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Cannibal Crickets Cause "Forced Marches" Through Crops, Study Says

Elizabeth Svoboda
for National Geographic News
February 28, 2006
 
Mormon crickets, close relatives of locusts and grasshoppers, can devour their way across 50 miles (80
kilometers) of farmland in a single season.

Stephen Simpson, an entomologist at the University of Sydney in Australia, had long wondered why crop-eating bugs like Mormon crickets, which live
in the western United States, cover so much ground so quickly.

He recently pinned down two driving forces, he says: a need for protein and a fear of cannibalism.

Simpson conducted a series of tests to determine what the crickets ate under different circumstances. The results showed that, when protein is scarce, Mormon crickets on the move are quick to devour each other.

This helps explain why the crickets showed a strong tendency to keep jumping forward, he says.

"They have an incentive to move, because they're least at risk of being cannibalized if they're going in the same direction as everybody else," Simpson said.

The result of this collective survival strategy is what Simpson dubs a "forced march."

Simpson reported his results yesterday in the Proceedings of the National Academy of Sciences.

Unusual Taste Test

Simpson noticed that Mormon crickets seemed very selective about what they ate, favoring seeds, other protein-rich plant parts—and weaker members of their own species.

"They produce these massive bands moving across the landscape," Simpson said. "But they don't strip everything bare as they go. They'll leave food behind and go to the next field."

To get a clearer picture of the insects' nutritional needs, Simpson presented a test group of crickets with food-filled petri dishes. One held high-protein and low-carbohydrate food, another held high-carbohydrate and low-protein food, and a third had equal amounts of both.

The results of this taste test were unambiguous: The crickets overwhelmingly chose the protein-rich food over the other options. They also showed a preference for foods with high salt content.

But when Simpson confined the crickets in boxes with access to both protein and carbs, the insects only gorged themselves on protein for about a day. They then scaled back to a half-and-half mix of both nutrients, apparently having leveled out a nutritional imbalance.

"That led us to ask, Is it really that these crickets naturally have very high protein requirements, or is it that their environment has let them down in some way?" Simpson said.

Suspecting that a lack of protein in their environment might indirectly be fueling the insects' punishing rate of travel, Simpson tried gluing a few crickets to stalks in the field to impair their movement.

Just as he'd anticipated, the tethered test subjects didn't last long. Other crickets quickly swooped in to gobble up the protein-rich morsels.

Crickets at the front of the pack, Simpson theorized, will move away from a partially decimated field in hopes of finding food elsewhere that has more protein and salt.

Once this initial flank of insects has started moving, the rest of the swarm has to match their pace—or risk being devoured by other protein-deficient bugs nearby.

John Capinera, an entomologist at the University of Florida, thinks Simpson's work will interest farmers looking for more effective ways to halt insect damage, helping forestall the modern-day equivalent of locust plagues. (Read about locust swarms in the Great Plains.)

"Knowing what drives these insects' migration patterns tells you two things," he said.

"One, they'll only feed on crops that contain a high amount of protein, which allows you to limit intervention to those particular crops. Two, whatever insecticides or other agents you use can be put into a protein-based bait."

Beyond suggesting pest control strategies, Simpson's work offers a plausible environmental explanation for the traveling-swarm phenomenon.

It also demonstrates how seemingly minute changes at the lowest levels of an ecosystem can give rise to larger, unanticipated ones higher up.

"In general, insect populations were very low in the fields we studied," Simpson said.

"It could be that these crickets normally supplement their plant-based diet with other insects—caterpillars, ants, whatever they come across. If those become rare, that may be when you start seeing more cannibalistic behavior and migration."

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