| Locust handbook |
|2. Desert Locust-Schistocerca gregaria|
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 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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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).
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.