Serotonin turns loner locusts into swarming, gregarious cereal killers: smells, sights, tickles boost levels of the neurotransmitter.

Neighbors say he seemed like such a nice, quiet locust. But a surge of serotonin, researchers now say, sent this solitary type to join a crop-destroying plague.

Desert locusts (Schistocerca gregaria) often live as shy loners that try to avoid others of their kind. If they do get crowded for several hours, though, the locusts start to switch behavior dramatically, almost becoming another animal.

"Party animal" is the way Stephen Simpson of the University of Sydney in Australia describes the new form. Loners get livelier. They move toward, rather than away from, other locusts. And if swarming persists, locusts can sweep across the landscape and devour pretty much all vegetation.

That switch in behavior turns out to rely on a compound known to be important in human moods, the neurotransmitter serotonin, Simpson says. Experiments show serotonin is both necessary and sufficient for the locusts' flip from solitary to gregarious forms, he and his colleagues from the University of Oxford and the University of Cambridge in England report in the Jan. 30 Science.

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Serotonin concentrations soar in a critical part of the locust nervous system during the first few hours of the transformation, the researchers say.

"That's pharmacologically similar to being on antidepressants, and on Ecstasy," Simpson says.

Serotonin itself won't make a good target for some new mass locust control, Simpson warns. The compound plays a major role in so many animal nervous systems, including people's, that researchers will have to look for more locust-specific parts of the swarming biochemistry, he says.

People certainly need better ways to control locust swarms, says Jeffrey Lockwood of the University of Wyoming in Laramie, an entomologist and writer whose book Locust discusses North America's now-vanished swarming pest. Desert locusts still devastate crops in their native North Africa, the Middle East and Asia. And the world is plagued by about a dozen species of locusts in total. They have considerable diversity, so Lockwood hesitates to speculate that all their chemistries would work exactly the same way.

Still, the new work "is a major step in understanding what makes locusts form swarms," says Paul A. Stevenson of Leipzig University in Germany. The research is the first to show that a particular nervous-system compound is necessary for the switch and can do the job by itself. The work also helps explain how social interactions can lead to changes in animal behavior, Stevenson says.

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The current study grows out of previous work on the social cues that change the locusts' behavior as well as their color. Simpson and his colleagues had discovered two triggers for the transformation. A loner locust that smells and sees other locusts for long enough will go gregarious. In the lab, researchers keeping solitary locusts pipe in air to individual locust chambers to prevent the smell of neighbors from initiating the change.

Physical jostling also changes loners, and Simpson tickled locust body parts with paintbrushes, discovering that the hind legs are critical. A five-second tickle on a loner's leg every minute for several hours will start the transformation.

Now exploring the underlying biochemistry, the collaborating researchers have found that smells and sights or tickles send serotonin concentrations soaring in the thoracic ganglia within a few hours. Injecting serotonin into that spot also initiated the change.

Yet locusts treated with serotonin-blocking chemicals didn't get social even when trapped in a crowd.