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close this folder 2. Desert Locust-Schistocerca gregaria
View the document Anatomy of a locust
View the document Life cycle
View the document Behaviour in relation to habitat
View the document Seasonal movements and breeding areas of desert locust during plagues and recessions
View the document Recession periods, outbreaks and the origin of plagues

2. Desert Locust-Schistocerca gregaria

 

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.


Fig. 6. The distribution of the Desert Locust.


Fig. 7. Annual fluctuations in number of territories infested by Desert Locusts from 1890 to 1990, and African Migratory Locusts and Red Locusts from 1890 to 1970.

 

Anatomy of a locust

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.

 

Life cycle

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).

 


Fig. 12. Life cycle of the Desert Locust.

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).

 


Fig. 24. Changes in pronotum and sternum of the Desert Locust.

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.

 

Behaviour in relation to habitat

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.


Table 3

 

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

282

1

267

150,000 (12.5 ha)

1

96 (88 m)

100

2

56

100

2

84

220

2

330

160,000 (13 ha}

2

579 1530 m)

70,000

3

274

165,000 (14 ha}

3

916 (838 ml

7400

5

382

9800

5

917

15,600

5

1216

One mile wide (260 ha)

5

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

1-1.5

10-15

Second week after fledging

1.7-2

40-60

4-5 weeks after fledging

2-2.5

250-300

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).


Fig. 31. Shapes of flying swarms of Desert Locusts.

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.

 


Fig. 33. Direction and speed of displacement of individual Desert Locust swarms in relation to the wind: eastern Africa 1951-1957. Out of the 42 best documented observations 26 swarm displacements were within 10º of downwind. (Rainey, R.C. 11963). Meteorology and the migration of Desert Locusts. Anti-Locust Memoir No. 7: 115pp ).

 


Fig. 34. Swarm displacements in a quasi-uniform wind field in Kenya, January 1954, during a period of predominantly northeasterly winds. During periods of effectively constant wind direction, swarms displace progressively and systematically downwind with a constancy exceeding 75%, and make displacements of 5-130 km a day (Rainey, R.C. (1963). Meteorology and the migration of Desert Locusts. Anti-Locust Memoir No. 7: 115pp.)

 

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.


Fig. 35. Spring breeding areas of the Desert Locust.


Fig. 36. Summer breeding areas of the Desert Locust.


Fig. 37. Winter breeding areas of the Desert Locust.


Fig. 38. Occurrence of the Desert Locust during recessions, February to June, from 1920 to 1948 and from 1963 to 1976. Spring generation hoppers.


Fig. 39. Occurrence of the Desert Locust during recessions, July to October, from 1920 to 1948 and from 1963 to 1976. Summer generation hoppers.


Fig. 40. Occurrence of the Desert Locust during recessions, November to January, from 1920 to 1948 and from 1963 to 1976. Winter generation hoppers.

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.

Seasonal movements and breeding areas of desert locust during plagues and recessions

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.

 


Fig. 41. Swarms in January.


Fig. 42. Swarms in February.


Fig. 43. Swarms in March.


Fig. 44. Swarms in April.


Fig. 45. Swarms in May.


Fig. 46. Swarms in June.


Fig. 47. Swarms in July.


Fig. 48. Swarms in August.


Fig. 49. Swarms in September.


Fig. 50. Swarms in October.


Fig. 51. Swarms in November.


Fig. 52. Swarms in December.


Fig. 53. Hopper bands in January.


Fig. 54. Hopper bands in February.


Fig. 55. Hopper bands in March.


Fig. 56. Hopper bands in April.


Fig. 57. Hopper bands in May.


Fig. 58. Hopper bands in June.


Fig. 59. Hopper bands in July.


Fig. 60. Hopper bands in August.


Fig. 61. Hopper bands in September.


Fig. 62. Hopper bands in October.


Fig. 63. Hopper bands in November.


Fig. 64. Hopper bands in December.

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.


Fig. 65. Adults in January.


Fig. 66. Adults in February.


Fig. 67. Adults in March.


Fig. 68. Adults in April.


Fig. 69. Adults in May.


Fig. 70. Adults in June.


Fig. 71. Adults in July.


Fig. 72. Adults in August.


Fig. 73. Adults in September.


Fig. 74. Adults in October.


Fig. 75. Adults in November.


Fig. 76. Adults in December.


Fig. 77. Hoppers in January.


Fig. 78. Hoppers in February.


Fig. 79. Hoppers in March.


Fig. 80. Hoppers in April.


Fig. 81. Hoppers in May


Fig. 82. Hoppers in June.


Fig. 83. Hoppers in July.


Fig. 84. Hoppers in August.


Fig. 85. Hoppers in September.


Fig. 86. Hoppers in October.


Fig. 87. Hoppers in November.


Fig. 88. Hoppers in December.

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

12

10-21

10-20

9-17

13-36

Pakistan

 

10-16

9-17

9-13

 

Arabia

15-16

 

10-12

10-15

9-12

Ethiopia, Somalia, Kenya (below 1500 m)

15-17

10-23

10-20

10-12

 

Ethiopia, Somalia, Kenya (above 1500 m)

 

13-23

14-20

   

Sudan, Chad, Mali, Niger, Mauritania, Senegal

9-16

9-14

9-25

9-23

17

 

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

10-13

10-19

10-17

9-18

9-17

11-15

Kenya 900-1500 m

15-19

15-22

16-17

11-20

9-20

12-17

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

12-29

11-22

13-15

10-15

15 15

12-16

10-16

9-17 1-18

9-25

 


Table 10. Egg incubation period (days) in the winter-spring breeding areas in Asia, Arabia and the Middle East

 

TABLE 11. Hopper development periods (days) recorded in the field

Summer and monsoon breeding areas

30-39

Short rains

31-45 (63% of records 35-391

Long rains

27-47

Red Sea coast/Gulf of Aden

24-48 (71% of records 30-39)

Winter-spring

25 - 57

Recession periods, outbreaks and the origin of plagues

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.

 


Fig. 89. The distribution of adult Desert Locusts during recessions.


Fig. 90. The distribution of Desert Locust hoppers during recessions.

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.


Fig. 91. Areas of actual gregarisation of the Desert Locust.

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.