Locusts are prompted to band together in enormous, destructive swarms by the same brain chemical that is linked to happiness in humans. A fascinating new study has found that locusts that are about to swarm experience a sudden surge of serotonin, the same neurotransmitter that’s targeted by antidepressant drugs. “Here we have a solitary and lonely creature, the desert locust. But just give them a little serotonin, and they go and join a gang,” observed Malcolm Burrows [AP], one of the study’s authors.
Researchers say the findings may lead to methods to block the formation of locust swarms. These infestations, which can cover hundreds of square miles and involve billions of vegetation-munching insects, can devastate agriculture and cost tens of millions of dollars to control [The New York Times].
Because locusts usually avoid each other, it’s only dire circumstances that bring them together in buzzing hordes. For instance, unpredictable desert rains cause vegetation blooms, which in turn makes locust populations skyrocket. But as the rains abate and fertile land shrivels up, locusts crowd together in the remaining green patches. Eventually, the swarm trigger goes off and the locusts take to the skies—”a strategy of desperation driven by hunger,” [National Geographic News], says coauthor Stephen Rogers. When they make that behavior shift they also change appearance dramatically, going from light green to dark brown.
In the study, reported in Science [subscription required], researchers were studying the changes in locust behavior 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 [AP].
The researchers also investigated what conditions spurred the serotonin surge, and found that the sight, smell, and touch of other locusts when the insects are crowded together are all cues. Indeed, the scientists found that tickling the insects’ back legs for a couple hours could induce the locusts to make more serotonin [AP], because they interpreted that stimulus as the jostling of other insects. Finally, when researchers injected a serotonin-blocking drug into locusts in a crowded enclosure, the insects didn’t change color and showed no inclination to swarm.
Image: Tom Fayle