Editing Locust (section)

== Swarming grasshoppers ==

{{main|Grasshopper|Swarm behaviour|Predator satiation}}

{{external media |width = 210px |float = right |headerimage=  |video1 = [https://knowablemagazine.org/article/living-world/2020/locusts-and-grasshoppers-things-know "Locusts and Grasshoppers - Things to Know"], ''[[Knowable Magazine]]'', 2020.}}

Locusts are the [[Swarm behaviour|swarming]] phase of certain species of short-horned [[grasshopper]]s in the family [[Acrididae]]. These insects are usually solitary, but under certain circumstances become more abundant and change their behaviour and habits, becoming [[Social animal|gregarious]].<ref name="CurrentBio">{{cite journal|author1=Simpson, Stephen J. |author2= Sword, Gregory A. |title=Locusts|journal=Current Biology|volume=18|issue=9|pages=R364–R366|doi=10.1016/j.cub.2008.02.029|pmid=18460311|year=2008|doi-access=free|bibcode= 2008CBio...18.R364S }}{{Open access}}</ref><ref>{{cite web |url=http://www.fao.org/ag/locusts/en/info/info/faq/ |title=Frequently Asked Questions (FAQs) about locusts |work=Locust watch |publisher=FAO |access-date=1 April 2015}}</ref><ref>{{cite web |url=http://www.animalcorner.co.uk/insects/grasshoppers/grasshopper_about.html |title=Grasshoppers |work=Animal Corner |access-date=1 April 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150408004426/http://www.animalcorner.co.uk/insects/grasshoppers/grasshopper_about.html |archive-date=8 April 2015 }}</ref>

[[File:Copulating desert locust pair.jpg|thumb|left|Desert locusts in copulation]]

No [[Taxonomy (biology)|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 [[morphology (biology)|morphologically]] and behaviourally on crowding, forming swarms that develop from bands of immature stages called hoppers. The change is described as density-dependent [[phenotypic plasticity]].<ref name="Pener Simpson 2009">{{cite book |last1=Pener |first1=Meir Paul |last2=Simpson |first2=Stephen J. |volume=36 |isbn=9780123814289 |series=Advances in Insect Physiology |title=Locust Phase Polyphenism: An Update |date=14 October 2009 |publication-date=23 September 2009 |edition=1st |page=9 |publisher=Academic Press}}></ref>

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]].<ref>{{cite journal |last=Baron |first=Stanley |title=The Desert Locust |journal=[[New Scientist]] |url=https://books.google.com/books?id=yhMXtivOrZ4C&pg=PA156 |date=1972 |page=156}}</ref> 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''.<ref name=Dingle1996>{{cite book |last=Dingle |first=Hugh |title=Migration: The Biology of Life on the Move |url=https://books.google.com/books?id=qzzJoVfgg0QC&pg=PA273 |year=1996 |publisher=Oxford University Press |isbn=978-0-19-535827-8 |pages=273–274}}</ref> These are called [[statary]] and migratory [[Polymorphism (biology)|morph]]s, though strictly speaking, their swarms are [[nomad]]ic rather than [[Animal migration|migratory]]. [[Charles Valentine Riley]] and [[Norman Criddle]] were involved in achieving the understanding and control of locusts.<ref>[[Wikisource:The Encyclopedia Americana (1920)/Riley, Charles Valentine]]</ref><ref>{{cite web |last=Holliday |first=N. J. |url=http://www.mhs.mb.ca/docs/mb_history/51/criddle_n.shtml |title=Norman Criddle: Pioneer Entomologist of the Prairies |date=1 February 2006 |work=Manitoba History |publisher=Manitoba Historical Society |access-date=16 April 2015}}</ref>

Swarming behaviour is a response to overcrowding. Increased tactile stimulation of the hind legs causes an increase in levels of serotonin.<ref>{{cite news |url=http://news.bbc.co.uk/2/hi/science/nature/7858996.stm |work=BBC News |last=Morgan |first=James |title=Locust swarms 'high' on serotonin |date=29 January 2009 |access-date=4 March 2014 |url-status=live |archive-url=https://web.archive.org/web/20131010043157/http://news.bbc.co.uk/2/hi/science/nature/7858996.stm |archive-date=10 October 2013}}</ref> 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.<ref>{{cite journal |year=2003 |title=Mechanosensory-induced behavioral gregarization in the desert locust ''Schistocerca gregaria'' |journal=[[Journal of Experimental Biology]] |volume=206 |issue=22 |pages=3991–4002 |doi=10.1242/jeb.00648 |pmid=14555739 |last1=Rogers |first1=S. M. |last2=Matheson |first2=T. |last3=Despland |first3=E. |last4=Dodgson |first4=T. |last5=Burrows |first5=M. |last6=Simpson |first6=S. J. |s2cid=10665260 |doi-access= |bibcode=2003JExpB.206.3991R }}{{Open access}}</ref> 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).<ref name=Showler/> When desert locusts meet, their nervous systems release serotonin, which causes them to become mutually attracted, a prerequisite for swarming.<ref>{{cite journal |last=Stevenson |first=P. A. |date=2009 |title=The key to Pandora's box |journal=[[Science (journal)|Science]] |volume=323 |issue=5914 |pages=594–595 |pmid=19179520 |doi=10.1126/science.1169280|s2cid=39306643 }}</ref><ref>{{cite news |url=https://www.newscientist.com/article/dn16505-blocking-happiness-chemical-may-prevent-locust-plagues.html |title=Blocking 'happiness' chemical may prevent locust plagues |publisher=New Scientist |date=29 January 2009 |access-date=31 January 2009 |author=Callaway, Ewen }}</ref><ref name="Antsey Rogers Swidbert 2009">{{cite journal |last1=Antsey |first1=Michael |last2=Rogers |first2=Stephen |last3=Swidbert |first3=R.O. |last4=Burrows |first4=Malcolm |last5=Simpson |first5=S.J. |title=Serotonin mediates behavioral gregarization underlying swarm formation in desert locusts |journal=Science |date=30 January 2009 |volume=323|issue=5914 |pages=627–630 |doi=10.1126/science.1165939 |pmid=19179529 |bibcode=2009Sci...323..627A |s2cid=5448884 }}</ref>

The formation of initial bands of gregarious hoppers is called an "outbreak"; when these join 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".<ref name=Showler2013>{{cite web |last=Showler |first=Allan T. |url=http://ipmworld.umn.edu/chapters/showler.htm |title=The Desert Locust in Africa and Western Asia: Complexities of War, Politics, Perilous Terrain, and Development |date=4 March 2013 |work=Radcliffe's IPM World Textbook |publisher=University of Minnesota |access-date=3 April 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150408034918/http://ipmworld.umn.edu/chapters/showler.htm |archive-date=8 April 2015 }}</ref> 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 {{convert|100000|km2}} to {{convert|5000|km2}}.<ref name=Krall453/>

=== Solitary and gregarious phases ===

[[File:DesertLocust.jpeg|thumb|''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 involves visual and [[Olfaction|olfactory]] cues.<ref>{{cite journal |last1=Guo |first1=Xiaojiao |last2=Yu |first2=Qiaoqiao |last3=Chen |first3=Dafeng |last4=Wei |first4=Jianing |last5=Yang |first5=Pengcheng |last6=Yu |first6=Jia |last7=Wang |first7=Xianhui |last8=Kang |first8=Le |doi=10.1038/s41586-020-2610-4 |title=4-Vinylanisole is an aggregation pheromone in locusts |year=2020 |journal=Nature |volume=584 |issue=7822 |pages=584–588 |pmid=32788724 |bibcode=2020Natur.584..584G |s2cid=221106319}}</ref> The bands seem to navigate using the sun. They pause to feed at intervals before continuing on, and may cover tens of kilometres over a few weeks.<ref name=Dingle1996/>

Locusts in the gregarious phase differ in morphology and development. 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 [[Metabolism|metabolic rates]]; they mature more rapidly and start reproducing earlier, but have lower levels of [[fecundity]].<ref name=Dingle1996/>

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.<ref name=Dingle1996/>