The biological key that transforms locusts from solitary and antisocial insects into marauding swarms has been identified for the first time.

The discovery, by British and Australian scientists, could lead to new approaches to preventing locusts from forming plagues of the sort described in the Book of Exodus. These devastate crops and threaten the livelihood and food supply of one in ten people around the world.

Desert locusts range over 20 per cent of the world’s surface, periodically forming swarms of billions of insects that stretch over many square miles at once. In November, a swarm 3.7 miles (6km) long devastated parts of Australia.

The insects can eat their own bodyweight every day, and can fly quickly, covering 60 miles in five to eight hours. They are thus one of the principal threats to crops in many countries, especially in the developing world where insect control is less effective.

Though famed for their swarming behaviour, locusts are usually solitary insects that live in a desert habitat, and have a strong aversion to mingling with others. When rain arrives, however, vegetation growns lusher, providing the conditions for them to breed in large numbers. As this vegetation withers or is grazed away, the insects become increasingly concentrated in a few patches of land.

This close contact and crowding prompts a change in their behaviour: they become mobile and seek the company of other insects. They also change colour and grow bigger muscles, and then start to swarm.

While the sensory trigger that prompts swarming behaviour is already known — it is possible to induce it by rubbing their hind legs to simulate the jostling of a crowd — the chemical cue that causes it has remained mysterious.

The new research has pinpointed the biological cause as the signalling chemical serotonin, which is involved in regulating mood in humans.

Scientists at Cambridge, Oxford and Syndey universities found that levels of this chemical rise when locusts are jostled and start to become gregarious. The transformation can also be stopped with chemicals that block the action of serotonin. Details of the research are published in the journal Science.

The findings suggest that it could be possible to stop locusts from swarming using chemicals that prevent serotonin from inducing behavioural changes.

Another prospect is the development of sprays that convert swarming locusts back into solitary insects, which would no longe present such a threat to crops.”

Michael, Anstey, of the University of Oxford, said: “Up until now, while we knew the stimuli that cause locusts’ amazing Jekyll and Hyde-style transformation, nobody had been able to identify the changes in the nervous system that turn antisocial locusts into monstrous swarms. The question of how locusts transform their behaviour in this way has puzzled scientists for almost 90 years, now we finally have the evidence to provide an answer.”

Professor Malcolm Burrows, of Cambridge University, said: “We hope that this greater understanding of the mechanisms causing such a big change in behaviour will help in the control of this pest, and more broadly help in understanding the widespread changes in behavioural traits of animals.”

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