However, under suitable conditions of drought followed by rapid vegetation growth, serotonin in their brains triggers dramatic changes: they start to breed abundantly, becoming gregarious and nomadic (loosely described as migratory) when their populations become dense enough.

More recently, changes in agricultural practices and better surveillance of locust breeding grounds have allowed control measures at an early stage.

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.

[5] 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.

[6] 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.).

[12] When desert locusts meet, their nervous systems release serotonin, which causes them to become mutually attracted, a prerequisite for swarming.

[16] 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.

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 a longer nymphal period; 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.

[19] The desert locust (Schistocerca gregaria) is probably the best known species owing to its wide distribution (North Africa, Middle East, and Indian subcontinent)[19] and its ability to migrate over long distances.

A major infestation covered much of western Africa from 2003 to 2005, after unusually heavy rain set up favourable ecological conditions for swarming.

[27] 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.

[32][33] The fossilized wing of an indeterminate locust has been found in Early Oligocene-aged sediments of the Pabdeh Formation in Iran, which were deposited in a deep marine environment.

The locust was likely migrating across the early Paratethys Sea, between the emergent Arabian Peninsula and central Iran, which were still separated by large areas of deep ocean at this time.

Other species of locusts caused havoc in North and South America, Asia, and Australasia; in China, 173 outbreaks over 1924 years.

[38] 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.

Albert's swarm of 1875 was estimated to contain 12.5 trillion insects covering an area of 198,000 square miles (510,000 km2) (larger than the state of California) and to weigh 27.5 million tons.

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,[43] destroying the underground eggs of the locust.

[50] 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.

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.

[17] 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.

[17] In years when locust control is needed, the hoppers are targeted early by applying water-based contact pesticides from tractor-based sprayers.

[56] 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.

[51] Other modern technologies for planning locust control include GPS, GIS tools, and satellite imagery with rapid computer data management and analysis.

[59] Dried fungal spores of a Metarhizium acridum sprayed in breeding areas pierce the locust exoskeleton on germination and invade the body cavity, causing death.

[65] At Tel Aviv University, scientists have been using the antennae's acute sensitivity of Sense of smell to detect different odors in various technologies.

Several cultures throughout the world consume insects, and locusts are considered a delicacy in many African, Middle Eastern, and Asian countries.

[68] The Bible records that John the Baptist ate locusts and wild honey (Greek: ἀκρίδες καὶ μέλι ἄγριον, romanized: akrídes kaì méli ágrion) while living in the wilderness.

[70][71] The Torah prohibits the use of most insects as food, but it permits consuming certain types of locust; specifically, those that are red, yellow, or spotted grey.

[80] 19th century European travellers observed Arabs in Arabia, Egypt, and Morocco selling, cooking, and eating locusts.

[86][87][88] Locusts yield about five times more edible protein per unit of fodder than cattle, and produce lower levels of greenhouse gases in the process.