Desert locust

The desert locust ("Schistocerca gregaria") 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.  Even a very small, 1-km2 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 Middle East, and South Asia, their populations have been routinely monitored through collaborations between countries through the United Nations Food and Agriculture Organization (FAO). Their 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 outbreaks, and extremely high densities or plagues. They have two to five generations per year. The desert locust risk increases with 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. 
In February 2020, the FAO announced that desert locusts are destroying tens of thousands of hectares of crops and grazing land in north east Africa in the worst invasion in 25 years. The UN warns that a dangerous situation is now facing the region and in particular eastern Ethiopia. Since this announcement, swarms have caused Somalia to declare a state of emergency and spread to other East African countries such as Kenya, Uganda, Tanzania, and most recently South Sudan.
Description.
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.
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 eggpod 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 (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.
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 (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 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.
Crop loss.
Desert locusts consume an estimated equivalent of their body weight () each day in green vegetation. They are polyphagous and feed on leaves, shoots, flowers, fruit, seeds, stems, and bark. Nearly all crops and noncrop plants are eaten, including pearl millet, maize, sorghum, barley, rice, pasture grasses, sugarcane, cotton, fruit trees, date palms, banana plants, vegetables, and weeds.
Crop loss from locusts was noted in the Bible and Qur'an; these insects have been documented as contributing to the severity of a number of Ethiopian famines. Since the early 20th century, desert locust plagues occurred in 1926–1934, 1940–1948, 1949–1963, 1967–1969, 1987–1989, 2003–2005, and 2019–2020. In March–October 1915, a plague of locusts stripped Ottoman Palestine of almost all vegetation. The significant crop loss caused by swarming desert locusts exacerbates problems of food shortage, and is a threat to food security.
Control.
Early warning and preventive control is the strategy adopted by locust-affected countries in Africa and Asia to try to stop locust plagues from developing and spreading. In the 1920s-1930s, locust control became a major field for international cooperation. The International Agricultural Institute developed several programmes aimed at exchanging data about the desert locust and international conferences were held in the 1930s: Rome in 1931, Paris in 1932, London in 1934, Cairo in 1936, and Brussels in 1938. Colonial empires were heavily involved in these attempts to control locust pests, which affected heavily the Middle East and parts of Africa. The USSR also used locust control as a way to expand its influence in the Middle East and Central Asia.
FAO's Desert Locust Information Service (DLIS) in Rome monitors the weather, ecological conditions, and the locust situation on a daily basis. DLIS receives results of survey and control operations carried out by national teams in affected countries, and combines this information with satellite data such as MODIS, rainfall estimates, and seasonal temperature and rainfall predictions to assess the current situation and forecast the timing, scale, and location of breeding and migration up to 6 weeks in advance. The situation assessments and forecasts are published in monthly locust bulletins that date back to the 1970s. These are supplemented by warnings and alerts to affected countries and the international community. Those since the 1990s are available on the FAO Locust Watch website. FAO also provides information and training to affected countries and coordinates funding from donor agencies in case of major upsurges and plagues.
The desert locust is a difficult pest to control, and control measures are further compounded by the large and often remote areas (16-30 million km2) where locusts can be found. Undeveloped basic infrastructure in some affected countries, limited resources for locust monitoring and control, and political turmoil within and between affected countries further reduce the capacity of a country to undertake the necessary monitoring and control activities.
At present, the primary method of controlling desert locust infestations is with insecticides applied in small, concentrated doses by vehicle-mounted and aerial sprayers at ultra-low volume rates of application. The insecticide is acquired by the insect directly or via secondary pickup (i.e. walking over or eating the residue on a plant). Control is undertaken by government agencies in locust-affected countries or by specialized organizations such as the Desert Locust Control Organization for East Africa (DLCO-EA).
The desert locust has natural enemies such as predatory wasps and flies, parasitoid wasps, predatory beetle larvae, birds, and reptiles. These may be effective at keeping solitary populations in check, but are of limited effects against gregarious desert locusts because of the enormous numbers of insects in the swarms and hopper bands.
Farmers often try mechanical means of killing locusts, such as digging trenches and burying hopper bands, but this is very labour-intensive and is difficult to undertake when large infestations are scattered over a wide area. Farmers also try to scare locust swarms away from their fields by making noise, burning tires, or other methods. This tends to shift the problem to neighbouring farms, and locust swarms can easily return to reinfest previously visited fields.
Biopesticides.
Biopesticides include fungi, bacteria, neem extract, and pheromones. The effectiveness of many biopesticides equals that of conventional chemical pesticides, but two distinct differences exist. Biopesticides in general take longer to kill insects, plant diseases, or weeds, usually between 2 and 10 days.
The two types of biopesticides are biochemical and microbial. Biochemical pesticides are similar to naturally occurring chemicals and are nontoxic, such as insect pheromones used to locate mates, while microbial biopesticides, come from bacteria, fungi, algae, or viruses that either occur naturally or are genetically altered. Entomopathogenic fungi generally suppress pests by mycosis - causing a disease that is specific to the insect.
Biological control products have been under development since the late 1990s; Green Muscle and NOVACRID are based on a naturally occurring entomopathogenic fungus, "Metarhizium acridum". Species of "Metarhizium" are widespread throughout the world, infecting many groups of insects, but show a low risk to humans, other mammals, and birds. The species "M. acridum" has specialised on short-horned grasshoppers, to which these locusts belong, so has been chosen as the active ingredient of the product.
The product is available in Australia under the name Green Guard and in Africa, it used to be available as Green Muscle. However, since Green Muscle seems to have disappeared from the market, another product, NOVACRID, was developed for Africa, Central Asia, and the Middle East. These products are applied in the same way as chemical insecticides, but do not kill as quickly. At recommended doses, the fungus can take up 2 weeks to kill up to 90% of the locusts. For that reason, it is recommended for use mainly against hoppers, the wingless early stages of locusts. These are mostly found in the desert, far from cropping areas, where the delay in death does not result in damage. The advantage of the product is that it affects only grasshoppers and locusts, which makes it much safer than chemical insecticides. Specifically, it allows the natural enemies of locusts and grasshoppers to continue their beneficial work. These include birds, parasitoid and predatory wasps, parasitoid flies, and certain species of beetles. Though natural enemies cannot prevent plagues, they can limit the frequency of outbreaks and contribute to their control. Biopesticides are also safer to use in environmentally sensitive areas such as national parks or near rivers and other water bodies.
Green Muscle was developed under the LUBILOSA programme, which was initiated in 1989 in response to environmental concerns over the heavy use of chemical insecticides to control locusts and grasshoppers during the 1987-89 plague. The project focused on the use of beneficial disease-causing microorganisms (pathogens) as biological control agents for grasshoppers and locusts. These insects were considered to be too mobile and to reproduce too fast to be readily controlled by classical biological control. Pathogens have the advantage that many can be produced in artificial culture in large quantities and be used with ordinary spraying equipment. Entomopathogenic fungi were traditionally seen as needing humid conditions to work well. However, the LUBILOSA programme found a way to avoid this by spraying fungal spores in an oil formulation. Even under desert conditions, Green Muscle can be used to kill locusts and other acridid pests, such as the Senegalese grasshopper. During trials in Algeria and Mauritania in 2005 and 2006, various natural enemies, but especially birds, were abundant enough to eliminate treated hopper bands in about a week, because the diseased hoppers became sluggish and easy to catch.
Desert locust upsurges.
1915 Ottoman Syria locust infestation.
From March to October 1915, swarms of locusts stripped areas in and around Palestine, Mount Lebanon and Syria of almost all vegetation. This infestation seriously compromised the already-depleted food supply of the region and sharpened the misery of all Jerusalemites.
2003–2005 desert locust upsurge (West Africa).
From October 2003 to May 2005, West Africa faced the largest desert locust outbreak in 15 years. The upsurge started as small, independent outbreaks that developed in Mauritania, Mali, Niger, and Sudan in the autumn of 2003. Two days of unusually heavy rains that stretched from Dakar, Senegal, to Morocco in October allowed breeding conditions to remain favourable for the next 6 months and the desert locusts rapidly increased. Lack of rain and cold temperatures in the winter breeding area of northwest Africa in early 2005 slowed the development of the locusts and allowed the locust control agencies to stop the cycle. During the upsurge, nearly 130,000 km2 were treated by ground and aerial operations in more than 20 countries. The costs of fighting this upsurge have been estimated by the FAO to have exceeded US$400 million, and harvest losses were valued at up to US$2.5 billion, which had disastrous effects on food security in West Africa.
The countries affected by the 2004 outbreak were Algeria, Burkina Faso, the Canary Islands, Cape Verde, Chad, Egypt, Ethiopia, the Gambia, Greece, Guinea, Guinea Bissau, Israel, Jordan, Lebanon, Libyan Arab Jamahiriya, Mali, Mauritania, Morocco, Niger, Saudi Arabia, Senegal, Sudan, Syria, and Tunisia. 
2019–20 East Africa and Asia infestation.
Since January 2020, a very large population of desert locusts gathered in Kenya. Agriculture officials estimate that 5,000 km2 of pasture and cropland have so far been destroyed by locusts in Mandera, Marsabit, Wajir, Isiolo, Meru, and Samburu counties. The insects had covered more than 10,000 km2 of land in Kenya by January 2020, and had reached the eastern regions of Machakos county, Ethiopia, Somalia, and South Sudan. As of January 2020, it is a serious problem in East Africa, particularly in Uganda and Kenya.
In February, the UN stated that the swarms are the largest in Somalia and Ethiopia in 25 years. The swarm spread into East Africa from war-torn Yemen, after heavy rainfall in late 2019 created ideal conditions for the insects to flourish, and as of February 2, Somalia is the first country in the region to declare an emergency over the infestation. 
On 1 February 2020, the Pakistani government declared a national emergency to counter an invasion of desert locusts that is destroying crops.
Locusts have been migrating from Africa to Pakistan, and have reached India, as of May 27, 2020. They have spread widely due to availability of crops from one place to another. The states widely affected are Rajasthan, Gujarat, Madhya Pradesh, Haryana, Maharastra, Uttar Pradesh, and Punjab. "There is a continuous locust invasion from Pakistan. It is not a new problem and we have been facing it for a long time. This year, the locusts attack is worse in 26 years. However, a coordinated effort has been made to curb its spread," a senior official from the Faridabad-headquartered LWO told news agency PTI.
Locust pheromone.
Guaiacol is produced in the gut of desert locusts by the breakdown of plant material. This process is undertaken by the gut bacterium "Pantoea" ("Enterobacter") "agglomerans". Guaiacol is one of the main components of the pheromones that cause locust swarming.
In culture.
Film.
Owing to the destructive habits of locusts, they have been a representation of famine in many Middle Eastern cultures, and are seen in the movies "The Mummy" (1999) and "The Bible" (1966).
Religious books.
This species has been identified as one of the kosher species of locusts mentioned in Leviticus 11:22 by several rabbinical authorities among Middle Eastern Jewish communities.