page_title: Desert locust
section_title: Ecology and swarming
hierarchical_section_title: Desert locust / Ecology and swarming
caption_attribution_description: English: Schistocerca gregaria, London Zoo.
context_page_description: The desert locust is a species of locust, a periodically swarming, short-horned grasshopper in the family Acrididae. They are found mainly in Africa, through Arabia and West Asia, and extending into parts of South Asia. During population surge years, they may extend into parts of western Spain. The desert locust shows periodic changes in its body form and can change, in response to environmental conditions, over generations, from a solitary, shorter-winged, highly fecund, nonmigratory 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 km² and fly in the direction of the prevailing wind, up to 150 km in one day.
context_section_description: 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 (immature) or yellow (mature). 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. This effect could aid locust control in the future.
  During quiet periods, called recessions, desert locusts are confined to a 16-million-km² 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 32 million km², 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 100 to 200 km (62 to 124 mi) in a day, and  fly up to about 2,000 m (6500 ft) above sea level (the temperature becomes too cold at higher altitudes). 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 1200 km² and can contain between 40 and 80 million locusts per km² (a total of around 50 to 100 billion locusts per swarm, representing 100,000 to 200,000 tons, considering an average mass of 2 g per locust). 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.
caption: Desert locusts preparatory to mating, London Zoo
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