Locust Swarms

Desert locusts are usually shy, solitary insects. [A] They are green or brown, avoid one another, and cause little harm. Yet under certain conditions, the same insects can form swarms of billions that travel long distances and destroy crops across entire regions. For a long time, scientists believed that solitary locusts and swarming locusts belonged to two different species, because they look and behave so differently. [B]

The change from one form to the other is triggered by crowding. After heavy rains, plants grow quickly and locusts reproduce in large numbers. When the rains end and the plants dry up, the insects are forced together on the few green areas that remain. [C] Experiments have shown that repeated touching of the back legs is enough to start the change: within a few hours, the insects become more active and are attracted to one another instead of avoiding contact. [D]

Over the following generations, crowded locusts also change color and shape, becoming yellow and black, with larger muscles for long flights. Studies have linked the change in behavior to a rise in serotonin, a chemical in the nervous system. Understanding this process has practical value. If authorities can detect the early signs of crowding, they can target small groups before they merge into enormous swarms, which are much harder to control.