little information on the swath width and no information on recovery with emissions above 15 m.
Never spray in very still air such as occurs under temperature inversions. These often set in before sunset and last until some hours after dawn.
Never spray on hot afternoons with strong convection when updraughts alternate with downdraughts. These conditions are indicated by frequent changes in both wind strength and direction, interspersed with calm periods.
If forced to spray in light winds reduce the track spacing to that recommended in Table 23 according to the type of aircraft. The emission rate has also to be reduced to give the correct area dosage. Control may be less effective in light winds.
Aerial spraying of milling swarms at the roost site. Spraying swarms at the roost site is an efficient and effective technique but it has not been studied in detail. Most of the locusts in such a swarm are likely to be flying, exposed on bare ground or exposed high in vegetation, so they should collect small drops. When spraying these swarms it is recommended:
- use a reduced emission rate, 40% of the standard emission rate for treatment of blocks of land (Table 23), with a 100 m track spacing, to reduce waste through overdosing the more exposed fliers;
- spraying the block twice at a 100 m track spacing to allow locusts to change position between runs.
Spraying swarms in flight. The aim in spraying flying swarms is to keep the spray within the swarms for as long as possible which means using a small drop. The locust passes through the spray cloud at its flying speed of about 3 m/second while its beating wings will be moving even faster. It will, therefore, collect small drops efficiently. Furthermore, the locust has narrow collecting surfaces, such as legs and antennae, that collect small drops better than wide surfaces.
Methods for spraying swarms have not been developed. However, even a less than ideal method should produce a good kill and uses small amounts of pesticides.
Low flying stratiform and high flying cumuliform swarms (see page 30) present substantially different control targets.
It is difficult to carry out a spray trial with flying swarms, especially cumuliform swarms. These following recommendations should be used as guidelines only.
Stratiform swarms. It is recommended that the stratiform swarm is treated like a settled target and so spraying should be carried out close to the top of the target.
Treat the target twice with a 100 m track spacing and half the emission rate recommended for block spraying, using drops in the 70-100 µm size range.
This treatment is essentially the same as that recommended for milling swarms. The aim is to produce drops which will fall sufficiently quickly to reach the swarm, but not so quickly that they will be carried through to the ground before they have a chance to be collected by the flying insects.
A further advantage of treating the whole swarm using a drop size near the upper limit recommended is that many of the locusts are likely to be settled. These would escape if spraying was carried out with only small drops, unless spraying continued for long enough to allow all the locusts to take off and pass through the airborne spray.
Cumuliform swarms. The basic technique for treating cumuliform swarms is to spray repeatedly above the densest part of the swarm using a small drop and a low emission rate. The spray should remain within the swarm for a long time and the movement of the locusts should bring them into the spray cloud. Treatment should be continued until the swarm disappears.
It is not easy when in an aircraft to see where you are in relation to a swarm when you are close to it. Flying into locusts can be dangerous since the aircraft's air intakes can become blocked, causing the engine to overheat and the windscreen to become obscured when locusts hit it.
There is no possible recommended area dosage for treating flying swarms. It is known how much insecticide should be used, although it will be much less than with any other treatment, except perhaps that needed to spray a settled swarm. One suggestion is that spraying continues until the swarm fall to the ground.
With a quick-acting insecticide such as bendiocarb, phoxim or chlorpyriphos, lethally dosed locusts should fall to the ground within about 30 min. A slower acting insecticide, such as fenitrothion, will give a more certain kill, but at the risk of significant overdosing; the locust might collect several times the amount needed to kill it before it became unable to fly.
Barrier spraying. With barrier spraying an extremely persistent stomach-acting insecticide is applied in swaths such that eventually a hopper band marches through the barrier, eats the poisoned vegetation and is killed. Only dieldrin is known to be satisfactory for this purpose but dieldrin is no longer generally permitted or available. Barrier spraying, therefore, cannot now be practised until a reasonably persistent alternative to dieldrin is found.
Insect growth regulators (IGRs), such as diflubenzuron or teflubenzuron, which disrupt hopper development, may prove to be a partial substitute for dieldrin but much testing still needs to be done. Even if they prove to be persistent their use will require more care than with dieldrin since the IGRs are not cumulative; that is, the barrier should be wide enough and dosed so heavily that the insect acquires a lethal dose whilst the band is crossing the barrier.
Spraying with helicopters. Spraying with a helicopter, whether applying ulv or ec formulations, is, in principle, no different from spraying with a fixed-wing aircraft. A helicopter is, however, more manoeuverable and can be used in restricted areas, such as narrow valleys, where it might be dangerous to use a normal aircraft. Helicopters are more expensive to operate, are difficult to maintain, have a shorter range and are slower than a normal aircraft, so they should be used only where there is no alternative.
Block spraying of bands using vehicle-mounted sprayers. The requirements for this ground application are the same as for aerial block spraying with the following recommendations:
use a 50 m track spacing and the appropriate emission rate
never spray unless there is sufficient wind to carry the spray away from the vehicle quickly.
Spraying individual hopper bands from the ground. It is rare that individual bands are large enough to be treated by aircraft or even by helicopter, so single bands are usually treated from the ground. The disadvantage of this method is the time needed to find and treat a hopper band and the high numbers of vehicles necessary to combat a major infestation. In practice, the total impact of individual band spraying is not large. It is, however, a very economical method because you are not spraying the uninfested vegetation between the hopper bands, as you do with block spraying.
1. Large bands. Hopper bands larger than about 50 ha can be treated as a block, although defining the limits of a large band is not easy. The edges of the band should not be marked but instead, start with a track across the downwind edge, then continue spraying subsequent tracks until you reach the edge of the band (Fig. 147). It will not be definitely known where to start and stop spraying, and consequently, individuals at the edge are likely to escape. Regardless of the direction of movement of the hopper band, spraying must proceed with crosswind runs, moving upwind into the band. Fortunately, most hopper bands move upwind, in which case start with a run across the well-defined leading edge of the band.
2. All other hopper bands. With vehicle-mounted sprayers a small to medium-sized hopper band will be smaller in diameter than the swath width. This means the band will have to be treated in one or two passes. A medium-sized hopper band will only be two swath widths across. With the settings for incremental spraying, one run will not produce a sufficiently high deposit to result in a reliable kill. For a small hopper band, say less than 0.5 ha, a single swath, 10 m from the upwind edge, returning along the same track so that there is a double deposit, should be used. For a larger hopper band, 0.5-50 ha, three passes, one about 10 m from the upwind edge, one through the middle of the band, and the third a repeat of the first, are recommended.
Do not apply this recommendation rigidly-use your judgement.
Small hopper bands are better treated individually with a hand-held sprayer. With handheld sprayers, such as the Micro-Ulva, the requirements are exactly the same as for block spraying. The whole process is scaled down so blocks as small as 0.5 ha can be treated by incremental spraying.
Calibration
Calibration is the measurement and adjustment of spray application equipment to achieve an appropriate emission rate. It is necessary with all equipment, all methods of application and classes of insecticides.
Equipment must be calibrated each time the sprayer setting or the insecticide is changed. How often it is necessary to calibrate afterwards depends on the type of equipment used. Using an aircraft fitted with a flow meter and totaliser, a single calibration at the beginning of each season should be sufficient. However, the flow meter should be cleaned every week to maintain accuracy. Flow regulators of some vehicle-mounted sprayers are unreliable so, initially, the emission rate should be checked daily and readjusted if necessary. If the setting does not need to be changed significantly, the number of checks can be decreased, but at least one check should be made every week.
It is important to calibrate at temperatures typical of those experienced during spraying since emission rates increase with temperature, because at high temperatures liquids flow more readily, due to their lower viscosity. More importantly, different insecticides have different viscosities so the calibration is only valid for a particular insecticide.
Calculating the emission rate
The overall emission rate is the volume of insecticide sprayed in a given period. This is equal to the sum of the individual emission rates for each spray head.
The required emission rate depends on three factors: area dosage, the speed of the sprayer and the track spacing (Fig. 148).
Area dosage. This is the amount of liquid which is spread over each unit of area. For ulv insecticides, area dosage is usually expressed in ml/ha (although with the higher volumes used with emulsifiable concentrates, l/ha is more sensible). The concept of area dosage does not apply to air-to-air spraying in which you are attempting to treat a volume, not an area.
Speed of the sprayer. This is usually measured in km/h, although with an aircraft, speed may be given in miles per hour or nautical miles per hour (knots). Speed can be controlled, although it is usual practice to fly or drive at a standard speed-about 200 km/in for aircraft (125 mile/h or 108 knots) or 7 km/in for ground-vehicle sprayers. For vehicles, higher speeds are difficult to maintain in rough terrain. With a hand-held sprayer, you need to measure the time it takes a person to spray a set distance (at least 100 m). The person should carry the sprayer and walk at a normal speed over the type of country where spraying will be carried out. The average of several passes should be calculated.
Track spacing. This is the distance between crosswind runs of the sprayer and is usually measured in metres. Normally, this is selected in advance and the emission rate is adjusted to give the desired area dosage. However, with some hand-held sprayers the emission rate can be altered only by changing the nozzles, which means only a few rates can be obtained. The track spacing that will give the exact area dosage required must then be determined. Do not confuse swath width and track spacing (see page 157).
Emission rate (ml/min)= Area dosage (ml/ha) x sprayer speed (m/min) x track spacing (m)/10,000
This speed unit may seem unusual, but the equation is in this form so that the answer needs only to be divided by a round figure, that is, 10,000. Therefore, if a mistake is made the answer is likely to be ten times too large or ten times too small, which should be obvious. Convert speeds from km/h, miles/h or knots, to m/min before using the equation (see conversion tables in the Appendix).
Measuring the actual emission rate. The emission rate must be measured if at all possible. To do this load the spray tank with sufficient ulv insecticide, or if emulsifiable concentrate is to be used, with a sufficient amount of water for the system to operate normally. The flow rate with spinning devices is roughly the same whether they are spinning or not. With a hand-held sprayer, such as the Microulva, allow the insecticide to drain into a measuring cylinder for a given length of time (flow rates will normally be less than 100 ml/min so the volume emitted during 5 min should be measured). If calibration is not possible, use the sprayer manufacturer's figures.
Hand-held sprayers. The flow for these sprayers is determined by which nozzle is used. Choose the nozzle which the manufacturer's handbook states gives the emission rate closest to the one required. Then measure the actual emission rate, and finally, calculate the track spacing which gives the recommended area dosage. The measured flow rate will vary only slightly with changing temperature.
Rotating-cage and stacked-disc sprayers. Where the pump is driven by a separate motor, hang a plastic bucket beneath the sprayer head and run the insecticide through the sprayer for a known time with the pump on, but without the spray head turning. The volume can then be measured. Do not put the bucket on the ground in case the wind suddenly blows the spray stream to the side of the bucket, or the insecticide splashes out of the bucket.
Wind-driven aircraft pumps. The only way to calibrate these systems is to spray for a known time and then to measure the decrease in the volume of insecticide in the tank. This method is impracticable with ulv insecticides and aircraft because of the cost of the insecticide. It can, however, be afforded if emulsifiable concentrates are to be used as water is a reasonable substitute for the insecticide. Aircraft spray tanks are usually wide, so it is difficult to mark the volume accurately. The only reliable method is to put a measured volume into an empty tank and note the time taken to spray it all; nevertheless, there will be a measurement error due to the volume of liquid remaining in the connecting pipes between the tank and the spray head.
Routine checks. Always make an estimate of the amount of insecticide expected to be used and compare this with what has been sprayed. This check will only be approximate since vehicle and aircraft speeds cannot be controlled precisely and the amount of insecticide cannot be measured accurately. If 1001 insecticide were put in the tank and an estimated 100 ha was treated at an intended 0.5 I/ha, but only about three quarters of the insecticide has gone, clearly something is wrong. The first thing to check is the flow rate. With ground spraying, the next most likely source of error is the sprayer speed, and the least likely to be wrong is the track spacing. With an aircraft, the track spacing is more likely to he wrong than the flying speed.
Track marking
If at all possible, flagmen should be used in locust spraying operations. They should stand at the ends of the spray run with large flags, preferably of 'day-glo' cloth, on poles about 2 m long. Even then it is difficult for the pilot to fly tracks that are more than 2 or 3 km long as he cannot see the far flag as he flies over the near one. As the aircraft approaches, the flagman should set off for his next position so that he can avoid being sprayed (as this could make him ill) and be in place by the time the pilot makes his turn. It is best to mark both ends of the spray run, but marking one end is still useful since this helps to avoid overdosing or underdosing.
Flagmen should 'calibrate' their pace by walking in a normal way over 100 m of the type of country where locusts are sprayed. Take the average of, say 10 runs. You will find that 100 m is somewhere between 1 10 and 140 paces, depending on the person.
It is useful to set up a flag with a long streamer or to light a small smoky fire to give the pilot an indication of wind speed and direction.
Track marking will always be possible with ground spraying and should always be carried out, unless you are using only a single swath to treat a small band.
Much aerial block spraying has been carried out successfully without flagmen. The track spacings will not be exactly the same, but with incremental spraying an irregular track spacing is not too important. Without marking, however, the average spacing is likely to be either too large, leading to underdosing, or too small, which means that overdosing may occur, and much expensive insecticide will be used.
Electronic guidance systems for track marking which do not require equipment to emit a signal on the ground at the spray site may become available within a few years at affordable prices. These should solve the problem of spraying an accurate track spacing without ground support.
Assessment of kill
You should attempt to assess the effectiveness of control, if only subjectively, at a sample of target places. This sampling can be done by either counting live and dead locusts sometime after spraying or by caging a sample of locusts collected immediately after spraying.
With band control, you should count the live and dead hoppers in quadrats within the band area: since you are counting both in the same quadrat, a precise quadrat size is not important. If you spread your feet apart that will form a base about 1 m wide to give you an estimate of a 1 m² quadrat. Then count about 20 quadrats. The ratio of dead hoppers to the sum of those alive and dead gives the proportional kill:
dead/(alive+dead) x 100= % kill
Alternatively, you can sweep some hoppers and cage them with sprayed grass. Try not to sweep the net through the vegetation since the net can collect insecticide from the grass and the hoppers then collect additional insecticide from the net. Clean plastic flasks, about 2 litres, with air holes in the lid can be used to make good cages. They may be better than gauze cages since, in gauze cages the locusts seem to spend most of their time on the gauze and not on the grass. This is important as the locusts may get much of their dose of insecticide from the grass even with a quick-acting insecticide. Also, grass dries quickly in a well-ventilated gauze cage.
You should keep about six cages, each containing 20-30 locusts, and at least two controls (unsprayed locusts with unsprayed grass). Collect the control upwind of the sprayed area using the net you have used to collect the treated sample (this will check if net contamination is contributing to the observed mortality). The cages should be kept in the shade. Dead and live hoppers should be counted at intervals up to about 48 h after spraying. If there is more than about 10% mortality in the control cages, the results must be treated with reserve, because they show that catching and caging has caused some additional mortality.
You can collect from settled swarms using a vehicle or a motorcycle, by holding a net just above the grass; this avoids contaminating the net. You should be able to collect samples from milling swarms and from stratiform swarms, but there is no obvious way of sampling a cumuliform swarm. With a swarm sprayed at the roost site, it is worth watching to see if many, or any, adults are capable of flying off.
Estimating kill in blocks sprayed with a slow-acting insecticide is difficult except by caging. The control team cannot wait the two or so days for the insecticide to have its full effect. Moreover, the dead and dying locusts will drop out from the bands as they move. The only good test for slow-acting insecticides is the reduction in the band area, that is, the reduction in the percentage of the block that is infested. This necessitates careful sampling before and after spraying, which is not practicable except for research trials.
Control records
A record of every aircraft spray sortie should be kept. What is needed is shown on the form in Fig. 149. It may not be possible to fill in everything on every occasion, but as much as possible should be completed each time. Do not omit to keep a record simply because all the details cannot be completed. Even the date, the type of target, the aircraft and the insecticide used, alone, are important details. A similar record should be kept for block hopper band ground control. With individual hopper band spraying, a daily record should be kept of the number of bands treated and the insecticide used.
<section>Safety in insecticide application and storage</section>
In locust control operations large quantities of insecticides are likely to be stored and used, with associated risks to man and the environment. Care is therefore required at all stages in their transport, storage and application. Siting, design, maintenance and day-to-day organisation of stores and transport operations must be planned to keep hazards to a minimum. Application of insecticide should be made in accordance with good pest control practice to ensure efficacy and safe use. Some insecticides are more hazardous than others and require very careful handling. It is important, therefore, to know which insecticides are most dangerous so that adequate safety precautions can be taken. Such information is given on the insecticide label.
Insecticide label
The insecticide label must always be read before handling insecticides and the instructions given heeded. Besides giving the name and quantity of the insecticide a good label has all the information needed to handle the insecticide efficiently and safely.
DANGER (usually used with the word POISON and a skull and crossbones) signifies a highly toxic pesticide. WARNING is used for a moderately toxic pesticide and CAUTION for a pesticide of low toxicity. Pesticides may be classified for GENERAL USE or RESTRICTED USE, the latter should only be used by qualified personnel.
The label should include directions for use, first aid procedure in case of poisoning and a statement on hazard to the environment. The safety period (time before harvesting crop or re-entering the treated area) should be given and also information on storage and disposal and date of manufacture.
Insecticide toxicity and user safety
The hazard presented by an insecticide depends on toxicity and exposure. Insecticides can enter the human body through the mouth (oral poisoning), skin (dermal poisoning) or lungs (poisoning by inhalation). Acute poisoning is the result of taking in a single large dose; chronic poisoning is the result of repeated small doses of insecticide.
Toxicity of an insecticide may be measured in milligrams of active ingredient per kilogram of body weight required to kill 50% of a test sample of living organisms (usually rats). This is the LD50 (LD = lethal dose). Insecticides are classified by the World Health Organization into four classes of hazard based on the oral or dermal LD50 of the active ingredient:
Class la-extremely hazardous; Class lb-highly hazardous; Class II-moderately hazardous; Class III-slightly hazardous.
Hazard varies with the method of contamination, i.e. oral or dermal, and whether the insecticide is in a solid or liquid form. Formulation can reduce the hazard of the active ingredient by: reducing the concentration and changing the physical state, e.g. an insecticide which is highly hazardous in liquid form may present a much reduced hazard in the dry state, so formulations such as dusts and granules would reduce the hazard to the user. Table 24 shows the hazard of those insecticides that have been used for locust control.
Protective clothing
Wear protective clothing when handling or applying an insecticide to prevent it entering the body through the mouth, skin or lungs. In the tropics, rubber gloves, overalls, respirators and face masks can become uncomfortable and unpleasant, and may present a threat to the health of the wearer in high humidities. An uncomfortable insecticide user then becomes a dangerous or careless user. The selection of protective clothing should take into account the degree of protection required for safe practice and the comfort of the wearer.
In tropical conditions, materials should be as light as possible. A pair of light, durable cotton fabric overalls (dungarees) is useful, but a shirt and long trousers (without turn-ups) may suffice. They should be washed immediately after use and kept separately from everyday clothing. Protective footwear like rubber boots (with trouser ends worn outside) and a hat reduce contamination from insecticide during spraying operations. Rubber/neoprene gloves (not cotton or leather) must be free from holes and overall and shirt sleeve cuffs should be worn outside the top of the gloves to prevent insecticides trickling inside. It is always advisable to wear a respirator or dust mask when handling dust and fine powder formulations. Where protective clothing is unavailable only insecticides in the lower hazard categories should be used. Remember, always have plenty of clean water and soap available for washing thoroughly after using insecticides. A general guide to clothing
recommended for handling and spraying insecticides is shown in Table 25.
Code of conduct for insecticide users
Mixing and applying insecticides
1. Wear appropriate protective clothing.
2. Read the instructions on the label and follow them.
3. Avoid contamination by pouring liquids carefully without splashing and transferring powders without spillage or puffing them up into the face.
4. Never eat, smoke or drink when handling insecticides.
5. Avoid inhaling toxic dusts and vapours.
6. Never work alone when handling very toxic substances.
7. Keep unauthorised people, especially children, away from insecticides.
8. Have plenty of soap and water available for washing.
9. Use insecticides in correct quantities.
10. Never blow out blocked nozzles or holes with your mouth.
11. Never leave insecticides unattended in an insecure place.
12. Do not apply insecticides if weather conditions (wind, time of day) and season are unsuitable.
13. Ensure that operators are adequately supervised and have sufficient rest periods.
14. Do not continue to work or remain in contact with insecticides if blood tests show that your cholinesterase level (see page 172) is below normal.
After applying insecticides
1. Return unused insecticides to the store and keep them locked away from unauthorised people and out of reach of children.
2. Render all empty containers unusable (pierce plastic and metal ones), and by burying or burning (keep clear of the smoke) safely dispose of them. It is impossible to clean an insecticide container well enough to make it safe for storing food or water or for use as a cooking, beer brewing or distilling utensil.
3. Unused insecticides must be removed from sprayers, and all equipment properly cleaned and returned to the store.
4. Remove and wash protective clothing.
5. Wash yourself and put on clean clothing.
6. Keep an accurate record of insecticide usage, including details of worker hours exposed to insecticide during application.
7. Prevent people entering treated areas until safe to do so.
Safety in storage of insecticides
Ideally, any store for insecticides should be designed and used solely for that purpose and located on a carefully selected site away from other buildings. Other commodities, especially food, drink, animal feed, seed and fertiliser, and also fuel (petrol, diesel fuel and oil) should not be stored with insecticides to avoid contamination and fire hazards. The site should be well drained, not subject to flooding, and well away from water courses, wells and other sources of domestic water to avoid contamination from leaks, spills and a major emergency such as fire. The site should be shaded to keep store temperature down (high temperatures reduce the shelf life of some insecticides) and have good access for delivery vehicles and fire-fighting equipment.
During the course of mobile locust control operations, a temporary store may have to be made in the open. The basic principles of insecticide storage still apply, and a temporary store must be secure (fenced in or locked in a vehicle), kept dry and cool, and well ventilated (especially if in a vehicle). All pesticide stores must have a prominently displayed notice outside: DANGER. INSECTICIDES. AUTHORISED PERSONS ONLY. Notices should be prominently displayed both on the inside and outside: NO EATING, DRINKING OR SMOKING.
The storeman's office should be outside or away from the store.
Storage systems
A permanent store especially should have a systematic method of storage, allowing rotation of stocks on a first in, first out basis. It is useful to write the date of receipt on the labels of new stocks.
Suitable stacking arrangements should be made, and recommended stacking heights observed. Older insecticides should be used first. The floor must be kept free of debris and passages between stacks should allow free access and air circulation. Insecticide containers should be stored on dunnage (timber, bricks) and not directly on the floor so that leaks are quickly seen and corrosion from damp on the floor or leaking insecticide is avoided.
Insecticide shelf life
Nearly all insecticides have a limited shelf life and methods of storage and packing should be designed to prolong this as much as possible. Shelf life rapidly declines once a container has been opened. Shelf life as well as rate of use must be taken into account when ordering quantities of insecticide. Outdated stocks may still be usable if the formulation has not broken down and advice for large quantities should be sought from the manufacturer.
Disposal of outdated or unusable insecticides presents many problems and should be avoided by good storekeeping.
Recording system
There must be a system of recording the quantities of insecticides already inside, entering or being removed from the store. A stock record sheet can be kept for this purpose recording the name of the insecticide, manufacturer, the formulation and other details, the quantity and date received, storage notes, the date and quantity issued and the balance in stock.
Transport of insecticide
There have been serious cases of poisoning following contamination of foodstuffs by insecticides being transported in the same truck. Insecticides should never be transported with foodstuffs or in open or leaky containers. Labels should not be allowed to rub or drop off. The driver should be informed of the contents of the load and have instructions provided on what action to take in the event of an emergency (crash, fire, spillage). People must be kept away from a major spill, which is then covered with earth, sand, etc.
Insecticide drums and other containers should be unloaded with care, using ramps if necessary. They should never be thrown off trucks. After unloading the insecticides, check the inside of the truck for evidence of spills or leaks and decontaminate immediately. Leaky containers in stores have usually been the result of transport damage. Torn or unreadable labels should be replaced.
Spills and leaks in stores
Spills will occasionally occur even in the best run stores. Liquid spills and leaks should be soaked up with an absorbent material, e.g. sawdust, sand or earth, supplies of which should always be kept in the store and swept up. Solid spills should also be swept up carefully, avoiding creating dust (made easier by adding damp sand, sawdust etc.). Sweepings of solid and liquid spills should be placed in marked containers for disposal. The affected area should be scrubbed with detergent or strong soap and water, not hosed down as this merely disperses the spill.
Disposal of insecticides
The need to dispose of unwanted or surplus insecticides should be kept to an absolute minimum by careful store management and stock rotation. For large quantities, advice should be sought from the supplier. When only a few litres or kilograms are involved, the best method of disposal is usually burial. Although very practical, burial can lead to problems with public health and environmental contamination, especially of water supplies. Burial sites should therefore be carefully chosen, permanently fenced and marked. Sometimes it may be possible to dispose of small quantities of surplus insecticides by diluting in the normal way and spraying at field application rates on waste ground away from people and livestock.
First aid procedure
If adequate safety precautions are taken, the following information will not be necessary.
Symptoms of insecticide poisoning
Mild. Headache, nausea, dizziness, fatigue, irritation of skin (rashes), eyes, nose and throat, diarrhoea, cold sweats and loss of appetite.
Moderate. Vomiting, blurred vision, stomach cramps, rapid pulse, difficulty in breathing, constricted eye pupils, heavy cold sweat, trembling and twitching of muscles, fatigue and nervous distress.
Severe. Convulsions, respiratory failure, loss of consciousness, loss of pulse, sometimes leading to death.
Treatment
1. Remove patient from source of contamination, including removal of contaminated clothing.
2. Wash skin with plenty of soap and water taking care not to become contaminated too.
3. Keep patient warm and quiet, preferably in a sheltered place.
4. If an insecticide is swallowed:
(a) for a highly toxic chemical (Class la) likely to prove fatal, and if medical assistance is not readily available, induce patient to vomit by giving him salt solution (2 tablespoons of salt in 1 pint, just under 0.5 litre, of water) or tickling back of throat;
(b) if the chemical is an emulsifiable concentrate or solution in an organic solvent, induce patient to eat a large quantity of a suitable, soothing de-mulcent, e.g. beaten egg, milk of magnesia, starch solution etc., to reduce irritation in the stomach.
5. If patient's breathing stops apply artificial respiration (for mouth-to-mouth resuscitation remember to avoid contamination by washing out patient's mouth or by placing an handkerchief over the patient's mouth); in extreme cases, heartbeat may stop and cardiac massage will be necessary.
6. Prevent patient from hurting himself if convulsions occur.
7. For severe or serious cases of poisoning, immediate professional medical attention is required. The doctor should be told the name of the active ingredient in the insecticide, if possible show him the label.
Occupational health
Some insecticides (organophosphates and carbamates) can inhibit an enzyme in the blood known as acetylcholinesterase (cholinesterase). This controls acetylcholine, an impulse transmitter substance in the nervous system. Blood cholinesterase levels are a useful indicator of the degree of exposure to insecticides and can be depressed even in the complete absence of external symptoms of poisoning. The level of cholinesterase in the blood should be determined in organophosphorus and carbamate insecticide users before exposure and checked at regular intervals subsequently, where spray operations are carried out using the same operators over a prolonged period.
<section>Environmental concerns and locust control</section>
The quality of the environment has become a major issue. Many chemicals previously accepted for locust control at national and international levels would not survive the rigorous environmental testing required of modern insecticides. Detailed field assessments are urgently needed to ascertain the impact of all insecticides. Regrettably, detailed environmental evaluations were not made after the upsurges in locust and grasshopper populations of 1977-1978 and 1985-1986, but recent studies in Mali, Sudan and Senegal have provided some information on the environmental side-effects associated with the use of insecticides.
Non-target organisms and ecological processes
Insecticides used for the control of specific agricultural pests will inevitably affect non-pest species, the so-called 'non-target organisms', for broad-spectrum insecticides affect many animal species and the methods of application are less precise than is desirable.
Other animals may be affected directly, through contact with pesticides during application, or indirectly as a result of eating a contaminated diet or having their food source reduced. Some of these organisms may be beneficial as parasites or predators of pest species. Wildlife, birds, livestock, invertebrates and microorganisms may all be susceptible to insecticides, the most common outcome of insecticide use being reduction in population size and community diversity. There are also sub-lethal effects such as reductions in breeding success for particular species and important ecological processes that regenerate nutrients or maintain the fertility and structure of soil may be affected.
Factors affecting pesticide hazard
1. Dosage. The concentration and rate and frequency of application of insecticide.
2. Persistence. The longer insecticides remain in the environment, the greater are the chances of side-effects. Persistence is affected by mobility, degradation and attenuation rate.
Mobility. The physico-chemical properties of the compound, e.g. solubility in water, and those of the medium on which it is deposited, e.g. cation exchange capacity, largely determine the speed of movement of the insecticide away from the sprayed area. Highly mobile compounds have the potential to affect a larger number of non-target organisms.
Degradation rate. Breakdown of a compound is affected by the interaction of its molecular structure with the chemical and biological characteristics of the environment in which it is applied. Water, sunlight and bacteria all lead to the breakdown of insecticides since hydrolysis, photo- and big-degradation are the main pathways of insecticidal breakdown. Compounds that resist rapid degradation are considered a greater threat to non-target organisms.
Attenuation rate. Volatilisation from a surface or dilution within soils and water quickly reduces the likelihood of non-target effects.
3. Movement. Insecticides stand more chance of affecting non-target organisms if dispersed by wind, e.g. aerosols or impacted dusts, or are transported by rainfall run-off.
4. Bioaccumulation. Non-target organisms can take in chemical residues directly from the soil and water, or indirectly with their food. In this way, residues may accumulate to
hazardous amounts in animal tissues. Carnivores (meat-eaters) are at particular risk, since they can be poisoned by a build-up of insecticide, especially organochlorines, through the food chain.
Impact of insecticides on non-target organisms
Studies of the side-effects of the insecticides used for locust control, i.e. acridicides, have just begun. Meanwhile it is reasonable to infer from laboratory and field observations of insecticides that have been used to control other pests the likely impact of the same insecticides only reformulated as acridicides. The following guide is based on recommended rates of application. In practice, these rates are not always adhered to, navigation is often poor and consequently overspraying is very common.
Mitigating against severe side-effects from over-dosing is the sporadic nature of locust outbreaks, and the infrequent, usually single applications made over any one area. Also, rapid immigration of non-target species from outside the relatively small treatment areas can be expected, reducing the acute effects of the acridicide on population structure and ecosystem function to a tolerable level. Aquatic fauna are often buffered against the effect of insecticides by organic material and sediment suspended in the water.
1. Birds, reptiles and mammals. The FAO recommended rate for fenitrothion, 400-500 g/ha, applied aerially, is near the threshold where it can cause immediate deaths among birds. Sub-lethal effects upon birds with diazinon (450-500 g/ha), carbaryl (1000-1200 g/ha), propoxur (80-150 g/ha), and perhaps malathion (900 g/ha) may also occur. HCH applied aerially at up to 500 g.ha is unlikely to kill birds and mammals, but tends to taint crops and milk taken from the area. When mixed as a bait (1.3 g/ha), it can be eaten and accumulate in the fat of mammals. It has been replaced by safer alternatives. Dieldrin is extremely toxic to birds and mammals and is no longer available. Chlorpyrifos (240 g/ha) is probably hazardous for birds and indirect mortalities can be expected in birds and reptiles from consumption of contaminated grasshoppers.
2. Fish and amphibia. HCH, chlorpyrifos and pyrethroid insecticides are likely to cause mortalities among fish populations and should not be sprayed over water bodies. However, pools and rivers carrying high sediment loads offer more protection from acridicides.
3. Arthropods. Deaths of honeybees and other insects have occurred with fenitrothion, chlorpyrifos, diazinon, carbaryl and propoxur. The pyrethroids are also likely to affect bees, spiders and aquatic insects. Spraying deltamethrin (12.5 g/ha) and lambdacyhalothrin (20 g/ha) can be expected to cause high mortalities of decapod crustaceans (shrimps, prawns and crabs) and should be avoided near water bodies (especially clear water). If sprayed over swamps, phoxim (240 g/ha) will also be toxic to aquatic insects and their larvae.
The pyrethroid insecticides, deltamethrin and lambdacyhalothrin, are less of a risk to the environment than many other insecticides. If they are to substitute for dieldrin as a barrier spray, formulations increasing their persistence will increase the chances of side-effects. The toxicity of insecticides varies with age, sex and weight of the non-target organisms and the time of year they are applied.
Impact of insecticides on non-target processes
Microorganisms have an important role in the breakdown of organic matter and the recycling of nutrients in natural and agricultural soils. These processes, and the contribution made by nitrogen-fixing microbes, can be vital in arid and desert areas for the maintenance of short-lived seasonal plants and their grazers. Important ecological processes that can be affected by insecticides include nitrification, biological nitrogen-fixation, respiration and ammonification, but the effects are very much related to soil types and climate, and generalisations are difficult to make.
1. Nitrification (oxidation of ammonia to nitrite and nitrate). HCH applied to certain tropical soils at a rate of 4400 g/ha has severely reduced the numbers of nitrifying bacteria and at a rate of 1500-3000 g/ha inhibited nitrification. Above 2500 g/ha, dieldrin and propoxur inhibit nitrification leading to a build-up of nitrite. Pyrethroids may inhibit nitrification for short periods, but recovery is normally rapid and complete.
2. Biological nitrogen fixation (microbial use of atmospheric nitrogen for growth which returns nitrogen to the soil). At the field rates used for locust control, inhibition of nodulation, nodule efficiency and fixation by free-living bacteria by diazinon is likely to be insignificant. In general, insecticides are not very disruptive to nitrogen-fixing organisms in soil.
3. Respiration and ammonification (oxygen utilisation and production of ammonia from organic matter respectively). Application of propoxur (500 g/ha) and diazinon (1100 g/ha) and pyrethroids have shown only minor effects on soil respiration and ammonification.
No lasting effect is likely at the application rates currently recommended for locust control.
Impact of insecticides in locust breeding sites
In flood plains and swampy areas, fisheries, shellfisheries and birds will be of local importance. Invertebrate animals eaten by fish may be affected by insecticides used to control locusts, and fish-eating birds may also be at risk. These wetlands will probably be seasonal or temporary habitats, and the animal communities will normally migrate, lay drought-resistant eggs and/or die. Risks associated with domestic animal watering and the likelihood of contaminated milk should be considered. Insecticides with the least effects upon aquatic species should be selected and organochlorines avoided.
In sandy areas, contamination of dune grasses and desert shrubs with persistent insecticides could lead to lethal and sub-lethal effects in herbivores, i.e. goats, camels etc., and contamination of milk with insecticide residues. Organochlorine insecticides should be avoided. With some control methods, it should be possible to treat specific areas to minimise the side-effects. In Desert Locust areas, an alternative to dieldrin for treating marching hopper bands is urgently required.
In crop areas, insecticides kill beneficial insects, e.g. the insect predators and parasites of crop pest species, pollinators of trees or crops, and soil animals involved in the maintenance of soil fertility. Under no circumstances should dieldrin be used.
Control decisions
The use of any insecticide has both benefits-the reduction of a pest problem, and dangers-the reduction of beneficial animals and the poisoning of food. While there is little detailed information on the environmental effects of locust control from past campaigns, it is hoped that the information given above will be a useful guide to the safest method of control in a given area in the future. The details are summarised in Table 26.
Locust control is based on the strategy of forecast and discovery of breeding populations. Insecticides are then used to reduce or destroy the populations before they encroach on crop areas. To have the right insecticide available at the right time requires considerable organisation and planning and is especially difficult with those species which recur at infrequent and irregular intervals. However, the information given in this handbook should help those responsible for locust control to plan ahead. Every support should be given to research leading to the provision of safer insecticides which have been fully tested and which are then widely available with associated training in their safe and economical use.
In the longer term, it is hoped that changes in land use may be the best method of locust control for many species, although they will not affect the Desert Locust. It has been shown above that locusts have ceased to be a significant pest in many areas where their habitats have been reduced by keeping agricultural areas free of weeds and grasses and varying the type of crops grown. In spite of the considerable knowledge of locust behaviour there is still much to learn. Many species have a different pattern of behaviour in different places and it is vital that this behaviour is fully understood so that the best place, time and method for local locust control can be identified. This should lead to safer, less damaging use of insecticides where they are needed.
The factors influencing control decisions and, in particular, choice of insecticide are complex. In practice, local constraints such as availability of insecticide, manpower and resources often limit the options open to those with responsibility for taking such decisions. Where the use of insecticides known to be associated with high risk to the environment is the only recourse a decision to proceed with control must be weighed against the likely environmental damage and human suffering which would result from a failure to do so, and application must be conducted in such a way that environmental impact of the insecticide is minimised.
<section>8. Natural control</section>
In an earlier chapter attention was drawn to the fact that sometimes there are plagues of locusts and at other times there are not. Nowadays, with modern insecticides, the availability of considerable control forces and the expenditure of large sums of money, war can be waged against swarming locusts. However, even before man developed such resources locust plagues eventually came to an end through natural factors, though sometimes not for 20 years.
Detailed studies of the biology and ecology of the main species of locust suggest that these plagues came to an end as a result of a rising mortality produced by a combination of numerous ecological factors, including the weather and natural enemies. The term natural enemies is applied to the many parasitic and predatory organisms which attack the locust in all stages of its life cycle.
This chapter begins with a brief consideration of some of the weather factors that limit locust numbers. Unfortunately there is insufficient knowledge to assess the full effect of such factors in the natural control of locust plagues, but it is probable that weather, acting directly and indirectly in a variety of ways and differently on different locusts, is a vitally important natural controlling influence. In the remainder of this chapter the more important natural enemies of locusts are mentioned and notes are given about their life cycles and the overall effect that they may have on locust populations.
Much more information on the natural enemies (including diseases) of locusts is required and accurate observations by locust officers in the field would be most valuable.
As elsewhere in this handbook, this chapter concentrates on the Desert Locust with notes on other species where appropriate.
<section>Weather factors</section>
Weather is probably the most important of the natural factors that limit locust populations. The Desert Locust is affected adversely in the conditions outlined below.
Temperature
Locusts are resistant to low temperatures, but adults buried in snow or exposed to prolonged frost may die. Near the upper limit of their temperature range, hoppers have been seen to die during the heat of the day if they are unable to find shade or get off the hot ground, particularly if the humidity is low. Excessive heat is probably sometimes the cause of death of newly hatched hoppers in very large numbers, but much more information is needed on this point.
Rainfall
Too little rain means that sometimes there is insufficient moisture in the soil for eggs to develop successfully, or even if the eggs hatch there may not have been enough rain to allow growth of suitable food plants for the young hoppers.
Too much rain can kill eggs by exposing them on the surface of the soil, washing them out of the ground or causing them to rot. Floods can drown large numbers of young hoppers.
Wind
Wind can carry flying locusts out to sea where many may drown, or up into the icy peaks of high mountains where they may die of cold. Strong winds sometimes blow the soil away from egg pods, which then dry out so that the eggs die (Fig. 150). Also strong winds occasionally cause drifting sand to bury settled locusts alive when they are too cold to move.
<section>Natural enemies of the desert locust</section>
During its long travels over a large part of the earth's surface the Desert Locust encounters many natural enemies. For much of the time these various enemies are not numerous enough to reduce the locust populations appreciably, so that man has to use chemicals and control organisations to protect his crops and pastures. On the other hand there are times when natural enemies play an important part locally in reducing locust numbers.
Most of the natural enemies of the Desert Locust are now known, but knowledge about the proportion of locusts killed by them is very scanty. Much more information is required so that we can have a better idea of how much assistance can be expected from these natural enemies at various times and places.
Insect enemies
Egg parasites and predators
Scelio species. There are several parasitic wasps of the genus Scelio (Fig. 151 ) which destroy the eggs of locusts.
When a female locust lays its eggs, the female Scelio digs through the froth plug of the egg pod and lays its own minute eggs inside the locust eggs. Only one Scelio egg is laid in each of several of the locust eggs. The parasite hatches and, feeding on the contents of the locust egg, develops through three instars to the adult stage, entirely inside it. The adult Scelio emerges by biting round the end of the locust egg. Affected locust eggs are easily recognised since they become more opaque and darker than normal eggs.
Scelio has only rarely been found in egg pods laid by swarms. It occurs more frequently in egg pods of scattered solitarious locusts, but is never plentiful.
Scelio spp. are also predators of the eggs of African Migratory Locusts, Red Locusts, Diabolocatantops axillaris, Kraussaria angulifera, Bombay Locusts and Javanese Grasshoppers.
Stomorhina lunate. This is a fly which resembles the housefly and is about the same size (Fig. 152). It is very often to be seen where swarms of Desert Locusts are laying and, because it is not found in these areas when there are no locusts, it has been suggested that it travels with the swarms.
The adult Stomorhina waits on a plant or on the ground for a female locust to lay nearby and then moves immediately to lay its own eggs in the top of the locust egg pod. The fly's eggs hatch in a few hours and the larvae feed on the locust eggs, making their way down through the pod as they grow. They are creamy-coloured grubs a few millimetres long, tapering at one end, and are easily seen with the naked eye (Figs 153 and 155). There may be one or several in a single egg pod. They always damage more locust eggs than they completely devour and usually prevent all the eggs in the pod from hatching. They are fully grown after about five days (in the warmer areas) and then they change to puparia (Fig. 154); they may be found in this state, as small brown or black capsules inside the remains of the egg pod. A week later an adult Stomorhina fly emerges from each puparium, usually before the undamaged locust eggs of the same egg field hatch.
Stomorhina occurs in many parts of the Desert Locust invasion area, but has never been found in the West African region south of the Sahara. It is not present in every egg field and is usually much more plentiful during the first series of layings in any particular season than during later ones. Even in egg fields where it is present its occurrence is very patchy; part of an egg field may be infested while another part is not, or one egg field may be infested all over while another only a few miles away may not have any Stomorhina.
This fly is probably the most important insect enemy of the Desert Locust. It often destroys up to 20% of locust eggs locally and on occasions has been known to destroy whole egg fields.
Stomorhina lunate has also been recorded in egg pods of Red Locust, Brown Locust and Migratory Locust in southwest France (Locusta migratoria gallica) and can destroy a significant proportion of eggs in each of these species. As with the Desert Locust, S. lunate is normally found only with swarms.
Systoechus some/i. This is a fly belonging to a group called the Bombyliidae, which are sometimes called 'bee flies' because of their hairiness which makes them resemble bees (Fig. 1 56).
The adult flies are active only on warm sunny days and fly rapidly, rarely settling. The females fly close to the ground and lay eggs in cracks and hollows in the soil. When the larvae hatch they move through the soil until they find locust egg pods. The second and third instars of the larvae feed in the egg pod. The third-instar larva is a creamy colour, markedly curved and legless (Fig. 157). (The difference between these and beetle larvae is that the latter have three pairs of legs at the front end of the body.)
Systoechus larvae feed on locust eggs by puncturing them and sucking the contents. The resulting flattened eggs are often found amongst healthy eggs in the same pod, damage being done only to those eggs actually eaten. A single locust egg pod may contain several larvae, and as many as 50% of the pods in an egg field have occasionally been found to be attacked and partially destroyed. Damage to more than 10% of the eggs, however, has never been recorded and is usually less even when large numbers of larvae are present.
Systoechus somali is confined to East Africa; other species occur elsewhere, e.g. West Africa and the Arabian Peninsula, but they seem to be relatively less important as predators of eggs of the Desert Locust.
Other species of Systoechus have been recorded feeding on the eggs of the Brown Locust and Senegalese Grasshopper.
Trox procerus. Several kinds of beetle larvae may be found on digging up locust eggs. One of the commonest is that of a beetle called Trox procerus (Fig. 158), which appears to be important as an egg predator in the summer breeding areas of West Africa south of the Sahara, Sudan, the Red Sea coasts and the Indo-Pakistan desert.
Tiny mounds of earth all over the egg field often indicate that Trox larvae are present. The full-grown larva is a relatively large creature (40 mm long) with three pairs of legs and a black head (Fig. 159). Sometimes the larvae eat locust eggs for a while and then wander away in the soil before they are full-grown, presumably to complete their feeding on other eggs or larvae. It is therefore difficult to decide exactly how much damage they do to locust egg fields. They have once been recorded as destroying a complete egg field.
Other enemies of eggs. There are many other insects, particularly the larvae of certain flies and beetles, that attack locust eggs, but on the whole their importance seems to be minor.
Parasites of hoppers and adults
Blaesoxipha filipjevi. This is one of the sarcophagid flies, commonly called flesh flies (Fig. 160). The adult flies may be seen sitting on plants or on the ground.
Blaesoxipha does not lay eggs; its eggs hatch inside the female's body. When the female contains larvae it becomes very active, and if a locust flies by it darts after it and strikes it. The locust falls to the ground and if examined carefully with a lens it will be found to have newly born fly larvae (Fig. 161) on the underside of its wings. The larvae burrow through the membranes at the base of the wings and then feed and grow (Fig. 162) inside the locust; when fully grown they leave the locust and pupate in the ground. The puparia are barrel-shaped and reddish-brown.
Blaesoxipha filipjavi is an effective control agent only against small stationary populations of Desert Locust. Blaesoxipha spp. are non-migratory and take some three weeks to complete their life cycles.
Blaesoxipha filipjavi has also been recorded parasitising Tree Locusts, Diabolocatantops axillaris, Kraussaria angulifera and Variegated Grasshoppers. Other Blaesoxipha spp. parasitise African Migratory Locusts, Red Locusts, Sudan Plague Locusts, Diabolocatantops axillaris, Cataloipus cymbiferus and Bombay Locusts.
Symmictus costatus. This is a fly belonging to a group called Nemestrinidae (Fig. 163).
It probably lays a large number of minute eggs in cracks in the soil or the bark of trees. These hatch and the young larvae are blown about by the wind. If they find locust hoppers they enter them through the soft membrane and pass through four instars inside the hoppers. This takes 9-14 days, after which they leave the hoppers and enter the ground where there is a resting period. Pupation is induced by rain and may not occur for several months or even years if the soil is dry. The adult emerges about 14 days after pupation. When they are about to emerge from the hoppers they are quite large and occupy most of the inside of fourth-instar hoppers (Fig. 1 64).
On one occasion in East Africa, in a band of fourth- and fifth-instar hoppers, 34% of the hoppers contained Symmictus larvae. The larvae were mainly in the fourth-instar hoppers, which were almost all at the rear of the band. It seems that moulting was delayed owing to the presence of the parasites and few of the affected hoppers survived. The hoppers containing parasites appeared swollen and had chalky pinkish-white markings instead of the usual yellow ones.
Symmictus costatus has also been reported parasitising Brown Locusts and Sudan Plague Locusts.
Predators of hoppers and adults
Cannibalism. Locusts eat each other. This happens particularly in the hopper stage and especially at hatching time. The first hoppers to hatch often eat some of those that hatch later. It is probable that this sometimes occurs on a large scale, especially in very dry conditions. Field observations are needed.
Ants (Formicidae). In some places ants are very numerous and they probably eat large numbers of young hoppers. Nothing is known, however, about their effect on locust populations, but they have been seen carrying away numerous hatchlings as soon as the latter appeared at the surface of the soil.
Wasps (Sphecidae). Wasps, particularly species of Sphex, have frequently been seen attacking Desert Locusts. They paralyse them, drag them away and bury them after they have laid eggs on them, one egg on each. Although this has often been seen the total numbers of locusts killed must be very small compared with the number in the swarm.
Sphex spp. also attack and parasitise Tree Locusts.
Ant lions (Myrmeliontidae). Both hoppers and adults of the Desert Locust have been seen crossing the pits in which ant lion larvae lie in hiding, but although a locust is occasionally caught on such occasions by the larger species of ant lions, these predators are neither specific to locusts nor significant in numbers.
There are many other kinds of insects that destroy locust adults and hoppers in small numbers.
Birds. Many kinds of birds, both large and small, feed on locust hoppers and adults. On one occasion in East Africa, when a swarm was attacked by several thousand large birds, some of these were shot and the locusts in them counted. The largest numbers of locusts found in single individuals of three kinds of bird were as shown in Table 27.
TABLE 27
Marabou Stork
White Stork
Eagle
Mouth
Oesophagus
Crop
Stomach
Total
In addition, many locusts were found on the ground with their abdomens ripped off, so that many more were killed than could be assessed from the numbers inside the shot birds. The large number of birds present on this occasion was considered exceptional by experienced locust officers; nevertheless, there is no doubt that the amount of damage done to bands and swarms by birds, though very variable, is sometimes great and occasionally amounts to total annihilation of local infestations.
Locust swarms moving on bird migration routes tend to collect large numbers of birds which may stay with them and destroy large proportions of them.
Birds have been seen feeding on hoppers on many occasions. Table 28 shows some figures which indicate the number of hoppers some of the larger birds may eat at a single meal. It is still not known how many such meals these birds take in a day, but it is probably at least two.
Nematodes. Nematodes, which are thin worms, commonly occur inside locust hoppers and adults. They are probably not often responsible for the death of locusts, but they may well reduce their fecundity in severe cases. More information about their effect on locusts is needed.
Reptiles and mammals. Lizards and snakes feed on locusts sometimes, as do foxes, dogs, hyenas, jackals, ant bears and smaller mammals like hedgehogs and rats. Nothing is known of their overall effect on locust populations; it is likely that they are rarely important alone.
Bacteria. Sometimes when locusts have been found dead in the field with no apparent reason for their death, specimens have been sent for examination to bacteriologists. Bacteria which could have been the cause of death have been discovered in specimens.
Fungi. Several kinds of fungi attack locusts. They generally do so when the locusts are living in humid, overcast conditions. When affected by fungi the locusts are found hanging with their front legs around stems or twigs and the hind legs hanging free. The fungus spreads through the body and eventually appears on the outside of the locust as a white cushion. Observations have indicated that whole swarms have been wiped out by fungal disease, but such occurrences are rare, depending on unusual combinations of weather factors.
One such fungus is Entomophaga grylli which attacks Bombay Locusts, Variegated Grasshoppers, Brown Locusts, Red Locusts and Tree Locusts.
Green Muscardine Disease is caused by the fungus Metarrhizium anisopliae. The spores stick to insects in warm humid conditions, they germinate and grow through the cuticle to infect the insect. Finally, within 2-4 days under favourable conditions, the fungus re-emerges through the cuticle to produce more spores. In 1959 in Eritrea, Desert Locusts were found dead and dying, hanging from trees by their front legs. In favourable conditions, Green Muscardine Disease can destroy complete swarms. However, insects show a resistance to the disease and the overall effect of the disease is small.
Green Muscardine Disease has also been recorded in Red Locusts, Tree Locusts and Javanese Grasshoppers.
<section>Biological control</section>
It may well be asked why, with all these natural enemies, chemicals must be used for controlling locusts. For instance, we might consider breeding bacteria or fungus spores in great quantities and releasing them on locust swarms. Unfortunately, this would be ineffective in most cases because special weather conditions are needed for these diseases to develop. In other words, local factors would have to be 'just right' at exactly the right time. Similar difficulties apply to the possibility of using other natural enemies, notably insect parasites and predators, to kill locusts. Moreover, the mass migrations of locusts usually make it impossible for any insect enemies to multiply sufficiently to prevent a developing locust plague from proceeding further.
From the knowledge so far available about the numbers of locusts killed by natural causes, it seems that the most important natural factor is the weather. This is the factor that probably operates over the widest area at any one time. The effect of the natural enemies, insects, birds and others, appears to be limited, usually being important only when the locust population is already reduced by other causes.
<section>Appendix</section>
<section>Campaign report</section>
A campaign report should be written by the locust officer in charge of each area, and the reports from all the locust officers should then be combined into a single report for the whole campaign. Copies should be made available to all who request them.
The campaign reports of locust officers should be compiled under the following headings.
Introduction
Summary of locust situation before and during the campaign. General description of preparations for the campaign.
Information
Summary of methods of obtaining information and their relative importance and efficiency.
Weather
General description of weather preceding and during the campaign, with particular reference to rainfall.
Infestation
Summary of laying, hatching and fledging by areas, and general description of the whole infested area (size, position, terrain, vegetation).
Any available details of intensity, e.g. sizes of largest hopper bands seen in each instar, maximum egg pod densities measured, number of eggs per pod etc.
General remarks about the behaviour and movement of swarms and/or hopper bands.
Map of infested areas.
Information about natural mortality if any striking predation or parasitisation observed, e.g. by large flocks of birds or attack on eggs by predators or parasites (Chapter 8).
Organisation
Staff.
Base, sub-bases, dumps of control materials.
Airfields used.
Supply arrangements, e.g. petrol, diesel, aviation fuel and water.
Operations and communications
Operations
Areas worked; starting and finishing dates.
Methods used, with comments on efficiency.
Labour employed.
Materials used-types and quantities.
Transport-aircraft, ground vehicles, animals.
Communications
Roads and tracks.
Radio-efficiency and schedules.
Particular reference to new areas opened up.
Assistance from other organisations
Note any joint surveys, control work and exchange of information.
Results
Number of swarms attacked; assessment of results, even if vague.
Estimated number and size of hopper bands destroyed.
Estimate of total area worked.
Estimate of any infested area not worked.
Areas from which reports not received.
Estimate of number of swarms that were not destroyed.
Summary
Describe where and when locust infestations were found. Note the age and extent of the populations. Also note those areas surveyed where no locusts were found. Describe any control undertaken. List the number of people involved, the transport required and the quantity of insecticide used. Make suggestions for improvements to be made in good time for the next campaign.
Even if the next campaign does not occur for several years, previous reports will provide the basis for planning and organisation.
<section>Latitude and longitude</section>
Locust officers are often required to indicate the exact locality in which observations have been made, or in which control operations have been carried out. Sometimes this can be done satisfactorily by giving the name of a nearby large town, but if the observation occurred near a small village it is unlikely that the organisation receiving the report, e.g. FAO, other regional organisations etc., will know where the village is. Localities can, however, be accurately determined from the latitude and longitude, which should, wherever possible, be given. They will enable a person reading the report to find the locality even on a map on which the small villages are not marked.
Lines of latitude are imaginary lines round the world running parallel to the equator, and lines of longitude run at right angles to them and through the North and South Poles. Lines of latitude and longitude are usually marked on maps; the lines of latitude run across the map from side to side, and the lines of longitude run from top to bottom. The latitude of a locality indicates its position relative to the equator, and the longitude indicates its position relative to the 'Greenwich Meridian', which is longitude 0º and runs through London.
Latitudes and longitudes are measured in degrees, which are each subdivided into minutes.
The symbol for a degree is º, e.g. 5º.
The symbol for a minute is ', e.g. 21'.
There are 90 degrees of latitude to the north of the equator and 90 to the south of it, and there are 180 degrees of longitude to the east of the Greenwich Meridian and 180 degrees to the west of it. Most of the Desert Locust invasion area lies to the north of the equator and to the east of the Greenwich Meridian (Fig. 165).
How to determine latitude and longitude
The first thing to be decided is whether you are reading latitudes north or south of the equator and longitudes east or west of the Greenwich Meridian. You can determine this either from Fig. 165 or by making an examination of the direction in which degrees of latitude and longitude increase along the edges of the map.
To determine the latitude of the point in question (Fig. 166) draw a line to the left or right side of your map, whichever is the nearer, and then read the smaller number of degrees marked. In Fig. 166 the latitude of the place chosen as an example is clearly more than 11ºN. The longitude is determined similarly by reading the smaller number of degrees on the top or bottom of the map. In the example the longitude is more than 50ºE. It is important that lines drawn from the point to the side and to the top of the map should be parallel to the top and side respectively, or to the nearest line of latitude or longitude. If they are not drawn parallel an inaccurate reading will be made.
To determine the number of minutes by which the latitude and longitude exceed the nearest degree you must first examine the marks along the edges of the map. The distance between degrees is usually divided according to one of two systems {Fig. 167). In the upper system, the degree is divided into six sections each of which equals 10', and in the lower system the degree is divided into 12 sections of 5' Bach. You must first decide which system is used on your map. Then make an estimate of the number of minutes. In Fig. 166 the line going to the side of the map crosses the third section up from 11º line six-tenths of the way along. The number of minutes is determined by adding up 10' for each whole section and 6' for the portion of the third section. The latitude is therefore 11º26'N. Similarly the longitude is 50º38'E.
Military grids
In some parts of the world the only maps available may be military maps. These are normally crossed by numerous lines forming a military grid. Locust officers should not use these grids as they cannot be used by another person unless he has the same map. Even on military maps with a conspicuous military grid, the latitudes and longitudes will be found on the borders of the map, but they are often less clearly marked than on non-military maps.
<section>Collecting, preserving and packing specimens</section>
It is often useful or necessary to collect specimens of locusts and grasshoppers or their natural enemies to send to suitable organisations for identification, examination or morphometric measurement. The equipment required is: killing jar, a pair of small scissors, cardboard boxes, cotton wool and a pair of fine forceps.
Killing
Insects are best killed by using a specially prepared glass jar. This should be about 15 cm tall and 7.5-10 cm in diameter with an easily removable, wide stopper or screw-top. In the bottom is placed a 1-inch layer of plaster-of-Paris mixed with a little water and about one teaspoonful of potassium or sodium cyanide. The plaster sets hard and the killing jar is then ready for use. The locusts or other insects are placed in the jar and the stopper put in place. A newly prepared killing jar should kill locusts in 5-10 min.
If a killing jar of this kind is not available the locusts can be killed in a closed container with a piece of cotton wool or rag soaked in petrol or by dipping their heads for a few minutes in petrol. The jar should be kept in the shade as far as possible; otherwise moisture will condense inside and the specimens will become messy and discoloured.
Cleaning and drying
If the specimens are to be kept for some time they should be de-gutted and thoroughly dried. This is done by slitting open the abdomen along one side, taking out the gut with a pair of forceps and then further cleaning the inside of the abdomen with small pieces of cotton wool on the end of a pair of forceps. Drying can be carried out in a low oven or in the hot sun; it must be done thoroughly. In the case of insects much smaller than locusts, de-gutting is not necessary, but thorough drying is always essential.
Packing
Cardboard boxes, not tin boxes, should be used for packing. Specimens decay very quickly in tin boxes. Place a layer of cotton wool in the bottom of the box and lay the specimens on it, not too close together. Then lay a sheet of paper over them and then another layer of cotton wool and more specimens, then another sheet of paper, and so on until the box is full. For morphometric measurements 50 males and 50 females should be collected, if possible, as a sample from a swarm or from an area of solitary locusts.
The box of specimens should always have a label inside giving at least:
the name of the collector;
the place where the specimens were collected, with a map reference if possible;
the date on which they were collected (day, month and year).
Collection of 'wet' specimens
Sometimes locust specimens are needed for dissection and examination of the internal organs. Specimens for this purpose should be preserved by dropping them into a jar containing medical spirit or 10% formaldehyde, if these are available; if not, an attempt should be made to send the specimens alive in the way described below.
Live specimens
Live specimens can be sent in a cage if a suitable one is available. Otherwise a very good way is to roll each individual in a tube of paper and then put them all in a suitable box. In either case the specimens must be despatched by air at the earliest possible moment.
Eggs
Live eggs can be packed in small tins or glass tubes with some damp soil or in damp blotting paper. If soil is used the container must be filled with it; otherwise the soil is thrown about within the container in transit and the eggs are crushed.
Egg predators
For transporting these as larvae, each should be placed in a cavity made in firmly packed damp soil in a glass specimen tube, which is then plugged with cotton wool. This is to prevent the larvae being killed through abrasion by soil particles.
<section>Sprayers suitable for locust control operations</section>
Exhaust nozzle sprayer
This sprayer provides an easy and cheap way of killing locust hoppers. Egg fields (if known) can also be sprayed a few days before hatching so that hoppers will die when they eat the sprayed vegetation. Locust control operations have mostly to be carried out in places where there are no workshops and time is limited. It is therefore highly desirable to cut down mechanical maintenance to a minimum. A machine like this exhaust sprayer is suitable because it uses the exhaust gases of the motor vehicle to atomise and disperse ulv spray and does not involve looking after another engine. It is simple to maintain because it has no moving parts, and is robust, relatively inexpensive and particularly suitable for Landrover, Unimog or equivalent vehicles. It is effective for a swath width of up to 300 m. Its lack of off-vehicle portability, more or less fixed drop size range and alleged tendency to shorten engine life are of some disadvantage.
It kills hoppers by depositing insecticide on the vegetation which they eat. This is achieved by drifting a very fine spray over and in front of the area which they occupy. The method uses only 0.5 litre of special insecticide solution per hectare.
While this method was designed primarily for target spraying of hoppers, i.e. direct application to the area occupied by each band, it can also be used for barrier spraying (page 1 60).
How the exhaust nozzle sprayer works
The components of the exhaust sprayer and how it operates are illustrated in Figs 168 and 169. The spray is produced by directing the exhaust gases from the engine of the vehicle through a restricted orifice surrounding a nozzle jet from which the liquid insecticide is expelled under pressure (Fig. 169). This pressure is produced by restricting the escape of the exhaust gases; the resultant back pressure is directed into the spray tank, thus forcing the liquid insecticide out of the tank and through the nozzle jet, where it is shattered into minute droplets by the escaping exhaust gases. Thereafter the resulting spray is carried and spread by the wind.
The sprayer can be fitted to most types of four-wheel drive reconnaissance vehicles used by locust control organisations and has been successfully used with pick-up versions of:
Landrover-long and short wheelbase, powered by both petrol and diesel engines (Fig. 1 43)
Willey's Jeep Models No. 6-726 and CJ 5/6-one-ton truck powered by a petrol engine
Dodge Power Wagon-powered by a petrol engine
Toyota Land Cruiser-powered by a petrol engine
Unimog 410, 411-powered by a diesel engine.
Whatever the vehicle used, it should have a double-ratio gear box, and an enclosed cab for the driver to protect him from the insecticide spray.
When placing orders for exhaust nozzle sprayers with the manufacturer*, locust control authorities should give full details of the make and type of vehicle to which the sprayer will be fitted, since the methods of fitting and the diameter of the restriction orifice vary with the type of vehicle.
All vehicles to be used for exhaust-nozzle spraying operations should be adapted before being sent out in the field. This can be done in any garage equipped with drilling and welding apparatus.
How to fit the exhaust nozzle sprayer to a Landrover
1. Remove the canvas canopy and its supporting metal frame from the back of the vehicle. These should be left off during spraying operations.
2. Before going into the field, cut off or heat up and bend the end of the exhaust pipe until it heads to the rear of the vehicle. Preferably extend the pipe until it is level with the back of the vehicle. The exhaust connection supplied must be welded or braised to the vehicle exhaust pipe.
3. Fitting the spray tank. Before fitting the spray tanks, always rinse out with kerosene to remove any solid particles that may be inside.
The tank unit must be fitted so that the driver can operate the ON/OFF tank selector valves, and have full view of the pressure gauge. The pressure gauge should be mounted on the cab of the vehicle and connected by the rubber hose provided.
4. Assembly of nozzle jet (Fig. 170). Take the nozzle jet and brass tube to suit the particular vehicle and engine. Screw on the l/. inch BSP back nut, as found in the tool bag.
Screw into ¼ inch BSP socket, found in centre of 1½ inch BSP socket on exhaust chamber, and lock up with ¼ inch BSP back nut.
Screw brass lock collar on to end of stack pipe with longest thread, for five turns, and hold up vertically, collar downwards. Carefully place stack pipe over the nozzle jet tube, and screw on to socket on exhaust chamber. Press spider into base of nozzle restrictor, flange downwards. Screw nozzle restrictor down into socket up to shoulder on nozzle. Now position the socket assembly carefully over nozzle jet, and screw socket down on to stack pipe until tight. Screw the stack pipe down into the socket on the exhaust chamber, so that the jet is in the centre of, and protruding 0.5-1 mm above, the restrictor orifice. Secure with the locking ring at the base of the stack pipe.
5. Connecting sprayer to exhaust system. Screw the plain large bend supplied into the hole on the exhaust chamber, and then attach one end of the flexible pipe. The other bend is then screwed into the other end of the flexible pipe, and the whole then connected to the exhaust fitting by means of the screwed union.
How to test the exhaust nozzle sprayer
The exhaust system should always be tested before the sprayer is used to ensure that
there are no gas leaks which would affect the spraying performance. The procedure for
testing is as follows.
1. Screw the protective cap on to the top of the nozzle restrictor.
2. Connect the exhaust feed pipe to the connection point of the sprayer and to the union on the exhaust pipe of the vehicle.
3. Put enough kerosene (or spray solution) into the spray tank to cover the bottom of the tank.
4. Close the shut-off valves.
5. Start the engine.
Allow the engine to run slowly in neutral, and watch the pressure gauge needle rise to 0.5 bar without the need to accelerate. After the engine is stopped, the needle should gradually return to zero. This will indicate that there are no air leaks.
If the needle fails to rise when the engine is running slowly, or the needle returns to zero quickly when the engine is stopped, this indicates air leaks which must be sealed.
The following procedure should be adopted to find and remedy leaks in the exhaust system.
1. Inspect all screws, gaskets on spraying equipment and screwed union on exhaust system,
tighten if necessary.
2. Re-start the engine and run as above, switch off when the pressure reaches 0.5 bar.
3. Inspect the pressure release valve and ensure proper functioning by cleaning and oiling as necessary. If the valve continues to leak, it is necessary to readjust the valve setting to release to 0.5 bar, or to regrind the valve seating.
4. Inspect the vehicle's exhaust system and check for leaks. These are most likely to occur in the following places:
around newly welded joints;
at the joints between the silencer and the exhaust pipe;
at the junction of the exhaust pipe to the engine due to faulty gasket or loose connecting bolts.
The exhaust system of old vehicles may be corroded and need complete replacement.
How to use the exhaust nozzle sprayer
1. Place a large funnel, with gauze filter, into filling hole in top of tank.
2. Fill the tank to indicated level, or below, do not overfill.
3. Remove the protecting cap from the nozzle restrictor and place in a safe place.
4. The flexible exhaust pipe supplied must be fitted to the union on the vehicle exhaust pipe and to the exhaust chamber of the sprayer.
5. Start the engine.
6. Engage in first gear, low ratio and let in the clutch.
7. Unscrew one of the wheel-type selector valves when the pressure in the spray tank rises to 0.2 bar.
If the vehicle is fitted with a hand throttle this can be used to regulate the speed of the vehicle to obtain a steady pressure of 0.3 bar.
8. During spraying operations, always maintain a steady pressure in the spray tank of 0.20.4 bar. Always avoid sudden acceleration of the vehicle. If the pressure falls below 0.2 bar, atomisation of the spray will be impaired and the emission rate reduced.
The pressure release valve, which is incorporated to prevent the build-up of an excessive back pressure in the engine, operates when the pressure in the spray tank reaches 0.5 bar.
9. Always close the ON/OFF valves at the end of each spray run.
10. Start downwind of the target and move upwind.
11. After spraying operations, remove the flexible exhaust feed pipe and replace the protecting cap on the nozzle orifice.
If the sprayer fails to operate when the above procedure is carried out, it is almost certainly due to a solid particle causing a blockage. Open and shut the selector valves several times. Blockages will seldom occur if a funnel with a gauze filter is always used to filter the insecticide when filling the spray tank. Stop the engine, when pressure has dropped, remove the cover from the filters, and check for cleanliness.
To ensure the sprayer will work properly whenever needed, the manufacturer's recommended maintenance procedures should be carried out regularly.
Micro-Ulva
The Micro-Ulva is a popular sprayer with smallholders and plant protection officers in the tropics. Its variable spray control means it is economical with insecticides and it minimises the risk of environmental contamination. It is ideal for locust control and some chemical companies have introduced special ultra-low volume formulations that can be used with both water and oil-based sprays.
The hand-held Micro-Ulva sprayer (similar to the Micron Ulva-8 illustrated in Fig. 171 ) is designed for controlled drop application of insecticides. A spinning disc produces a range of spray drop sizes for different applications so making this machine an easy, safe and economical sprayer.
Torchlight batteries (R20S D-cells) fit into the carrying handle of the Micro-Ulva and power a small motor which spins the disc. The spray liquid is gravity fed on to the disc from a O. 5 or 1 litre insecticide bottle. The speed of the spinning disc determines the size of the spray drops. When ready to use, the Micro-Ulva weighs only 1.7 kg. The 0.5 litre bottle gives 30 min spraying time and is sufficient to cover 1 ha. A 10 litre backpack reservoir can be used to treat larger areas. The natural forces of wind and gravity distribute the spray drops and so the sprayer must always be held downwind of the operator.
Ulvamast
The Mark 2 Ulvamast (Fig. 172) is a tractor-mounted sprayer designed specifically for wind-blown ultra-low volume application. It is a simple machine that requires little maintenance. The spray-head atomises chemicals in oil to produce a non-variable uniform drop size.
The rotating atomiser head of 15 discs is powered by a 12 volt battery. Chemicals are pumped from the 270-litre tank to the spray-head. The height of the arm can be adjusted to 3.5 m for high-level spraying of trees and bushes. The application rate for insecticides can be set as low as 1 I/ha.
The small drop size improves impaction on the target and greatly reduces the volume of chemicals needed for sufficient coverage.
CDA Boom
This sprayer is suitable for locust control on crops or grassland.
The CDA Boom sprayer (Fig. 173) is mounted behind a tractor and produces a variable spray droplet size suitable for ultra-low volume application. It is economical with chemicals.
The sprayer is engineered to be driven at 24 km/in without losing boom stability. The boom is 12 m wide and has 10 atomiser heads. The sprayer unit is powered by a 12 volt battery. The tank capacity is 250 litres and the flow rate can be set from 5 to 501/ha.
Micronair AU5000
Micronair spray heads produce a uniform drop size that can be varied by changing the rotational speed of the atomiser. This means that the drops are not so small that they evaporate before impaction or so large that they are wasteful and contaminating.
The Micronair AU5000 {Fig. 174) is an atomising spray head that is usually attached to aircraft but it can also be used on air-blast ground sprayers. It consists of a gauze cage which is rotated about a fixed spindle by a three-bladed fan in the aircraft slipstream. The rotational speed of the atomiser can be adjusted by varying the pitch of the fan blades. Each atomiser is attached to the aircraft boom by a lightweight streamlined mounting clamp. The flow of insecticide can be regulated by a variable restrictor unit. It is designed to deliver up to 56 litres of insecticide per minute.
This sprayer is usually used by large organizations for widespread locust control. It requires sophisticated equipment and a suitable landing strip for the aircraft.
Micronair AU7000
The Micronair AU7000 (Fig. 175) spray head has been developed from aerial sprayers. It has been adapted for use in ground spraying applications and is easily fitted to appropriate machinery. It utilises less chemicals and carrier liquid than conventional nozzles and is simpler to maintain.
The device consists of a rotating cage behind a fan driven by hydraulic motor connected to the hydraulic system on a tractor, or alternatively to an hydraulic pump fitted to the P.T.O. The rotation of the fan atomiser imparts a swirling motion, ensuring good penetration of dense foliage. The Micronair AU7000 can be calibrated for any application, including low volume spraying, by changing a restrictor unit. The maximum flow rate is 15 1/mint
This sprayer is suitable for use on locusts invading cropping systems and low-level vegetation. It can also be fitted with a 13 foot extension for spraying into trees and bushes.
Micronair AU8000
The Micronair AU8000 (Fig. 176) is a hand-held backpack sprayer used with a mist-blower. It is economical with chemicals and carrier liquid and produces spray droplets of a uniform size.
It incorporates a 14 litre chemical tank and a small engine, which drives the air blower. A flexible air duct connects the blower to the spray head. This contains a rotary atomiser fitted with a cylindrical wire gauze, which is driven by adjustable fan blades in the air-stream. The application rate can be controlled by a restrictor unit. Maximum flow rate is 1.2 1/mint The complete kit empty weighs 12 kg.
The air blower carries spray droplets away from the operator and ensures good penetration and coverage of the target. It is suitable for ultra-low volume applications and can be used for the control of locusts by farmers or plant protection officers.
<section>Conversion tables</section>
Temperature
Freezing point of water 32ºF, 0ºC; boiling point of water 212ºF, 100ºC
Length
BritishMetric
12 inches= 1 foot 10 millimetres (mm)= 1 centimetre NOTE 1 micron (µm) =0.001 mm
3 feet = 1 yard 100 centimetres (cm)= 1 metre (This is the unit used to measure the size
1760 yards = 1 mile 1000 metres (m) = 1 kilometre (km) of spray droplets.)
British to Metric
Metric to British
inch=2.54 cm or 0.025 m
1 mm=0.039 inch
foot=30.48 cm or 0.3048 m
1 cm=0.394 inch
yard=91.44 cm or 0.9144 m
1 m=39.37 inches=3.28 feet
mile = 1.609 km
1 km = 1093.6 yards = 0.62 mile
Area
British
Metric
144 square inches= 1 square foot (ftý)
10,000 square centimetres (cm²)= 1 square metre (m²)
9 square feet = 1 square yard (ydý)
10,000 square metres = 1 hectare (ha)
4840 square yards = 1 acre
British to Metric
Metric to British
1 square inch =6.452 square centimetres
1 square centimetre =0.155 square inches
1 square foot (144 sq in)=929.0 square centimetres 1 square metre
=1.196 square yards
1 square yard (9 sq ft) =0.836 square metre
1 hectare (10,000 square metres) = 11,960 square yards=2.471 acres
1 acre (4840 sq yd) =0.405 hectare
1 square mile (640 acres)=259.0 hectares
1 square kilometre (100 hectares)=247.11 acres = 2.59 square kilometres = 0.386 square mile
Other units
Ethiopia
British
Metric
1 Gasha =
99 acres
=40 hectares
Sudan and Egypt
1 Feddan
= 1.038 acres
=0.42 hectares
Weight
Solids
Wind speed
The use of the following terms was recommended by the Desert Locust Information Service for non-instrumental records of wind strength. They constitute a simplified Beaufort scale.
<section>Bibliography</section>
CENTRE FOR OVERSEAS PEST RESEARCH (1982).
The locust and grasshopper agricultural manual. London: Centre for Overseas Pest Research. 690pp.
FISHPOOL, L. D. C. and POPOV, G. B. (1984).
The grasshopper faunas of Mali, Niger, Benin and Togo savannas. Bulletin de l'lnstitut Fondamental d'Afrique Noir, 43A: 275-410.
LAUNOIS, M. (1978).
Manuel practique d'identification des principaux acridiens du Sahel. Paris: MinistSre de la Cooperation de la Republique Francaise and GERDAT, 297pp. (English edition available, Practical manual of identification of the principal locusts and grasshoppers of the Sahel. )
MACCUAIG, R. D. (1983).
Insecticide index: an index giving details of tests made to assess the usefulness of particular insecticides for control operations against locusts. (2nd edn) Rome: Food and Agriculture Organization, 191 pp.
PEDGLEY, D.[E.] (1981).
Desert Locust forecasting manual. ( 2 volumes) London: Centre for Overseas Pest Research. 268pp. + 142pp.
POPOV, G.B. (1988).
Sahelian grasshoppers: a summary of the work of the COPR/OCLALAV Research and Development Project on Sahelian grasshoppers in the middle Niger valley, August 1976-December 1978. Overseas Development Natural Resources Institute Bulletin No.5, 87pp. (French edition, Les sauteriaux du Sahel, available as Stations de recherche acridienne sur le terrain, series techniques No. AGP/DL/TS/25 from Food and Agriculture Organization, Rome.)
POPOV, G. B. (1989).
Nymphs of the Sahelian grasshoppers: an illustrated guide/Les larves des criquets du Sahel. Chatham: Overseas Development Natural Resources Institute, 158pp.
UVAROV, B.[P.] (1977).
Grasshoppers and locusts. A handbook of general acridology. Volume 2. Behaviour, ecology, biogeography, population dynamics. London: Centre for Overseas Pest Research. 613pp.
