What makes harmless little green grasshoppers turn into brown, crop-chomping clouds of swarming locusts? Serotonin, according to a study published this week in Science. Researchers from universities in the UK and Australia found that that neurotransmitter (a chemical compound that sends impulses between nerve cells and affects everything from sleep to aggression in humans) spurs a cascade of Dr. Jekyll-to-Mr. Hyde–like changes in at least one species of grasshopper — the desert locust (Schistocerca gregaria). This species is infamous for wreaking havoc from Africa to Asia.

Knowing what causes this swift metamorphosis may help governments and farmers develop methods to control future locust outbreaks with chemicals that would suppress the offending serotonin.

It took just two to three hours for timid grasshoppers in a lab to morph into gregarious locusts after they were injected with serotonin. Conversely, if they were given serotonin blockers, they stayed solitary even in swarm-inducing conditions.

"These little guys changed from a shy creature that actively avoided making contact with other grasshoppers [into a creature] actively seeking out other insects and joining a gang," says study co-author Malcolm Burrows, a zoology professor at Cambridge University in England. And we're not just talking about a gaggle of grasshoppers: Just last year, a swath of locusts more than three and a half miles (six kilometers) long tore through Australia, devastating crops in its path.

"They eat everything in sight," says Sean Mullen, an assistant professor of evolutionary genetics at Lehigh University in Bethlehem, Pa., about swarming locusts.

When these insects go into swarm mode, they don't just get super social, they also completely change physically, becoming stronger, darker and much more mobile, says study co-author Swidbert Ott, a research fellow at Cambridge. In fact, he says, the before-and-after bugs look so different that, until the 1920s, they were assumed to be two unique species.

In the wild, swarms usually appear after a rainy period followed by a time of drought. After rains, populations of grasshoppers explode, Burrows says, because there is food aplenty. But when the land becomes parched and grass scarce, the populations get pushed into smaller and smaller areas, becoming more packed as desirable pasture diminishes, he says. At a certain point of density, the swarm-inducing serotonin gets triggered and the locusts set off en masse to find greener pastures. After that, few things — other than an end to the food supply or an ocean — can stop them.

Burrows says that locusts can switch out of swarm mode, though it takes days rather than hours. He notes, however, that the about-face rarely happens in the wild, because the offspring of locusts that breed while swarming are born swarmers.

Today, locust invasions are controlled with pesticides that also wipe out other insects, note Burrows and Ott. This new research, however, paves the way for development of a chemical that would specifically inhibit serotonin production in the solitary grasshoppers, says Hojun Song, a postdoctoral researcher at Brigham Young University in Provo, Utah.

But remember, as Kung Fu's Master Po advised his young charge in the 1986 movie, "Do not go in fear, Grasshopper." Of the approximately 8,000 species of grasshoppers, only about 10 of them are likely to morph into swarming locusts, Burrows says. But, Song adds, more research should be conducted to determine whether other types of locusts also get hopped-up on serotonin.