Desert Locusts: Serotonin Triggers Swarming

In a report published in the journal Science, researchers have linked the radical transformation of desert locusts -- from harmless, solitary creatures to gregarious, swarm-forming insects -- to the brain chemical serotonin. This discovery reveals a mechanism within locusts that initiates their switch from aversion for other locusts to attraction, and may open the door to new methods of pest control.

With desert locusts (Schistocerca gregaria), the expression of this swarming characteristic generally means serious trouble for any nearby farmers caught in the path of the swarm. Locusts are known to sometimes swarm by the billions, and they often devastate crops.

Adult solitarious phase locust. copyright Tom Fayle. Used with permission.

Cambridge and Oxford universities' Stephen Rogers, an author of the paper, said that the desert locusts they studied are "probably the worst kind. About twenty percent of the world is affected by this particular species."

Another of the authors, Malcolm Burrows of Cambridge University, went on to say that, "This is a current problem. In the last few years, there have been major, devastating swarms in China, Africa, and Australia."

Although researchers had previously identified the sensory stimuli that trigger swarming behavior in locusts, this new finding reveals a neurochemical mechanism linking interactions among individuals to large-scale changes in population structure and the beginning of mass migration.

Although the discovery does not provide an immediate pest control solution, Paul Anthony Stevenson writes in a Perspective that these new insights "harbor considerable potential" for dealing with these harmful insects, if scientists can find viable ways to chemically convert swarming locusts back to their solitary phase.

Michael Anstey from Oxford University and colleagues including Swidbert Ott of Cambridge and Rogers monitored the levels of serotonin in desert locusts while they triggered both solitary and gregarious behavior in the creatures. Their results show that locusts behaving the most gregariously (in swarm-mode) had approximately three times more serotonin in their systems than the calm, solitary locusts.

Dwindling food sources seem to be one of the cues for dispersal, migration, and swarming. Rogers says, "As their desert environment dries up, they look for food, which eventually brings them all closer together. They are looking for anything to eat, and when they run out of options, a swarm is basically inevitable."

Physically, locusts can be stimulated into swarming, gregarious behavior by either stimulation of the hind legs as they crawl over and jostle each other or by the combined sight and smell of other locusts. After enough crowding, the locusts stop trying to avoid each other and begin coming together in a swarm.

Once Anstey and the team of researchers observed elevated levels of serotonin in swarming desert locusts, they tested whether or not both of these physical sensory pathways to swarming caused an influx of serotonin, and found that they both did. They also demonstrated that serotonin-inhibiting agents could allow locusts to remain calm and solitary despite the physical stimulation of crowding. On the other hand, injecting serotonin promoters into the locusts could induce swarming behavior even without that physical stimulation.

Flow diagram summarizing the major findings of the paper. copyright Steve Rogers. Used with permission.

Serotonin is present in all multi-cellular organisms, and serotonin receptors are often targeted by antidepressant drugs in humans to increase its availability. Ott says that, "many of the chemical agents that we used in this study to manipulate serotonin were at one time or another tested or used in clinical applications, such as the treatment of depression."

"It's really interesting," says Burrows. "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!"

Locust Facts:

    --Locusts are grasshoppers that swarm.

    --Of the 8,000 types of grasshoppers treated as species only about 12 are swarm-forming locusts.

    --An adult Desert Locust is 2-2.5 inches long and weighs 0.05-0.07 oz.

    --A Desert Locust adult can consume roughly its own weight in fresh food per day.

    --They are prodigious fliers, covering 60 miles in 5-8 hours.

    --The two phases are so different in appearance and behavior that they were treated as separate species until 1921.