The desert locust Schistocerca gregaria leads a largely reclusive life. Under the right conditions, however, these creatures experience a major mood swing, swarming into ravenous masses that devour crops. Now, scientists have found that the neurotransmitter serotonin is behind this Jekyll-and-Hyde switch (Science 2009, 323, 627). A team led by Stephen M. Rogers of the Universities of Cambridge and Oxford discovered the chemical's critical role in initiating the locusts' shift from its shy "solitarious" phase to its devastating "gregarious" phase. In the wild, the phase change occurs when locust populations explode, increasing the locust concentration and competition for food. External cues, such as seeing or jostling against other locusts, trigger the shift. Rogers' group was able to use serotonin to make the insects change phase without such external stimulation. Although the discovery suggests a possible route for pest control, there's not yet an effective serotonin-blocking chemical that's designed to pass through the cuticle and sheath surrounding the locust's nervous system. Furthermore, such a substance would need to be used while the locusts are in the hard-to-find solitarious phase.