Tickle a locust’s hind legs and two hours later it will be transformed into an insect ready to form a crop-devastating swarm.

While researchers know why — the tickling simulates the jostling that usually solitary locusts experience when limited food suppliers force them to crowd — they have puzzled for decades over how the radical biological transformation occurs.

A study released Thursday by the journal Science found that the brain chemical serotonin triggers the switch from aversion to attraction.

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

The researchers discovered that locusts in swarm mode — called gregarious locusts — had serotonin levels three times higher than those in a solitary behavior phase.

Once in this phase, the green locusts turn bright yellow, gain large muscles that equip them for prolonged flight and actively seek the company of other locusts. They can develop into swarms of billions and fly 60 miles (96 km) in five to eight hours in search of food.

But when they were injected with serotonin-blocking chemicals, locusts still in their antisocial phase remained calm and did not transform into the swarm phase in response to the leg tickling or presence of a crowd.

And when the locusts were injected with chemicals that stimulated serotonin they were transformed into the swarm phase without the stimulus.

“Up until now, whilst 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,” said study co-author Michael Anstey of University of Oxford.

“The question of how locusts transform their behavior in this way has puzzled scientists for almost 90 years, now we finally have the evidence to provide an answer.”

While the discovery “harbors considerable potential” for dealing with the harmful insects, it will not likely to a short-term pest control solution, said Paul Anthony Stevenson of Germany’s Leipzig University.

“To be effective, antiserotonin-like chemicals would need to be applied when the animals are solitary locusts and scarce targets in vast expanses of desert — about three locusts per 100 square meters (1,076 sq ft),” Stevenson wrote in an accompanying article.

“Current serotonergic drugs are not designed for passing through the insect cuticle and sheath encasing the nervous system, nor are they insect-selective, hence their use is ecologically unjustifiable.”