Desert locust
The desert locust is a species of locust in the grasshopper family, Acrididae. A periodically swarming, short-horned bird grasshopper, it is found primarily in the deserts and dry areas of northern and eastern Africa, Arabia, and southwest Asia. During population surge years, its range may extend north into parts of Southern Europe, Eastern Africa, and Northern India. The desert locust shows periodic changes in its body form and can change in response to environmental conditions over several generations. It begins life as a solitary, shorter-winged, highly fecund, non-migratory form, to a gregarious, long-winged, and migratory phase in which it may travel long distances into new areas. Due to this traveling, it may join groups, thus, forming locust plagues. This involves invading new areas where it may consume all vegetation including valuable crops. Although at other times, it may live unnoticed in small numbers in various areas.
During plague years, when the desert locust does group with others, it can cause widespread damage to crops as it is highly mobile and feeds on large quantities of any kind of green vegetation. A typical swarm can be made up of and fly in the direction of the prevailing wind, up to in one day. Even a very small, 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, its population has been routinely monitored through a collaborative effort between countries and the United Nations Food and Agriculture Organization Desert Locust Information Service, 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 its 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, 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 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.
Taxonomy
The desert locust is a species of orthopteran in the family Acrididae, subfamily Cyrtacanthacridinae. There are two subspecies, one called Schistocerca gregaria gregaria, the better known and of huge economic importance, located north of the equator, and the other, Schistocerca gregaria flaviventris, which has a smaller range in south-west Africa and is of less economic importance, although outbreaks have been observed in the past.Description
The genus Schistocerca consists of more than 30 species, distributed in Africa, Asia, and North and South America. Many species are difficult to identify due to the presence of variable morphs. Schistocerca 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 tibia have smooth margins with numerous spines, however, there are no apical spine on the outer margin. 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 morphology and colour of Schistocerca differ depending on whether individuals are solitary or gregarious.
Morphology - Adults: Solitary Females are 6–9 cm long; Solitary males are 4.5–6 cm; gregarious female 5–6 cm long; gregarious males 4.5–5 cm. The prosternal tubercle is straight, blunt and slightly sloping backwards. The Male subgenitial plate is biloped, ceri, flat and blunt. Elytra are marked with large irregular spots, and the pronotum is not crested, and is narrower and saddle-shaped in the gregarious phase. The eyes are striated, and the number of striae increases after each moult and are only clearly visible in solitary individuals.
Coloration - Nymph: Solitary nymphs are greenish or pale beige and may go through six instars. Gregarious nymphs are typically yellow, with a black head and pronotum, black lateral stripes on the abdomen and pass through five instars. During the first instar, gregarious nymphs are almost entirely black. Adults: Immature solitary adults are sandy, pale grey or beige in colour; this colouration evolves to pale yellow in mature male adults and to pale beige with brown patterns in mature females. Immature gregarious adults are pink/reddish in colour, changing to bright yellow in mature males; in mature females the yellow is less bright, mainly on the upper parts of the body, with the lower parts being more of a pale beige. The hindwings are transparent or light yellow.
Lifecycle
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 long and the lower end is about 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. 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 imaginal moult, the young adult is initially soft with drooping wings, but within a few days, the cuticle hardens and haemolymph is pumped into the wings, stiffening 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.
Ecology and swarming
Desert locusts have a solitary phase and a gregarious phase, a type of polyphenism. Solitary locusts nymphs and adults can behave gregariously within a few hours of being placed in a crowded situation, while gregarious locusts need one or more generations to become solitary when reared in isolation. Differences in morphology and behaviour are seen between the two phases. In the solitary phase, the hoppers do not group together into bands but move about independently. Their colouring in the later instars tends to be greenish or brownish to match the colour of their surrounding vegetation. The adults fly at night and are also coloured so as to blend into their surroundings, the immature adults being grey or beige and the mature adults being a pale yellowish colour. In the gregarious phase, the hoppers bunch together and in the later instars develop a bold colouring with black markings on a yellow background. The immatures are pink and the mature adults are bright yellow and fly during the day in dense swarms.The change from an innocuous solitary insect to a voracious gregarious one normally follows a period of drought, when rain falls and vegetation flushes occur in major desert locust breeding locations. The population builds up rapidly and the competition for food increases. As hoppers get more crowded, the close physical contact causes the insects' hind legs to bump against one another. This stimulus triggers a cascade of metabolic and behavioral changes that causes the insects to transform from the solitary to the gregarious phase. When the hoppers become gregarious, their colouration changes from largely green to yellow and black, and the adults change from brown to pink or yellow. Their bodies become shorter, and they give off a pheromone that causes them to be attracted to each other, enhancing hopper band and subsequently swarm formation. The nymphal pheromone is different from the adult one. When exposed to the adult pheromone, hoppers become confused and disoriented, because they can apparently no longer "smell" each other, though the visual and tactile stimuli remain. After a few days, the hopper bands disintegrate and those that escape predation become solitary again.
During quiet periods, called recessions, desert locusts are confined to a belt that extends from Mauritania through the Sahara Desert in northern Africa, across the Arabian Peninsula, and into northwest India. Under optimal ecological and climatic conditions, several successive generations can occur, causing swarms to form and invade countries on all sides of the recession area, as far north as Spain and Russia, as far south as Nigeria and Kenya, and as far east as India and southwest Asia. As many as 60 countries can be affected within an area of, or about 20% of the Earth's land surface.
Locust swarms fly with the wind at roughly the speed of the wind. They can cover from in a day, and fly up to about above sea level. Therefore, swarms cannot cross tall mountain ranges such as the Atlas, the Hindu Kush, or the Himalayas. They do not venture into the rain forests of Africa nor into central Europe. However, locust adults and swarms regularly cross the Red Sea between Africa and the Arabian Peninsula, and are even reported to have crossed the Atlantic Ocean from Africa to the Caribbean in 10 days during the 1987–89 plague. A single swarm can cover up to and can contain between . The locust can live between 3 and 6 months, and a 10- to 16-fold increase in locust numbers occurs from one generation to the next.