2. LIFE CYCLE

The life cycle of all species of locusts and grasshoppers comprises three stages:

EGG ------->HOPPER -----------> ADULT

The time spent in each stage, the number of hopper instars and the time required for the adult to become sexually mature vary greatly between species and even show considerable variation within a single species, depending upon the weather.
The life cycle of the Desert Locust, as it occurs in swarms, will be described in some detail, and in many respects it may be regarded as illustrative of grasshoppers and locusts in general.
It is intended that from time to time supplements shall be issued giving details of the life cycles of other species.


2.1. LIFE CYCLE OF THE DESERT LOCUST
(Schistocerca gregaria)

The life cycle may be summarised thus:-

2.1.1. IMMATURE ADULT

Colour pink, lighter or darker according to whether the locusts have been bred under high or low temperatures. The bright pink may change to a brownisli red if the locusts have spent more than two months in this immature stage

2. 1. 1. 1. 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 6 months. If they have the right kind of food and weather, maturation can take place rapidly in 2 to 4 weeks. We do not know exactly what are the conditions that cause locusts to mature, but the process is usually associated with the start of the rainy season. 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. Eggs then grow to full size in about one week and the abdomens of the females become distended. 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 the females and also in any males in which it has not already begun.

2..I. 2. MATURE ADULT
Colour 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.

2. 1. 2. 1 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. The tips of their abdomens come into contact and the male sex cells (spermatozoa) are passed into the body of the female where they fertilize the eggs. The time spent in copulation varies from 3 to 14 hours. Several females can be fertilized by one male and the spermatozoa can be stored inside the female's body and used to fertilize 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.

2.1. 2.2.. 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 temperature, hardness, moisture and salinity 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 vegetatioil 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 which 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. The. whole process of probing, digging and laying takes 1V2 to 2 hours. A copulating and laying swarm usually stays in the same area for one or two 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 layings of this kind, especially when on a large scale, constitute 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. The egg pod is 3 to 4 cm. long, the bottom being usually about 10 crn. 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 yards to a square mile 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 laboratory cages can lay many more. There is some evidence that in the field non-swarming locusts lay more pods than swarming ones, about 5 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 5 days if it is available at that time, and this is enough to allow them to develop successfully. 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.

2.1. 3. INCUBATION PERIOD AND HATCHING
The rate at which eggs develop varies according to the soil temperature. The period of egg development, between laying and hatching, is called the INCUBATION PERIOD.
The lengths of this period recorded for different areas and seasons are as follows:-

Summer breeding in Sudan, Ethiopia and West Africa............................................10 to 14 days

Summer breeding in lowlands of India .....................................................................10 to 14 days

Summer and winter in Somali Peninsula................................................................... 10 to 14 days

Summer and winter on Red Sea Coast.................................................................... 10 to 14 days

Spring breeding in central Arabia, southern Iran,
Pakistan and North Africa....................................................................................... 25 to 30 days

Winter/spring breeding elsewhere in Middle East
and in North Africa, in exceptionally cold weather.................................................... 60 to 70 days

In warmer weather ..................................................................................................20 to 30 days


You can add to this knowledge by making careful notes of the laying and hatching dates in the area where you work.
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 hours after 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 eggfield 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.

2. 1. 4. 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. The behaviour of hopper bands is described later (Sections 3.1.1.1.. and 3.1.1.2.).

2. 1. 4. 1. 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).

2. 1. 4. 2. Instars
The hopper stage of the life cycle is thus divided into five INSTARS.'(Iioppers 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. '5th stage hoppers'; it should,
however, be restricted to the three main stages of the life cycle, viz. egg, hopper, adult).

2. 1. 4. 3. How to recognise the different instars
The photographs and captions on the following pages will enable you to distinguish between
the different instars.

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.

2. 1- 5. FLEDGLING
Colour pink. Wings, head and body relatively soft. Activity limited to walking and short

descending flights.
Fledglings gradually become hard and able to fly strongly. Locusts in this condition are
called IMMATURE ADULTS (Section 2.1.1.).

2. 1. 6. DURATION OF LIFE CYCLE
In the following table the average time spent in each stage of the life cycle and the duration of each hopper instar are given for eastern Ethiopia. The periods given are only approximate and may vary from year to year according to the weather.

Figures like these are needed from all areas.. Fill in the spare column from observations made in the field in your own country, making a note of the place and date (month and year) of each observation.

Thus, the total period from the laying of eggs of one generation to the laying of the first eggs of the next generation was, in this example, 14 + 38 + 45 = 97 days, and the total duration of life of an individual was 14 + 38 + 75 = 127 days.
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 2Y2 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.