The brain chemical serotonin has been linked to mood in people. It plays a role in sexual desire, appetite, sleep, memory and learning, too.

Under certain conditions, locusts triple the amount of serotonin in their systems, changing the insects from loners to pack animals, Burrows and his co-authors report in Friday's edition of the journal Science.

These packs can be devastating. They affect 20 per cent of the Earth's land. Last year a swarm nearly 6.5 kilometres long plagued Australia. They also occur in Africa and Asia and have affected western parts of North America.

"Serotonin profoundly influences how we humans behave and interact, said co-author Swidbert Ott of Cambridge, "so to find that the same chemical in the brain is what causes a normally shy anti-social insect to gang up in huge groups is amazing."

Now that they know what causes the swarming behaviour, scientists can begin looking for ways to prevent it.

"It opens up a whole line of inquiry into what we could do to break apart these swarms before they develop," said co-author Stephen Rogers, who is affiliated both with Cambridge and the University of Oxford in England.

But, he added, "you need to get it at an early stage. Once you have several million or billion locusts, there is a limit to what you can do."

Calling the report a "breakthrough," Paul Anthony Stevenson of Leipzig University in Germany said it "harbours considerable potential" for finding ways to block swarming. But that will require a lot more research, said Stevenson, who was not part of the research team.

Researchers led by Michael Anstey of Oxford were studying the changes in locust behaviour and tested them for a variety of chemicals. The only change they found was that when the insects were swarming, they had about three times more serotonin in their systems than when they were living as solitary creatures.

So the scientists took some solitary locusts and injected serotonin into them. Sure enough they changed in appearance and flocked together.

The Dr. Jekyll to Mr. Hyde transformation took only a few hours.

It turns out that locusts produce more serotonin when circumstances force them together and they are stimulated by the sight, smell and touch of many other locusts. This can happen, for example, when drought reduces their food supply and causes locusts to gather at a few remaining sources of food.

Indeed, the scientists found that tickling the insects' back legs for a couple hours could induce the locusts to make more serotonin.

Once researchers determined that serotonin causes the change, they gave locusts drugs that blocked serotonin and then exposed them to situations that normally cause swarming. But the change didn't occur.

"To actually be able to stop it from happening, that was very exciting," Anstey said.

Now the question is how to target locusts without affecting humans or other animals.

Also part of the research team was Stephen Simpson of Oxford and the University of Sydney in Australia.

The research was funded by the Biotechnology and Biological Sciences Research Council of England, the Natural Sciences and Engineering Research Council of Canada, England's Royal Society and the Australian Research Council Federation.