Locusts eat plants and vegetation. When conditions are just right and there are plenty of green plants available for eating and reproducing, locusts gather into swarms that can devastate the area.
Locusts are related to grasshoppers and look similar to them. They can be solitary insects until the environment encourages them to congregate. This phase is called the gregarious phase. Locusts in the gregarious phase gather in large swarms to devour any and all plant life in the area and breed as much as possible. A swarm or plague of desert locusts can measure up to 460 square miles across. Each locust can eat its weight in plants each day.