Eat or be eaten.
Mormon crickets swarm to find protein and salt, and those who stop become meals themselves.

S. Simpson / U. Sydney

A Plague of Cannibals

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.

To test these three factors, the team used the swarm as a living laboratory. They placed pellets of high protein or high carbohydrate food in a swarm's path, as well as cotton balls soaked in water with a range of saltiness. And in a grisly test of the danger of cannibalism, they either glued or tethered crickets to the ground, plucking one or both legs off some and leaving others intact.

When the dust settled, the researchers found the mob had preferred the protein pellets and the saltier cotton balls, leaving behind the carbohydrate and low salt offerings. And sure enough, crickets that had been stuck in place were almost guaranteed to be eaten by their former mates. Only the tethered crickets with both their legs had a chance of survival by kicking away the hungry swarm, the team reports 27 February in Proceedings of the National Academies of Science. Simpson concludes that Mormon crickets swarm to find new sources of protein and salt, but this is "a forced march," he says, because those that stop become food themselves.

The study fully explains the cricket swarming, says David Raubenheimer, an ecologist at the University of Auckland, New Zealand. "This is an extraordinary piece of work," he says, because the simple experiments with individuals explained a population's "otherwise insoluble" behavior.

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