From Science:
The wrath of god is the traditional explanation for plagues of marauding insects that devour everything in their path. What really drives the swarm, according to a new study of crickets, is a hankering for protein and salt, along with the fear of getting cannibalized. Every few years, Mormon crickets march across the western United States by the millions. Last spring, a team led by Stephen Simpson, an ecologist at the University of Sydney, Australia, found some clues to their motivations in the trail blazed by a 1-kilometer long Mormon cricket marching band. For one thing, the crickets were not starving because they left most edible plants untouched. But they gobbled anything high in protein, such as seed pods, flowers, and even mammal feces. Salt also seemed to be on the menu; the crickets swallowed soil if it was soaked with urine. And a strange clue was the discovery that many crickets were eating each other.
More here.