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These changes are examples of phase polyphenism; they were first analysed and described by Boris Uvarov, who was instrumental in setting up the Anti-Locust Research Centre.[10] He made his discoveries during his studies of the migratory locust in the Caucasus, whose solitary and gregarious phases had previously been thought to be separate species (Locusta migratoria and L. danica L.). He designated the two phases as solitaria and gregaria.[11] 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.[12][13]



Swarming behaviour is a response to overcrowding. Increased tactile stimulation of the hind legs causes an increase in levels of serotonin.[14] 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.[15] 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 per square kilometre (200 million per square mile).[16] When desert locusts meet, their nervous systems release serotonin, which causes them to become mutually attracted, a prerequisite for swarming.[17][18]

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".[19] 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).[20]

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.[11]

Several species of grasshoppers swarm as locusts in different parts of the world, on all continents except Antarctica:[22][23][24][a] For example, the Australian plague locust (Chortoicetes terminifera) swarms across Australia.[22]

North America is currently the only sub-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, leaving North America with no regularly swarming locusts.

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. A devastating plague in Egypt is also mentioned in the Book of Exodus in the Bible.[20][34] The Iliad mentions locusts taking to the wing to escape fire.[35] Plagues of locusts are also mentioned in the Quran.[16] In the ninth century BC, the Chinese authorities appointed anti-locust officers.[36] In the New Testament, John the Baptist was said to survive in the wilderness on locusts and wild honey; and human-headed locusts appear in the Book of Revelation.[37]

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 the northwestern provinces of 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 carcasses.[36]

During the last two millennia, desert locust plagues have appeared sporadically in Africa, the Middle East, and Europe. Other species of locusts caused havoc in North and South America, Asia, and Australasia; in China, 173 outbreaks over 1924 years.[36] 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.[38]

In the spring of 1747 locusts arrived outside Damascus eating the majority of the crops and vegetation of the surrounding countryside. One local barber, Ahmad al-Budayri, recalled the locusts "came like a black cloud. They covered everything: the trees and the crops. May God Almighty save us!"[39]

Several organisations around the world monitor the threat from locusts. They provide forecasts detailing regions likely to suffer from locust plagues in the near future. In Australia, this service is provided by the Australian Plague Locust Commission.[44] It has been very successful in dealing with developing outbreaks, but has the great advantage of having a defined area to monitor and defend without locust invasions from elsewhere.[45] In Central and Southern Africa, the service is provided by the International Locust Control Organization for Central and Southern Africa.[46] In West and Northwest Africa, the service is co-ordinated by the Food and Agriculture Organization's Commission for Controlling the Desert Locust in the Western Region, and executed by locust control agencies belonging to each country concerned.[47] The FAO also monitors the situation in the Caucasus and Central Asia, where over 25 million hectares of cultivated land are under threat.[48] In February 2020, in an effort to end massive locust outbreaks, India decided to use drones and special equipment to monitor locusts and spray insecticides.[49]

Historically, people could do little to protect their crops from locusts, although eating the insects may have been some compensation. By the early 20th century, efforts were 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.[20] By the 1950s, the organochloride dieldrin was found to be an extremely effective insecticide, but it was later banned in most countries because of its persistence in the environment and its accumulation in the food chain.[20]

A biological pesticide to control locusts was tested across Africa by a multinational team in 1997.[54] Dried fungal spores of a Metarhizium acridum sprayed in breeding areas pierce the locust exoskeleton on germination and invade the body cavity, causing death.[55] The fungus is passed from insect to insect and persists in the area, making repeated treatments unnecessary.[56] 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 without harm to the local elephants, hippopotamuses, and giraffes.[46]

Locusts have been used as food throughout history. They are considered meat. Several cultures throughout the world consume insects, and locusts are considered a delicacy in many African, Middle Eastern, and Asian countries.[60]

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 permissible.[65][66] Islamic jurisprudence deems eating locusts to be halal.[67][66] The Prophet Muhammad was reported to have eaten locusts during a military raid with his companions.[68]

Locusts are eaten in the Arabian Peninsula, including Saudi Arabia.[69] In 2014, consumption of locusts spiked around Ramadan especially in the Al-Qassim Region, since many Saudis believe they are healthy to eat, but the Saudi Ministry of Health warned that pesticides made them unsafe.[70][71] Yemenis also consume locusts, and expressed discontent over governmental plans to use pesticides against them.[72] ʻAbd al-Salâm Shabînî described a locust recipe from Morocco.[73] 19th century European travellers observed Arabs in Arabia, Egypt, and Morocco selling, cooking, and eating locusts.[74] They reported that in Egypt and Palestine locusts were consumed.[75] 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.

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 whole by Bedouins.[76] 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.[77][78] Persians use the Anti-Arab racial slur Arabe malakh-khor (Persian: عرب ملخ خور, literally "Arab locust eater") against Arabs.[79][80][81] 041b061a72


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