Migratory locust

Migratory locust
Female migratory locust
Conservation status
Least Concern  (IUCN 3.1)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Orthoptera
Suborder:	Caelifera
Family:	Acrididae
Subfamily:	Oedipodinae
Tribe:	Locustini
Genus:	Locusta Linnaeus, 1758
Species:	L. migratoria
Binomial name
Locusta migratoria (Linnaeus, 1758)
Synonyms
Acridium migratorium Acridium plorans Pachytylus australis (Saussure, 1884) Pachytylus migratorius (Linnaeus, 1758) Pachytylus migratorioides (Fairmaire & L.J. Reiche, 1849)

Close-Up of a Locusta migratoria

The migratory locust, Locusta migratoria (Linnaeus, 1758), is the only species in the genus Locusta. It belongs to the locusts–species of grasshoppers with a phasis polymorphism–just like the desert locust or the red locust. The migratory locust can form dense, mobile swarms of adults and immense bands of hoppers. It is the most widely distributed species of locust throughout the Old World, from sea level to over 4,000 m in the mountains of Central Asia. The migratory locust is a major pest in many regions–mainly in the tropics–and regarded as the most important agricultural pest in some countries (Lecoq, 2023).

At 6.5 Gbp, the migratory locust possesses one of the largest known insect genomes.

Migratory locusts (mating of solitary-phase individuals) (green morph)

Taxonomy

According to the Orthoptera Species File two subspecies are currently recognised: Locusta migratoria migratoria (Linnaeus, 1758) and Locusta migratoria migratorioides (Reiche & Fairmaire, 1849).

In the past, numerous subspecies (up to twenty, see below) were described, some of which were never named but simply referred to by their geographical origin. Since the 1980s, various authors have questioned these distinctions and demonstrated that simple morphometry, distribution, number of generations, etc. are unsuitable criteria for identifying the subspecific status of a given population of migratory locusts. Recent molecular biology analyses (in particular Ma et al., 2012 based on a large geographical sample) have clearly divided Locusta migratoria into just two subspecies, L. m. migratoria (in temperate zones) and L. m. migratorioides (pantropical).

Distribution area of Locusta migratoria with indication of the approximate boundary between the two subspecies, L. m. migratoria and L. m. migratorioides. Based on Ma et al (2012) modified from Latchininsky et al (2002), Latchininsky and Sivanpillai (2010) and GBIF (2022) for the northern limit. The main subspecies recognised until the work of Ma et al. (2012) are mentioned in small type. The red circles indicate the location of the main outbreak areas of the migratory locust. The boundary between the two subspecies is approximate (after Lecoq, 2023).

Distribution

The migratory locust is the most widely distributed species of locust. It is found throughout the Old World, from sea level to altitudes of over 4,000 m in the mountains of Central Asia. The two subspecies cover distinct climatic regions, with geographical barriers and temperature playing an important role. The subspecies Locusta m. migratoria is distributed throughout the temperate zones of the Eurasian continent: its northern limit coincides roughly with the southern limit of the coniferous forests of Europe and Asia. L. m. migratorioides is widespread in tropical areas of Africa, southern Europe, Arabia, India, southern China, south-east Asia and Australia.

Habitats

Migratory locusts are mainly graminivorous. In temperate zones, subspecies L. m. migratoria is generally associated with flood plains and river deltas (the Yangtze and Yellow rivers in China, the Volga, Kuma and Terek deltas in southern Russia, the Amu-Dar'ya and Syr-Dar'ya deltas in Kazakhstan, and the Danube delta in Romania). In the tropics, the L. m. migratorioides subspecies is also often associated with flood plains (inland Niger delta in Mali, Lake Chad basin, Okavango delta in Botswana). In other regions, such as Madagascar, the habitats are grassy savannahs that become favourable during the warm season under the effect of the monsoon rains.

Polyphenism

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 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 nymphs have a yellow to orange covering with black spots; solitary nymphs 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.

Biology [1]

In tropical areas, the subspecies L. m. migratorioides develops continuously, with 4-5 generations per year during the solitary phase and only 3 generations during the gregarious phase. The eggs are laid in soil, preferably moist soil. Light sandy soils are the most favourable for egg-laying. During the warm season, embryonic development lasts around 10 days (up to 50 depending on temperature or drought conditions). There are five nymphal instars (rarely 6 or even 7), and nymphal development is completed in around 3 weeks (slightly longer in gregarious populations, depending on temperature). Young solitary adults undertake nocturnal migrations to find areas where conditions are favourable for reproduction. When environmental conditions are optimal, sexual maturation is rapid and the first eggs are laid around 15 days after the last nymphal moult. In the wild, each female lays just 1 to 3 egg pods (6 being a maximum rarely reached in the wild). These egg pods are laid at intervals of 5 to 30 days, depending on the temperature. The number of eggs per female varies according to the phase: 66-116 in solitary females, 39-66 in gregarious females. The entire life cycle in the tropics (from egg to first egg laying) takes around 2 months. Adults live up to 3 months.  

In temperate zones, the subspecies L. m. migratoria completes one generation per year and undergoes embryonic diapause. In populations between temperate and tropical zones, diapausing and non-diapausing eggs may be present in the same egg pod. Egg-laying takes place in August-September, in light sandy soil. The eggs spend the winter in diapause. There are at least 1 to 3 egg pods per female (2-3 in the southern parts of the range and only 1 in the northern parts). Each egg pod contains 60-80 eggs (extremes 40-120). The incubation period of the eggs depends on the latitude and the year, in particular the extent of flooding. On average, the eggs remain in the ground for around 9 months before hatching the following spring. They hatch between early May and early June (late April to mid-July depending on the area and temperature). There are 5 nymphal instars, which take an average of 40 to 45 days to develop in the north of the range, and 35 to 40 days in the south. The adults appear from June to early July, sometimes remaining until November in the warmer regions. Mating begins 2 to 4 weeks after the emergence of the imagos, and the females start laying eggs 2 to 3 weeks later (generally at the end of July). Depending on the length and severity of the winter, a second generation may appear, but generally all the nymphs of this second generation die by late autumn. The longevity of adults is around 90 days in the northern part of the species' range and 120-150 days in the southern parts. Populations of L. m. migratoria are considered to be relatively sedentary. Most migratory flights are local and egg-laying takes place in the same area, but occasionally, depending on weather conditions, massive movements of swarms can occur, which can spread over hundreds of kilometres into surrounding territories. In the past, the distances covered in a single generation by L. m. migratoria swarms in temperate Europe - from the hatching area in the Danube delta (an important gregarisation area) - reached 2,500 km and were comparable to those covered by L. m. migratorioides in tropical Africa.

Neurochemistry

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

Economic impact and management

Adult female (top), adult male (bottom left), fifth instar nymph (bottom right)

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. 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 best way to control migratory locust swarms is to try to prevent them from developing in the first place. Monitoring known outbreaks areas is of vital importance if early control measures, such as targeted chemical control, are to be used to keep the population below the outbreak threshold. Swarms of migratory locusts can travel hundreds of kilometres in just a few days, which often makes them an international challenge. Therefore, where necessary, early warning systems and control strategies must involve close cooperation between the countries concerned.

At present, the management of migratory locusts still relies on chemical pesticides, with ultra-low volume (ULV) spraying being the main method of application. Despite the many natural enemies of the migratory locust, only two have been developed as biological control agents: the microsporidian protozoan Nosema locustae and the deuteromycete fungus Metarhizium acridum. They are already widely used in some countries, such as China, and applied in the field like traditional pesticides. In addition, the Locust Pesticide Referree Group (LPRG), set up by the FAO, issues recommendations on pesticides effective against locusts and the doses to be used either as a barrier treatment or for total coverage applications, for the migratory locust and other locust species.  It also takes account of their environmental impact and the risks to non-target wildlife. Guides to good practice have also been published by the FAO, and are available online, to help organise locust control campaigns and reduce the risks to humans and the environment.

Overall, the migratory locust now seems to be better controlled than in the past, but major outbreaks continue to occur regularly in various parts of the Old World, despite numerous prevention and control operations.

As food

The migratory locust is an edible insect. It is part of the local diet in many countries in its geographical range, mainly in Africa and Asia. 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 ground 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.

Extension de l'invasion du Criquet migrateur qui a débuté en 1928 dans le delta central du fleuve Niger au Mali (zone en noir). Cette invasion n'a pris fin qu'en 1941 (d'après Batten 1966, in Duranton et al. 1982).

Subspecies of Locusta migratoria [2]

L. m. migratorioides female (solitary)

L. m. migratorioides male (solitary)

L. migratoria is found in a vast geographical area and its range covers many different ecological zones. For this reason, many subspecies–as many as twenty–are now grouped into just two subspecies: L. m. migratorioides and L. m. migratoria (see ‘Taxonomy’ above). The names of the former subspecies are still sometimes used in various parts of the world (thus maintaining some confusion), but correspond more to a facility for specifying a geographical origin than to a biological reality. An overview is given here below:

L. m. migratoria (Linnaeus, 1758) – Synonyms: cinerascens (Fabricius, 1781); australis (Saussure, 1884); brasiliensis (Walker, 1870); danica (Linnaeus, 1767); gallica Remaudière, 1947; remaudierei Harz, 1962; rossica Uvarov & Zolotarevsky, 1929; solitaria Carthy, 1955.

L. m. migratorioides (Reiche & Fairmaire, 1849) – Synonyms: affinis (Sjöstedt, 1931); burmana Ramme, 1951; capito (Saussure, 1884); manilensis (Meyen, 1835); morio (Sjöstedt, 1931); punctifrons (Dirsh, 1961); tibetensis Chen, 1963; arabian spp. (COPR, 1982); indian spp. (COPR, 1982); australian spp. (COPR, 1982).

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, Pantanga 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.

Photos (more photos at [3])

• 
  First instar nymph (gregarious)
• 
  Second and fourth instar nymphs (gregarious)
• 
  Third instar nymphs (gregarious)
• 
  Fourth instar nymph (gregarious)
• 
  Part of a hopper band in Kazakhstan
• 
  Hopper band in Kazakhstan More photos on the migratory locust.

See also

• 2013 Madagascar locust infestation
• Australian Plague Locust Commission (APLC)

References

• Lecoq, Michel (2023). Locusta migratoria (Migratory locust). In: Crop Protection Compendium. Wallingford, UK: CAB International. DOI:10.1079/cabicompendium.31151
• Steedman, Alison, ed. (1988). Locust Handbook (2nd ed.). London: Overseas Development Natural Resources Institute. ISBN 978-0-85954-232-6.
• Walker, Annette; Heath, Eric (2000). The Reed Handbook of Common New Zealand Insects. Auckland: Reed. ISBN 978-0-7900-0718-2.

External links

• Food and Agriculture Organization (FAO) Archived 2004-10-30 at the Wayback Machine
• The phenomenon of phases
• Biolib
• Fauna Europaea
• Genus Locusta at Orthoptera Species File on Line
• Sound recordings of Migratory Locust at BioAcoustica