Study shows what makes locusts swarm

LONDON, Jan 29 (Reuters) - A brain chemical that lifts people out of depression can transform solitary grasshoppers into swarming desert locusts, a finding that could one day help prevent the devastating plagues, researchers said on Thursday.

Increases of serotonin, the nerve-signalling chemical targeted by many antidepressants, appears to spark the behaviour changes needed to turn the normally harmless insects into bugs that gang up to munch crops, they said.

"Our paper shows how this change in behaviour changes what are essentially large grasshoppers living in the desert into swarming, destructive pests," said University of Cambridge researcher Stephen Rogers, who worked on the study.

"For a swarm to develop the locusts must transform from a solitary phase into a gregarious phase."

Vast swarms containing billions of locusts stretching over dozens of square kilometres periodically devastated parts of the United States when the West was settled and they continue to inflict economic hardship on parts of Africa and China.

The last big African swarm in 2004 cost $400 million simply to eradicate the pests, a tab that did not included money lost to destroyed crops, Rogers added.

"The gregarious phase is a strategy born of desperation and driven by hunger, and swarming is a response to find pastures new," he said.

Rogers and colleagues, who published their findings in the journal Science, wanted to find out what triggered the behaviour change, which occurs when the insects gather in close quarters.

The researchers said they looked at serotonin because it is a well-known chemical that affects the way nerve cells communicate with each other.

"The question of how locusts transform their behaviour in this way has puzzled scientists for almost 90 years, now we finally have evidence to provide the answer," said Michael Anstey, a University of Oxford researcher who worked on the study.

In their experiment, the team injected some grasshoppers with drugs that blocked serotonin and others with the chemical itself.

Locusts injected with serotonin turned gregarious even in the absence of other pests while insects exposed to the drugs did not, even in a situation in which they should have, the researchers said.

The findings do not provide an immediate pest control solution but could one day help find a way to stop the devastating swarms before they form, researchers said.

"For this to be effective we need to get it at a early stage, University of Cambridge's Rogers said. "Once you have several million or a billion locusts they are by de facto a swarm." (Editing by Maggie Fox)