When a specific region of the insect's back leg was stimulated, it caused a shift from solitary to gregarious behaviour.
Scientists say they have identified the biological trigger that causes locusts to swarm.
The discovery could lead to new insecticides capable of stopping the vast "clouds" of insects that can devastate crops.
For the first time, zoologists at the University of Oxford, UK, have pinpointed touch-sensitive hairs on the insects' hind legs which activate swarming.
"It's the G-spot of the locust - G for gregarisation, in this case," team leader Stephen Simpson told BBC News Online.
Crowd behaviour
Locusts are normally shy, solitary creatures. But in a swarm they become a menace, capable of stripping fields of crops in a matter of hours.
The Oxford team has found that "hot-spots" on the hind legs of the locusts, stimulated when they group together, turn the relatively harmless insects into destructive pests.
The discovery is important because it could lead to new insecticides capable of stopping insects from swarming.
"The nervous system communicates with chemicals," Professor Simpson said. "If you can work out which neural pathways are involved and what signalling chemicals they use to change locusts' behaviour from solitary to swarming, it might be possible to develop new control chemicals."
Tickle technique
The discovery was made by tickling lab locusts with a paintbrush.
When a specific region of the insect's back leg was stimulated, it caused a shift from solitary to gregarious behaviour.
Tickling other parts of the locust, such as its antennae, mouthparts or abdomen, had no effect on behaviour.
The research is reported in the current edition of the journal Proceedings of the National Academy of Sciences.