|Female migratory locust|
The migratory locust (Locusta migratoria) is the most widespread locust species, and the only species in the genus Locusta. It occurs throughout Africa, Asia, Australia and New Zealand. It used to be common in Europe but has now become rare there. Because of the vast geographic area it occupies, which comprises many different ecological zones, numerous subspecies have been described. However, not all experts agree on the validity of some of these subspecies.
The migratory locust is polyphenic. It transitions between two main phenotypes in response to population density; the solitary phase and the gregarious phase. As the density of the population increases the locust transforms progressively from the solitary phase towards the gregarious phase with intermediate phases:
- Solitaire = solitary phase → transiens congregans (intermediate form) → gregarious phase → transiens[check spelling] dissocians (intermediate form) → solitaire = solitary phase.
Pigmentation and size of the migratory locust vary according to its phase (gregarious or solitary form) and its age. Gregarious larvae have a yellow to orange covering with black spots; solitary larvae are green or brown. The gregarious adult is brownish with yellow, the latter colour becoming more intense and extensive on maturation. The solitary adult is brown with varying extent of green colour depending on the colour of the vegetation. Gregarious adults vary in size between 40 and 60 mm according to the sex; they are smaller than the solitary adults.
The phase transition may be mediated by DNA methylation in the brain. Expression of the DNA methyltrasferase gene Dnmt3 is high in the brain of the gregarious form, decreases in gregarious locusts when they are isolated, and increases in solitary locusts when they are crowded. Knock-down reduces phase-related locomotor activity. Transcriptionally, Dnmt3 is linked with phase-core transcriptional factor, hormone receptor HR3.
Increased extracellular K+ was found to cause membrane depolarization in muscle activating nerves by Hoyle 1953. This then in turn reduces the nerve potential, with the final result of reducing the force output of said muscle. He also found chronic cold temperatures to increase K+ in the haemolymph. These changes affect L. migratoria's nerve states because – as with insects and animals in general – nerve cells have a high K permeability, which allows K+'s transmembrane distribution to determine most cellular diffusion potential. This is shaping and will continue to shape the distribution of L. migratoria's range under climate change.
Relationship with humans
Locusts are highly mobile, and usually fly with the wind at a speed of about 15 to 20 kilometres per hour (9.3 to 12.4 mph). Swarms can travel 5 to 130 km or more in a day. Locust swarms can vary from less than one square kilometre to several hundred square kilometres with 40 to 80 million individuals per square kilometre. An adult locust can consume its own weight (several grams) in fresh food per day. For every million locusts, one ton of food is eaten.
In Africa, the last serious widespread plague of L. m. migratorioides occurred from 1928 to 1942. Since then, environmental transformations have made the development of swarms from the African migratory locust unlikely. Nevertheless, potential outbreaks are constantly monitored as plagues can be devastating. The Malagasy migratory locust (L. m. capito) still regularly swarms (roughly twice every ten years). The desert locust, which is very similar to the African migratory locust, remains a major threat too.
Locust survey and control are primarily the responsibility of the Ministry of Agriculture in locust-affected countries and are operations undertaken by national locust units. The Food and Agriculture Organization (FAO) of the United Nations provides information on the general locust situation to all interested countries and gives warnings and forecasts to those countries in danger of invasion.
The migratory locust is an edible insect. In Europe, the migratory locust is officially approved for the use in food in Switzerland (since May 2017). On 2 July 2021, the European Food Safety Agency published a scientific opinion stating that the consumption of migratory locust in frozen, dried or grounded state is safe for humans. On 12 November 2021, the EU member states gave their green light for the EU Commission to authorize the placing on the market of migratory locust as a food. This is one of the final steps in the novel food authorization procedure. As a next step, the Commission will now adopt a legal act.
Subspecies of Locusta migratoria
L. migratoria is found over a vast geographic area, and its range covers many different ecological zones. Because of this, numerous subspecies have been described; however, not all experts agree on the validity of some of these subspecies.
- L. m. burmana Ramme, 1951
- L. m. capito Saussure, 1884 (Malagasy migratory locust: Madagascar)
- L. m. cinerascens Fabricius, 1781 (Italy, Spain)
- L. m. manilensis (Meyen, 1835) 1 (eastern Asia)
- L. m. migratoria (Linnaeus, 1758) (Eurasian migratory locust: West and Central Asia, eastern Europe)
- L. m. migratorioides (Fairmaire & L.J. Reiche, 1849) (African migratory locust: Africa and Atlantic islands)
- L. m. tibetensis Chen, Yonglin, 1963
- L. m. danica (Linnaeus, 1767) = L. m. migratoria (Linnaeus, 1758)
- L. m. gallica Remaudičre, 1947 = L. m. migratoria (Linnaeus, 1758)
- L. m. solitaria Carthy, 1955 = L. m. migratoria (Linnaeus, 1758)
Other species called 'locusts'
Other species of Orthoptera that display gregarious and migratory behaviour are called locusts. This includes:
- American locust, Schistocerca americana
- Australian plague locust, Chortoicetes terminifera
- Bombay locust, Nomadacris succincta
- Brown locust, Locustana pardalina
- Desert locust, Schistocerca gregaria
- Egyptian locust, Anacridium aegyptium
- Italian locust, Calliptamus italicus
- Moroccan locust, Dociostaurus maroccanus
- Red locust, Nomadacris septemfasciata
- Rocky Mountain locust, Melanoplus spretus – extinct
- Sahelian tree locusts, Anacridium melanorhodon
- Spur-throated locust, Austracris guttulosa (note: "spur-throated grasshoppers/locusts" may also refer to spp. in other genera)
- Sudan plague locust, Aiolopus simulatrix
The Senegalese grasshopper (Oedaleus senegalensis) also often displays locust-like behaviour in the Sahel region.
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