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Desert locusts (Schistocerca gregaria) are normally solitary insects, but in crowded conditions, they undergo physical and behavioral changes and become 'gregarious', clustering in swarms that can include billions of individuals. These swarms devastate crops over large areas of land, causing economic hardship and affecting the livelihood of as many as one in ten people worldwide. The locusts' transition from solitary to gregarious is triggered by sensory stimuli: either jostling from or the sight and smell of other locusts nearby. Until recently, however, the mechanism of action of these stimuli was not known. Now, scientists report that an increase in serotonin concentration in a specific region of the locust's nervous system underlies the behavioral changes that lead to swarming.

The scientists, headed by Michael Anstey (University of Oxford, UK) and Stephen M. Rogers (University of Cambridge, UK), began by showing that solitary locusts could become fully gregarious within 2 h of forced crowding and that this 2-h period also featured an increase in serotonin concentration in the thoracic ganglia, but not in the brain or other regions of the nervous system (Science 323, 627–630; 2009). Using a series of experiments, the researchers set out to establish whether the increase in serotonin was the causative factor for the behavior change. First, they analyzed the degree of gregariousness in relation to the amount of serotonin in locusts crowded for various lengths of time. They found that serotonin concentration was positively correlated with the degree of gregariousness. Next, they assessed the effects on locust behavior of blocking either the action or the production of serotonin and found that either block prevented locusts from becoming gregarious. Then, they evaluated whether administering serotonin or serotonin agonists would induce locusts to become gregarious, even in the absence of sensory stimuli associated with other locusts, and found that locusts treated with the compounds were more gregarious than untreated controls. Finally, they asked whether boosting the locusts' natural synthesis of serotonin would encourage them to become more gregarious when exposed to sensory stimuli from other locusts and found that the treatment did enhance gregariousness.

The combined results of the experiments show that an increase in serotonin concentration in the thoracic ganglia of locusts is both necessary and sufficient to trigger the transformation from solitary to gregarious. The identification of this trigger may facilitate development of methods of suppressing or inhibiting serotonin in order to control swarming in locusts.