Bulletin 7 - April 1979: The Desert Locust

The Desert Locust

It seems that man has been plagued by locusts ever since he first began to grow crops eight or nine thousand years ago. Bas-reliefs on tombs at Saggara in Egypt dating from the Sixth Dynasty, about 2400 BC, vividly depict locusts, and the Bible provides further early evidence of the plagues that regularly afflicted Egypt. Two passages in particular (Exodus 11, verses 12 and following, and Joel 1, verses 5 and following) contain good descriptions of plagues that are by no means entomologically inaccurate. The Exodus verses include a description of how the eighth plague of Egypt came, like the swarms still do, on the east wind from Arabia, and was eventually blown back into the Red Sea on a strong westerly wind. Other passages are clearly based on observation at first hand of how a vast army of hoppers marched along in their gregarious phase eating every green thing in their path.

The passages referred to above describe the desert locust (Schistocerca gregaria), which is the species that occurs in the Arabian Peninsula and with which this article is primarily concerned. It is unique among the fourteen or so species of its kind in that it has no permanent outbreak areas, but affects an area of roughly 11 million square miles (the 'invasion area') stretching from the west coast of Africa to Burma and from the shores of Europe to Central Africa. There are other true locust species like the brown locust of South Africa, the Moroccan locust, and the South American and Australian varieties; but none of these have the range nor the destructive power of the desert locust. Some other species like the bush locusts of Africa, and the spotted grasshoppers of South East Asia swarm only in the hopper or early immature stage. Other species swarm only in the adult stage and some swarm very rarely even when present in great numbers.

What gives Schistocerca gregaria its baneful influence is its extraordinary mobility and its ability to take rapid advantage of the climate and geography in which it finds itself. It can survive the wide range of temperatures found in the desert, between 0° and 60°C on the surface. It can speed up its life cycle in favourable conditions and slow it down if it finds itself in cold weather or is affected by shortage of food. For example, the passage of food through the gut of the desert locust can take as little as one hour or two when food is plentiful, but the process can be slowed to three or four days when times are hard. Likewise, the incubation period of the eggs in the soil can be as little as fourteen days or as much as seventy days if cold weather is encountered, or the soil becomes too dry for the eggs to hatch.

Once the locust has hatched, which usually takes place in the early morning; it moults immediately, shedding its enclosing membrane. The hopper as it is now called passes through five stages or instars and at the end of each it sheds its skin. This is done usually by hanging on upside down to a twig and wriggling out of the old skin that splits down the back. There is little change between the first two stages, but at the third stage wing buds appear. These grow markedly during the next two stages. The whole hopper stage takes from under thirty days to well over fifty, depending mainly on the temperature. The final moult, known as fledging, brings the locust to the adult stage. The main characteristic of this moult is that the wings, which emerge crumpled, are pumped up to their final adult size. The locusts then gain strength steadily and, as the days go by, they can make longer and longer flights.

It is during this immature adult stage that the locust travels the longest distances and eats the most. It is generally pink coloured until it becomes sexually mature. This change, which occurs somewhere between three weeks and a few months of fledging, usually coincides with the advent of rain and can be very rapid; the locusts then become bright yellow. A whole swarm can change colour in a few days. As soon as the female locust matures, copulation takes place and the female immediately starts looking for a suitable place to lay the eggs. The soil for this must be damp, but not too wet, sandy and cool. By probing with the tip of her abdomen (her ovipositor) the female can locate suitable sites below a dry surface. She then stretches her abdomen down into the sand, forming a vertical hole up to three times the length of her own body. Eggs are then laid in large numbers, usually up to one hundred or more, and the hole is filled in with a frothy secretion that seals it off, and then hardens binding the eggs together. A female locust can lay two or even three times this way, though the number of eggs decreases with successive layings and many females die each time.

The eggs in ideal conditions absorb water from the surrounding soil; if insufficient water is available, the eggs can lie dormant for several weeks. Allowing for an average of 200 eggs laid per female and for equal numbers of males and females hatching out, there is a theoretical possibility of multiplication in each generation of one hundred times. Luckily, natural factors contribute to reduce this fearful possibility to more manageable proportions. Conditions can alter while the eggs are still in the ground; they can dry out if the ground does not stay suitably moist or, alternatively, they can be washed away by floods. Egg pods are sometimes exposed to the air by wind erosion.

Even when they have hatched, the hoppers can die in large numbers if they do not find bushes or plants in which to roost at intervals while on the march, so as to escape the temperatures on the ground in the middle of the day. Many will also die if food in sufficient quantities is not constantly available. In addition to these problems, there are numerous enemies who prey on the locusts at all stages of their life cycle. There is a fly, called Stomorhina, which resembles the house-fly, and which lays its eggs on top of the locust egg pod. When the fly grubs hatch out, they feed off the locust eggs, destroying the whole pod. There are other species of flies, and also beetles, whose larvae eat locust eggs. Ants too often carry off young hoppers, and many species of birds feed off them avidly.

We can be thankful that nature herself sees to the decimation of most locust swarms. The survivors tend nonetheless to be very numerous and are extraordinarily mobile. They can migrate enormous distances between breedings; recorded instances of swarms travelling 1000 - 3000 miles are common. Wind tunnel tests have shown that a locust can fly continuously for 17 hours. Even more impressive is the evidence of an actual recorded flight of locusts from the Canary Islands to the British Isles in 1954, a distance of 1600 miles, although only a small proportion of those that set out completed the whole flight. Nonetheless, swarms have been followed travelling across Africa for distances of more than 2000 miles in a month. Locusts in general fly at about 10 or 12 miles per hour and tend to move down-wind. They can rise to heights of several hundred meters on up-currents of air in hot conditions. Their general movement down-wind tends to concentrate them in areas where winds meet and rains fall. Adult locusts are thus always arriving in regions where the soil is moist and the vegetation green, enabling them to breed successfully. The most interesting and distinctive aspect of the locust's life cycle is what is known as the phase change. This behavioural rather than anatomical difference is what distinguishes a locust from a grasshopper. It was not until 1912 what the great Russian entomologist, Boris Uvarov (later to become Sir Boris), who was working on the Russian Migratory Locust, began to suspect that two kinds of grasshopper or locust were one and the same species. He began work on the African Migratory Locust in 1920, noting the marked change in size and colour before the locust swarmed. It was not in fact until 1929 that it was finally established that the Desert locust was one and not two different species. Locusts as opposed to grasshoppers can be induced to swarm by being collected together. They are not born with an inherited desire to swarm; they learn it afresh in each generation. If young hoppers find themselves in groups they will inspect and touch each other constantly and eventually crowd together more closely. As many as 20,000 per square meter have been recorded, where swarms are particularly dense. Even if they are born black showing that they are born of gregarious parents they have to go through this learning process, though this can take as little as four hours. Thus the winds and cyclonic movements tend to concentrate adults or hoppers together, as does a contracting amount of vegetation at certain times of the year. Solitary hoppers, born green, can in these circumstances turn black and go on to swarm. Conversely, a black hopper can be induced to turn green again if separated from its fellows at a sufficiently early stage. If this is left too late, the locust turns yellow and matures into a 'gregarious' adult.

Out of the sixty years from 1910 to 1970, forty had desert locust plagues somewhere or other. Severe plagues affected most of the years 1940 to 1963, though there was a recession in 1948. The plagues of 1967/8 were probably the first to be dealt with effectively by scientific methods, though one can be sure that the weather helped man's efforts in bringing these outbreaks under control. Initially, man's only weapons against the depredations of the locust had been to shout, bang tin drums, and light fires, to prevent them settling. Digging up the eggs and trampling the hoppers is obviously not very effective as a means of control. It was not until 1885 that poison baiting was tried using sodium arsenate; the most popular method of that period was to dig trenches in the path of a swarm and bury or burn the locusts. Now, with radar, aerial photography and satellite weather pictures to help the governments of the affected areas and the use of effective insecticides sprayed from aircraft or by vehicles on the ground, man at last stands a reasonable chance of controlling one of his oldest foes.

A footnote: There is plenty of evidence of locusts being eaten as food, indeed they are still eaten from time to time in many parts of the world. St. John survived in the wilderness on locusts and honey. Reliefs at Ninevah show locusts carried on skewers to a royal banquet. Diodorus Siculus and Herodotus both mention several recipes for preparing locusts for the table. However, the awful implications of the arrival of locust swarms each year and their direct threat to the livelihood of man throughout most of the invasion area, far outweigh any attraction the locust may have as a delicacy or a dietary supplement.

References:

I have drawn heavily on Stanley Baron's useful book The Desert Locust (London 1972) and on several publications of the Centre for Overseas Pest Research, notably C.F. Hemmings detailed booklet The Locust Menace (HMSO 1974).

This article is based on a talk given on 8 May 1978 to the Natural History Group. Appropriately enough, it coincided with another series of outbreaks of locust swarms in the Horn of Africa, where the Ogaden war effectively prevented routine control methods being mounted to search out and destroy hopper bands. If conditions are favourable, swarms can therefore be expected to spread into Arabia, to the West into the Southern Sahara region and perhaps farther afield.

Schistocerca gregaria (Desert Locust)