Sky-darkening swarms. Ruined harvests. Until the late 1800s, billions of Rocky Mountain locusts periodically descended on the Great Plains, ravaging prairie plants and decimating farmers’ crops. But then—the swarms stopped flying, and the locusts, last seen in 1902, never returned.

What caused this sudden, mysterious extinction? Not long ago, a University of Wyoming research team proposed an answer, which involves the locusts’ life cycle and its interaction with humans. Locusts are grasshoppers, and they have the capacity for two distinct adult phases: a solitary phase, and a gregarious, swarming one. (They only develop into a gregarious phase when specific weather, population and other conditions align.)

The Rocky Mountain locusts’ demise likely came during a solitary phase, when their numbers were low. Starting in the mid-1800s, as Euro-American settlers moved into Native lands in the West, they plowed and planted over the insects’ nesting areas. Today, these locusts no longer fly over North America, but other species sometimes swarm in vast numbers across parts of Africa, Australia, Asia and the Middle East.