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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)

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.