Serotonin, the brain chemical involved in depression, anger, and a variety of other human behaviors, turns out to have another surprising role: It transforms desert locusts from solitary, innocuous bugs into swarming, voracious pests that can ravage orchards and fields in a matter of hours. The findings, published in tomorrow's issue of Science, could point the way to new locust-control methods that don't rely on insecticides.
Most of the time, the desert locust (Schistocerca gregaria) is a bland, greenish insect that lives an inconspicuous life, shunning other members of its species and flying only by night. But when their densities reach a certain threshold, locusts become gregarious: They seek out one another's company, start reproducing explosively, and eventually form massive swarms that can move thousands of kilometers beyond their usual habitats and create havoc of biblical proportions. The behavior changes are accompanied by a complete physical makeover, taking several generations, during which the insects first turn pink and eventually black and bright yellow.
A team of researchers based at three universities in the United Kingdom and Australia had previously discovered that the change from solitary to gregarious starts when locusts see and smell one another, or when their hind legs touch one another, a stimulus researchers can imitate in the lab by gently tickling them. In a 2004 paper, the group also showed that levels of 13 brain chemicals differ between insects in the two stages (Science, 10 December 2004, p. 1881). Now, the researchers have singled out serotonin as "the first domino to fall, the one that sets the entire process in motion," says lead author Michael Anstey of the University of Oxford in the U.K.
When the team injected locusts with drugs that block serotonin's action or a compound that inhibited their own serotonin production, they didn't become gregarious, even when confronted with other insects or after leg tickling. But when the team treated solitary locusts with serotonin, or gave them a drug that boosted their own production, the locusts became gregarious, even in the absence of those stimuli. That shows that the chemical is both "necessary and sufficient" to kick-start the transformation, says neuroscientist R. Meldrum Robertson of Queen's University in Kingston, Canada, who studies locust flight. "It seems pretty clear that they have nailed down serotonin here," he says.
Currently, African countries spray millions of liters of insecticides over their fields every time locusts swarm; opinions vary on how effective this is (Science, 10 December 2004, p. 1880). Drugs targeting the serotonin pathway might provide an alternative, says Anstey, but others are skeptical. For one thing, serotonin signaling is so ubiquitous in the animal kingdom that other species might suffer major collateral damage, says neuroscientist Hans Hofmann of the University of Texas, Austin. He thinks a lot more work would be needed to find locust-specific targets. "At this point," says Hofmann, "I'm not sure that's more than science fiction."