The desert locust (Schistocerca gregaria) is a species of locust, a periodically swarming, short-horned grasshopper in the family Acrididae. They are found primarily in the deserts and dry areas of northern and eastern Africa, Arabia, and southwest Asia. During population surge years, they may extend north into parts of Southern Europe, south into Eastern Africa, and east in northern India. The desert locust shows periodic changes in its body form and can change in response to environmental conditions, over several generations, from a solitary, shorter-winged, highly fecund, non-migratory form to a gregarious, long-winged, and migratory phase in which they may travel long distances into new areas. In some years, they may thus form locust plagues, invading new areas, where they may consume all vegetation including crops, and at other times, they may live unnoticed in small numbers.
During plague years, desert locusts can cause widespread damage to crops, as they are highly mobile and feed on large quantities of any kind of green vegetation, including crops, pasture, and fodder. A typical swarm can be made up of 150 million locusts per square kilometre (390,000,000 per square mile) and fly in the direction of the prevailing wind, up to 150 kilometres (93 mi) in one day. Even a very small, 1-square-kilometre (0.39 sq mi) locust swarm can eat the same amount of food in a day as about 35,000 people.
As an international transboundary pest that threatens agricultural production and livelihoods in many countries in Africa, the Near East, and southwest Asia, their populations have been routinely monitored through a collaborative effort between countries and the United Nations Food and Agriculture Organization (FAO) Desert Locust Information Service (DLIS), which provides global and national assessments, forecasts, and early warning to affected countries and the international community. The desert locust's migratory nature and capacity for rapid population growth present major challenges for control, particularly in remote semiarid areas, which characterize much of their range.
Locusts differ from other grasshoppers in their ability to change from a solitary living form into gregarious, highly mobile, adult swarms and hopper bands, as their numbers and densities increase. They exist in different states known as recessions (with low and intermediate numbers), rising to local outbreaks and regional upsurges with increasingly high densities, to plagues consisting of numerous swarms. They have two to five generations per year. The desert locust risk increases with a one-to-two-year continuum of favourable weather (greater frequency of rains) and habitats that support population increases leading to upsurges and plagues.
The desert locust is potentially the most dangerous of the locust pests because of the ability of swarms to fly rapidly across great distances. The major desert locust upsurge in 2004–05 caused significant crop losses in West Africa and diminished food security in the region. The 2019–2021 upsurge caused similar losses in northeast Africa, the Near East, and southwest Asia.
Given the long history of desert locust, it is to be expected that references of the world's most dangerous migratory pest have crept into popular film and literature as well as many of the world's religions.
The genus Schistocerca consists of more than 30 species, distributed in Africa, Asia, and North and South America, and many species are difficult to identify due to the presence of variable morphs. It is the only genus within the Cyrtacanthacridinae that occurs in both the New and Old World. Most species have the fastigium deflexed and lack lateral carinae on the pronotum. The hind tibiae have smooth margins with numerous spines, but have no apical spine on the outer margin. The second tarsal segment is half as long as the first. Males in the genus have broad anal cerci and a split subgenital plate. The genus is thought to have originated in Africa and then speciated in the New World after a dispersal event that took place 6 to 7 million years ago.
The solitary phase insects (or solitaria morph) are greenish in early instars, while older adults are greyish. The solitary phase grasshoppers lack black patterning as present in the gregarious (or gregaria morph) phase, where the body is pinkish in early instars and yellow in adults.
The second and third pairs of spiracles are permanently open.
The lifecycle of the desert locust consists of three stages, the egg, the nymph known as a hopper, and the winged adult. Copulation takes place when a mature male hops onto the back of a mature female and grips her body with his legs. Sperm is transferred from the tip of his abdomen to the tip of hers, where it is stored. The process takes several hours and one insemination is sufficient for a number of batches of eggs.
The female locust then seeks suitable soft soil in which to lay her eggs. It needs to be the right temperature and degree of dampness and be in close proximity to other egg-laying females. She probes the soil with her abdomen and digs a hole into which an egg pod containing up to 100 eggs is deposited. The egg pod is.mw-parser-output.frac{white-space:nowrap}.mw-parser-output.frac.num,.mw-parser-output.frac.den{font-size:80%;line-height:0;vertical-align:super}.mw-parser-output.frac.den{vertical-align:sub}.mw-parser-output.sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}3 to 4 cm (1+1⁄8 to 1+5⁄8 in) long and the lower end is about 10 cm (4 in) below the surface of the ground. The eggs are surrounded by foam and this hardens into a membrane and plugs the hole above the egg pod. The eggs absorb moisture from the surrounding soil. The incubation period before the eggs hatch may be two weeks, or much longer, depending on the temperature.
The newly hatched nymph soon begins to feed, and if it is a gregarious individual, is attracted to other hoppers and they group together. As it grows, it needs to moult (shed its exoskeleton). Its hard cuticle splits and its body expands, while the new exoskeleton is still soft. The stages between moulting are called instars and the desert locust nymph undergoes five moults before becoming a winged adult. Immature and mature individuals in the gregarious phase form bands that feed, bask, and move as cohesive units, while solitary-phase individuals do not seek conspecifics.
After the fifth moult, the insect is not yet mature. It is at first soft and pink with drooping wings, but over the course of a few days, the cuticle hardens and haemolymph is pumped into the wings, which stiffens them. Maturation can occur in 2–4 weeks when the food supply and weather conditions are suitable but may take as long as 6 months when they are less ideal. Males start maturing first and give off an odour that stimulates maturation in the females. On maturing, the insects turn yellow and the abdomens of the females start swelling with developing eggs.
