Desert Locust

The **desert locust** (*Schistocerca gregaria*) is a species of locust. Plagues of desert locusts have threatened agricultural production in Africa, the Middle East, and Asiafor centuries. The livelihood of at least one-tenth of the world’s human population can be affected by this voracious insect.

The desert locust is potentially the most dangerous of the locust pests because of the ability of swarms to fly rapidly across great distances. It has two to five generations per year. The last major desert locust upsurge in 2004–05 caused significant crop losses in West Africa and had a negative impact on food security in the region. While the desert locust alone is not responsible for famines, it can be an important contributing factor.

## Description

Desert locusts have two phases, the solitary phase and the gregarious phase. This is a type of polyphenism. It has been shown that solitary locusts nymphs and adults can behave gregariously within a few hours of being placed in a crowded situation, while it takes gregarious locusts one or more generations to become solitary when reared in isolation. There are differences in morphology and behaviour 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 the herbage. 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 cause 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. Interestingly, 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. It is possible that this effect could aid locust control in the future.

During quiet periods, called recessions, desert locusts are confined to a 16-million-square-kilometer 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 square kilometers, or approximately 20 percent 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 kilometers in a day, and will fly up to about 2,000 meters above sea level (thereafter, it becomes too cold). Therefore, swarms cannot cross tall mountain ranges such as the Atlas, the Hindu Kush or the Himalayas. They will 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 ten days during the 1987-89 plague.[4] A single swarm can cover up to 1200 square kilometers and can contain between 40 and 80 million locusts per square kilometer (a total of around 50 to 100 billion locusts per swarm, representing 100,000 to 200,000 tons, considering an average mass of 2 grams per locust). The locust can live between three and six months, and there is a ten to 16-fold increase in locust numbers from one generation to the next.

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