"Long before it's in the papers"
January 28, 2015


Chemical found to trigger locust swarming

Jan. 30, 2009
Courtesy American Association 
for the Advancement of Science
and World Science staff

A com­mon brain chem­i­cal in hu­mans al­so sparks the Jekyll-and-Hyde trans­forma­t­ion of des­ert lo­custs from harm­less, lone in­sects to mem­bers of huge, vo­ra­cious and very dam­ag­ing swarms, sci­en­tists say.

The find­ing, they add, il­lu­mi­nates a mech­an­ism be­hind the lo­custs’ switch from mu­tu­al avoid­ance to at­trac­tion, and may lead to new pest-control strate­gies.

An adult locust. (© Tom Fayle)

“Here we have a sol­i­tary and lonely crea­ture, the des­ert lo­cust. But just give them a lit­tle ser­o­to­nin, and they go and join a gang!” said Mal­colm Bur­rows of the Uni­ver­s­ity of Cam­bridge, co-author of a re­port on the find­ings in the Jan. 30 is­sue of the re­search jour­nal Sci­ence.

The swarms gen­er­ally spell big trou­ble for any farm­ers caught in their path. The in­sects can swarm by the bil­lions, of­ten dev­as­tat­ing crop yields. 

Ste­phen Rog­ers, an­oth­er co-author al­so from the Uni­ver­s­ity of Cam­bridge and from the Uni­ver­s­ity of Ox­ford, said the des­ert lo­custs they stud­ied are “probably the worst kind. About 20 per­cent of the world is af­fect­ed” by the spe­cies.

“In the last few years, there have been ma­jor, dev­as­tat­ing swarms in Chi­na, Af­ri­ca, and Aus­tralia,” said Bur­rows.

Sci­en­tists spec­u­lat­ed that fu­ture pest-control strate­gies might in­volve find­ing ways to chem­ic­ally con­vert swarm­ing lo­custs back to sol­i­tary mode.

Re­search­ers have known of a few phys­i­cal fac­tors that trig­ger swarm­ing. Dwindling food seems to be one, said Rog­ers. “As their des­ert en­vi­ron­ment dries up, [the lo­custs] look for food, which even­tu­ally brings them all clos­er to­geth­er. They are look­ing for any­thing to eat, and when they run out of op­tions, a swarm is bas­ic­ally in­evitable.”

Des­ert lo­custs can al­so be stim­u­lat­ed in­to swarm­ing, gre­gar­i­ous be­hav­ior by ei­ther stimula­t­ion of the hind legs as they crawl over and jos­tle each oth­er or by the com­bined sight and smell of oth­er lo­custs. Af­ter enough of this “crowd­ing,” the lo­custs stop try­ing to avoid each oth­er and beg­in swarm­ing. 

The stu­dy’s au­thors took steps to trig­ger both sol­i­tary and gre­gar­i­ous be­hav­ior in des­ert lo­custs, then mon­i­tored their ser­o­to­nin lev­els. Lo­custs in swarm mode had about three times more ser­o­to­nin in their sys­tems than the calm, sol­i­tary lo­custs, they found. 

The in­ves­ti­ga­tors al­so tested wheth­er these phys­i­cal-sen­so­ry path­ways to swarm­ing caused an in­flux of ser­o­to­nin to the brain, and found that both path­ways did. They al­so found that ser­o­to­nin-in­hibit­ing chem­i­cals would al­low lo­custs to stay calm and sol­i­tary de­spite crowd­ing. On the oth­er hand, in­ject­ing ser­o­to­nin pro­mot­ers in­to the lo­custs could in­duce swarm­ing be­hav­ior even with­out the phys­i­cal stimula­t­ion. 

In­di­vid­ual brain cells driv­ing this swarm­ing be­hav­ior may al­so be iden­ti­fied and tar­geted, based on their use of ser­o­to­nin, ac­cord­ing to the in­ves­ti­ga­tors.

Ser­o­to­nin ex­ists in every mul­ti­cellular or­gan­ism. An­ti­de­pres­sant drugs tar­get ser­o­to­nin re­cep­tors—the mo­lec­u­lar gate­ways for ser­o­to­nin trans­mis­sion—in hu­mans to boost ser­o­to­nin use in the brain. “Many of the chem­i­cal agents that we used in this study to ma­ni­pu­late ser­o­to­nin were at one time or an­oth­er tested or used in clin­i­cal ap­plica­t­ions, such as the treat­ment of de­pres­sion,” said co-author Swid­bert Ott of the Uni­ver­s­ity of Cam­bridge.

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A common brain chemical in humans also sparks the Jekyll-and-Hyde transformation of desert locusts from harmless, lone insects to members of huge, voracious and very damaging swarms, scientists say. The finding, they add, illuminates a mechanism behind the locusts’ switch from mutual avoidance to attraction, and may lead to new pest-control strategies. “Here we have a solitary and lonely creature, the desert locust. But just give them a little serotonin, and they go and join a gang!” said Malcolm Burrows of the University of Cambridge, co-author of a a report on the findings in the Jan. 30 issue of the research journal Science. The swarms generally spell big trouble for any farmers caught in their path. The insects can swarm by the billions, often devastating crop yields. Stephen Rogers, another co-author also from the University of Cambridge and from the University of Oxford, said the desert locusts they studied are “probably the worst kind. About 20 percent of the world is affected” by the species. “In the last few years, there have been major, devastating swarms in China, Africa, and Australia,” said Burrows. Scientists speculated that future pest-control strategies might involve finding ways to chemically convert swarming locusts back to solitary mode. Researchers have known of a few physical factors that trigger swarming. Dwindling food seems to be one, said Rogers. “As their desert environment dries up, [the locusts] look for food, which eventually brings them all closer together. They are looking for anything to eat, and when they run out of options, a swarm is basically inevitable.” Desert locusts can also be stimulated into swarming, gregarious behavior by either stimulation of the hind legs as they crawl over and jostle each other or by the combined sight and smell of other locusts. After enough of this “crowding,” the locusts stop trying to avoid each other and begin swarming. The study’s authors took steps to trigger both solitary and gregarious behavior in desert locusts, then monitored their serotonin levels. Locusts in swarm mode had about three times more serotonin in their systems than the calm, solitary locusts, they found. The investigators also tested whether these physical sensory pathways to swarming caused an influx of serotonin to the brain, and found that they both did. They also found that serotonin-inhibiting chemicals would allow locusts to stay calm and solitary despite crowding. On the other hand, injecting serotonin promoters into the locusts could induce swarming behavior even without the physical stimulation. Individual brain cells driving this swarming behavior may also be identified and targeted, based on their use of serotonin, according to the investigators. Serotonin exists in every multi-cellular organism. Antidepressant drugs target serotonin receptors—the molecular gateways for serotonin transmission—in humans to boost serotonin use in the brain. “Many of the chemical agents that we used in this study to manipulate serotonin were at one time or another tested or used in clinical applications, such as the treatment of depression,” said co-author Swidbert Ott of the University of Cambridge.