Locust Swarms Switched On by Brain Chemical

Christine Dell'Amore
National Geographic News
January 29, 2009
The social life of a desert locust is a study in extremes: One moment it's an irritable loner, the next it's happily rubbing elbows with huge swarms of its own species.

(See locust swarm pictures.)

What turns this love-hate relationship on and off, a new study says, is a brain chemical common to most animal species—serotonin.

In a recent experiment, scientists found a close connection between how sociable the insects became and the levels of the neurotransmitter in their bodies.

"This really is a new discovery," said study co-author Stephen Rogers of the University of Oxford in the U.K.

"We had a good idea that [locust] behavior changed rapidly, so we did a broad sweep of neurotransmitters and found that serotonin changes in a very narrow window [of time]."

The results may someday lead to strategies for controlling swarms of the critters, which appear every few decades and munch their way through crops in arid lands from West Africa to India.

(Related story: "Cannibal Crickets Cause 'Forced Marches' Through Crops, Study Says" [February 28, 2006].)

Strategy of Desperation

About 90 percent of the time desert locusts live in small numbers scattered across the desert, avoiding as best they can other locusts.

It's only when the insects are forced to crowd together that they take on a swarm mentality.

Locusts switch into swarm behavior based on two cues: when they see and smell other locusts for an extended period or when their hind legs are constantly jostled.

In the laboratory, Rogers and colleagues tickled the hind legs of insects in solitary mode with paintbrushes for two hours, which simulated jostling from fellow locusts.

As the bugs became gregarious—a term to describe swarm tendency—scientists found the amount of serotonin in their bodies spiked.

Though serotonin has a wide range of roles in animals and plants, Rogers said, it generally impacts how animals perceive and interact with their environment. In humans, for instance, low serotonin levels are linked to depression.

Because locusts usually avoid each other, it's only dire circumstances that bring them together in buzzing hordes.

For instance, unpredictable desert rains cause vegetation blooms, which in turn makes locust populations skyrocket. But as the rains abate and fertile land shrivels up, locusts crowd together in the remaining green patches.

Eventually, the swarm trigger goes off and the locusts take to the skies—"a strategy of desperation driven by hunger," said Rogers, whose study appears tomorrow in the journal Science.

(Watch a video of locust swarms plaguing Australia.)

The weakened insects are largely carried by the wind, which takes them to regions of low atmospheric pressure where it's more likely to rain again.

Not Black and White

Rogers and colleagues don't know if other locusts, or other swarming species, experience the same serotonin spike.

But the new study may help countries most devastated by crop swarms to develop more effective control methods, Rogers pointed out.

The current strategy for most countries is to observe locust behavior, wait for them to become gregarious, and then kill them with pesticides.

Based on this new discovery, people could instead spray a compound on the gathering locusts that blocks their serotonin receptors and thus prevents them from swarming, the study authors note.

"I think there is wide appreciation for having something more specific to locusts than indiscriminate application of pesticides," Rogers said.

But simply thwarting gregariousness is not so black and white, said Keith Cressman, senior locust forecasting officer for the UN's Food and Agriculture Organization in Rome, Italy.

That's because locust-control teams will actually have more difficulty killing off locusts if they're not in organized swarms.

"This doesn't belittle the findings," said Cressman, who was not involved in the study.

"They are extremely important and help us fill in the gaps in our knowledge on understanding how this gregarious process works."

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