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Desert.locust.arp
Desert locust Schistocerca gregaria
SGR laying
Desert locust (Schistocerca gregaria) ovipositing in sand
Copulating desert locust pair
Desert locusts in copulation

Locust are the swarming phase of certain species of short-horned grasshoppers in the family Acrididae. Such swarms are usually made of a great number of locusts. They do a lot of damage to the places where they pass, by eating the crops.

These are species which breed rapidly under suitable conditions. The migratory locust is the best-known example. They become gregarious and migratory when their populations become dense enough. They form bands as nymphs and swarms as adults. Both the bands and the swarms are nomadic and rapidly strip fields and greatly damage crops. The adults are powerful fliers; they can travel great distances, consuming practically all green material wherever the swarm settles.

The origin and apparent extinction of certain species of locust—some of which grew to 6 inches (15 cm) in length—are unclear.

Locusts are edible insects, and are considered a delicacy in some countries. There have been references to their consumption as food throughout history.

In the Book of Joel in the Old Testament locust swarms are described as a plague.

Swarming grasshoppers

SGR laying
A desert locust ovipositing in sand

Locusts are the swarming phase of certain species of short-horned grasshoppers in the family Acrididae. These insects are usually solitary, but under certain circumstances become more abundant and change their behaviour and habits, becoming gregarious.

Copulating desert locust pair
Desert locusts in copulation

No taxonomic distinction is made between locust and grasshopper species; the basis for the definition is whether a species forms swarms under intermittently suitable conditions. In English, the term "locust" is used for grasshopper species that change morphologically and behaviourally on crowding, forming swarms that develop from bands of immature stages called hoppers.

These changes are examples of phase polymorphism; they were first analysed and described by Boris Uvarov, who was instrumental in setting up the Anti-Locust Research Centre. He made his discoveries during his studies of the desert locust, whose solitary and gregarious phases had previously been thought to be separate species. He designated the two phases as solitaria and gregaria. These are also referred to as statary and migratory morphs, though strictly speaking, their swarms are nomadic rather than migratory. Charles Valentine Riley and Norman Criddle were also involved in achieving the understanding and control of locusts.

Swarming behaviour is a response to overcrowding. Increased tactile stimulation of the hind legs causes an increase in levels of serotonin. This causes the locust to change colour, eat much more, and breed much more easily. The transformation of the locust to the swarming form is induced by several contacts per minute over a four-hour period. A large swarm can consist of billions of locusts spread out over an area of thousands of square kilometres, with a population of up to 80 million individuals per square kilometre (200 million per square mile). When desert locusts meet, their nervous systems release serotonin, which causes them to become mutually attracted, a prerequisite for swarming.

The initial bands of gregarious hoppers are known as "outbreaks", and when these join together into larger groups, the event is known as an "upsurge". Continuing agglomerations of upsurges on a regional level originating from a number of entirely separate breeding locations are known as "plagues". During outbreaks and the early stages of upsurges, only part of the locust population becomes gregarious, with scattered bands of hoppers spread out over a large area. As time goes by, the insects become more cohesive and the bands become concentrated in a smaller area. In the desert locust plague in Africa, the Middle East, and Asia that lasted from 1966 to 1969, the number of locusts increased from two to 30 billion over two generations, but the area covered decreased from over 100,000 square kilometres (39,000 sq mi) to 5,000 square kilometres (1,900 sq mi).

Solitary and gregarious phases

DesertLocust
Solitaria (grasshopper) and gregaria (swarming) phases of the desert locust

One of the greatest differences between the solitary and gregarious phases is behavioural. The gregaria nymphs are attracted to each other, this being seen as early as the second instar. They soon form bands of many thousands of individuals. These groups behave like cohesive units and move across the landscape, mostly downhill, but making their way around barriers and merging with other bands. The attraction between the insects seems to be largely visual, but also involves olfactory cues, and the band seem to navigate using the sun. They pause to feed at intervals before resuming their march, and may cover tens of kilometers over a few weeks.

Also, differences in morphology and development are seen. In the desert locust and the migratory locust, for example, the gregaria nymphs become darker with strongly contrasting yellow and black markings, they grow larger, and have longer developmental periods. The adults are larger with different body proportions, less sexual dimorphism, and higher metabolic rates. They mature more rapidly and start reproducing earlier, but have lower levels of fecundity.

The mutual attraction between individual insects continues into adulthood, and they continue to act as a cohesive group. Individuals that get detached from a swarm fly back into the mass. Others that get left behind after feeding, take off to rejoin the swarm when it passes overhead. When individuals at the front of the swarm settle to feed, others fly past overhead and settle in their turn, the whole swarm acting like a rolling unit with an ever-changing leading edge. The locusts spend much time on the ground feeding and resting, moving on when the vegetation is exhausted. They may then fly a considerable distance before settling in a location where transitory rainfall has caused a green flush of new growth.

Distribution and diversity

Several species of grasshoppers swarm as locusts in different parts of the world, on all continents except Antarctica and North America: For example, the Australian plague locust (Chortoicetes terminifera) swarms across Australia.

The desert locust (Schistocerca gregaria) is probably the best-known species owing to its wide distribution (North Africa, Middle East, and Indian subcontinent) and its ability to migrate over long distances. A major infestation covered much of western Africa in 2003-4, after unusually heavy rain set up favourable ecological conditions for swarming. The first outbreaks occurred in Mauritania, Mali, Niger, and Sudan in the autumn of 2003. The rain allowed swarms to develop and move north to Morocco and Algeria, threatening croplands. Swarms crossed Africa, appearing in Egypt, Jordan and Israel, the first time in those countries for 50 years. The cost of handling the infestation was put at US$122 million, and the damage to crops at up to $2.5 billion.

The migratory locust (Locusta migratoria), sometimes classified into up to 10 subspecies, swarms in Africa, Asia, Australia, and New Zealand, but has become rare in Europe. In 2013, the Madagascan form of the migratory locust formed many swarms of over a billion insects, reaching "plague" status and covering about half the country by March 2013. Species such as the Senegalese grasshopper (Oedaleus senegalensis) and the African rice grasshopper (Hieroglyphus daganensis), both from the Sahel, often display locust-like behaviour and change morphologically on crowding.

North America is currently the only continent besides Antarctica without a native locust species. The Rocky Mountain locust was formerly one of the most significant insect pests there, but it became extinct in 1902. In the 1930s, during the Dust Bowl, a second species of North American locust, the High Plains locust (Dissosteira longipennis) reached plague proportions in the American Midwest. Today, the High Plains locust is a rare species and no longer swarms, leaving North America with no swarming locusts.

Interaction with humans

Ancient times

Maler der Grabkammer des Horemhab 002
Locust detail from a hunt mural in the grave-chamber of Horemhab, Ancient Egypt, circa 1422–1411 BC

Study of literature shows how pervasive plagues of locusts were over the course of history. The insects arrived unexpectedly, often after a change of wind direction or weather, and the consequences were devastating. The Ancient Egyptians carved locusts on tombs in the period 2470 to 2220 BC, and a devastating plague is mentioned in the Book of Exodus in the Bible, as taking place in Egypt around 1300 BC. The Iliad mentions locusts taking to the wing to escape fire. Plagues of locusts are also mentioned in the Quran. In the ninth century BC, the Chinese authorities appointed anti-locust officers.

Aristotle studied locusts and their breeding habits and Livy recorded a devastating plague in Capua in 203 BC. He mentioned human epidemics following locust plagues which he associated with the stench from the putrifying corpses; the linking of human disease outbreaks to locust plagues was widespread. A pestilence in China in 311 AD that killed 98% of the population locally was blamed on locusts, and may have been caused by an increase in numbers of rats (and their fleas) that devoured the locust carcases.

More recent times

Diagrams of Locusts which swarmed over England in 1748
Locusts which swarmed over England in 1748: Drawing by De la Cour; engraved by R. White, in Thomas Pennant's A Tour in Wales, 1781

During the last two millennia, locust plagues continued to appear at irregular intervals with the main recorded outbreaks of the desert and migratory locusts occurring in Africa, the Middle East, and Europe. Other species of locusts caused havoc in North and South America, Asia, and Australasia; 173 outbreaks over a period of 1924 years have been recorded in China. The Bombay locust (Nomadacris succincta) was a major pest in India and southeastern Asia in the 18th and 19th centuries, but has seldom swarmed since the last plague in 1908.

The extinction of the Rocky Mountain locust has been a source of puzzlement. It had swarmed throughout the west of the United States and parts of Canada in the 19th century. Albert's swarm of 1875 was estimated to cover 198,000 square miles (510,000 km2) (greater than the area of California) and to weigh 27.5 million tons, with some 12.5 trillion insects. The last specimen was seen alive in Canada in 1902. Recent research suggests the breeding grounds of this insect in the valleys of the Rocky Mountains came under sustained agricultural development during the large influx of gold miners, destroying the underground eggs of the locust.

Control

Flaming Locusts in 1915
Preparing to flame locusts in Palestine, 1915

Historically, people could do little to protect their crops from being devastated by locusts, although eating the insects may have been some consolation. By the early 20th century, efforts were being made to disrupt the development of the insects by cultivating the soil where eggs were laid, collecting hoppers with catching machines, killing them with flamethrowers, trapping them in ditches, and crushing them with rollers and other mechanical methods. By the 1950s, the organochloride dieldrin was found to be an extremely effective insecticide, but it was later banned from use in most countries because of its persistence in the environment and its bioaccumulation in the food chain.

Cessna spraying red locusts in Iku Katavi NP
Cessna of the International Red Locust Control Organization spraying red locusts in Iku Katavi National Park, Tanzania, 2009

In years when locust control is needed, the hoppers are targeted in spring by applying water-based, contact pesticides using tractor-based sprayers. This is effective but slow and labour-intensive, and where possible, spraying concentrated insecticide solutions from aircraft over the insects or the vegetation on which they feed is preferable. The use of ultralow-volume spraying of contact pesticides from aircraft in overlapping swathes is effective against nomadic bands and can be used to treat large areas of land swiftly. Other modern technologies used for planning locust control include GPS, GIS tools, and satellite imagery, and computers provide rapid data management and analysis.

CSIRO ScienceImage 1367 Locusts attacked by the fungus Metarhizium
Locusts killed by the naturally occurring fungus Metarhizium, an environmentally friendly means of biological control

A biological pesticide to control locusts was tested across Africa by a multinational team in 1997. Dried fungal spores of a Metarhizium acridum sprayed in breeding areas pierce the locust exoskeleton on germination and invade the body cavity, causing death. The fungus is passed from insect to insect and persists in the area, making repeated treatments unnecessary. This approach to locust control was used in Tanzania in 2009 to treat around 10,000 hectares in the Iku-Katavi National Park infested with adult locusts. The outbreak was contained and the elephants, hippopotamuses, and giraffes present in the area were unharmed.

The ultimate goal in locust control is the use of preventive and proactive methods that disrupt the environment to the least possible extent. This would make agricultural production easier and more secure in the many regions where growing crops is of vital importance to the survival of the local people.

As food

Skewered locusts
Skewered locusts in Beijing, China

Several cultures throughout the world consume insects, and locusts are considered a delicacy and eaten in many African, Middle Eastern, and Asian countries. They have been used as food throughout history.

They can be cooked in many ways, but are often fried, smoked, or dried. The Bible records that John the Baptist ate locusts and wild honey (Greek: ἀκρίδες καὶ μέλι ἄγριον, akrides kai meli agrion) while living in the wilderness; attempts have been made to explain the locusts as suitably ascetic vegetarian food such as carob beans, but the plain meaning of ἀκρίδες is the insects.

The Torah, although disallowing the use of most insects as food, permits the consumption of certain locusts; specifically, the red, the yellow, the spotted grey, and the white are considered permissible. In Islamic jurisprudence, eating locusts is considered halal. The Prophet Muhammad was reported to have eaten locusts during a military raid with his companions.

Locusts are eaten in the Arabian Peninsula, including Saudi Arabia, where consumption of locusts spiked around Ramadan especially in the Al-Qassim Region in 2014, since many Saudis believe they are healthy to eat; however, the Saudi Ministry of Health warned that pesticides they used against the locusts made them unsafe; and Kuwait. Yemenis also consume locusts, and expressed discontent over governmental plans to use pesticides to control them. ʻAbd al-Salâm Shabînî described a locust recipe from Morocco. 19th century European travellers observed Arabs in Arabia, Egypt, and Morocco selling, cooking, and eating locusts. They reported that in Egypt and Palestine locusts were consumed. They reported that in Palestine, around the River Jordan, in Egypt, in Arabia, and in Morocco that Arabs ate locusts, while Syrian peasants did not eat locusts.

However, in the Haouran region, Fellahs who were in poverty and suffered from famine ate locusts after removing the guts and head, while locusts were swallowed wholesale by Bedouins. Syrians, Copts, Greeks, Armenians, and other Christians and Arabs themselves reported that in Arabia locusts were eaten frequently and one Arab described to a European traveler the different types of locusts which were favored as food by Arabs. Persians use the Anti-Arab racial slur "Arabe malakh-khor" (Persian: عرب ملخ خور, literally Arab locust eater) against Arabs.

Locusts yield about five times more edible protein per unit of fodder than cattle, and produce lower levels of greenhouse gases in the process. The feed conversion rate of orthopterans is 1.7 kg/kg, while for beef it is typically about 10 kg/kg. The protein content in fresh weight is between 13–28 g/100g for adult locust, 14–18 g/100g for larvae, as compared to 19–26 g/100g for beef. The calculated protein efficiency ratio is however low, with 1.69 for locust protein compared to 2.5 for standard casein. A serving of 100 g of desert locust provides 11.5 g of fat, 53.5% of which is unsaturated, and 286 mg of cholesterol. Among the fatty acids, palmitoleic, oleic and linolenic acids were found to be the most abundant. Varying amounts of potassium, sodium, phosphorus, calcium, magnesium, iron, and zinc were present.

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