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Experiments provide proof of how traveling in groups protects insects


Few events involving animals are more dramatic than when they band together and head out on the march cross-country. Among examples are the many thousands of wildebeests and other hoofed mammals that form herds and migrate across the African plains.

Countless millions of Mormon crickets and young locusts also sometimes unite with their own kind and form teeming, hungry islands of life that devour everything in their path that’s edible. Some spectacular marching packs stretch several miles wide and extend 10 miles or so in length. And they can travel a mile or so a day.

Why such groups of insects form has mystified humans for thousands of years. One firmly held belief was that God was punishing men and women for their sins. A more recent, scientific theory has been the "safety-in-numbers" idea -- that the small animals congregate periodically as a way of protecting themselves from predators such as birds and rodents.

Now, a trio of insect experts has developed what they believe is strong new evidence that the latter theory is correct. By gluing radio transmitters -- each weighing less than half a gram -- to the backs of Mormon crickets in northeastern Utah and northwestern Colorado, Drs. Gregory A. Sword, Patrick D. Lorch and Darryl T. Gwynne showed experimentally that band formation indeed boosted insect survival.

Sword is a research ecologist with the U.S. Department of Agriculture’s Agricultural Research Service, Lorch is a postdoctoral fellow in biology at the University of North Carolina at Chapel Hill and Gwynne is a biology professor at the University of Toronto at Mississauga. Their work, reported in the Feb. 17 issue of Nature, is unique since previously it has been too difficult to monitor what happened to specific insects among untold millions.

"With these remarkable new light-weight transmitters, we could keep track of individual crickets and find out what happened to them when they were in the band and also when we removed them and put them in other locations by themselves," Lorch said.

"In repeated experiments, we found that within two days, 50 percent to 60 percent of insects transplanted out of the band were dead because something ate them," he said. "On the other hand, we found no deaths during the same period among the crickets we monitored that stayed with the band."

That individuals removed from the group suffered gory deaths was clear from partially chewed transmitters the scientists recovered, often with body parts still attached, Lorch said. Researchers retrieved several devices from trees and burrows. Two were lost entirely and, presumably because Mormon crickets can’t fly, carried off a long way by birds and out of radio receiver range.

"What this new work appears to show is that even though being part of the band has its own costs -- such as greater competition for food and cannibalism of injured crickets – overall, there’s a clear benefit to band members," Lorch said. "Predation occurs anyway, but any given individual is far better protected than it would be if it were on its own."

The research, supported by the USDA, is part of a larger study designed to learn whether scientists can predict and possibly change the path of massive insect bands, he said.

"In the United States nowadays, Mormon crickets just cost farmers and others a lot of money, but in parts of Africa and elsewhere, locusts can cause widespread famine because they consume crops, and the people there sometimes have nothing else to eat."

UNC’s biology department is part of the College of Arts and Sciences.

David Williamson | EurekAlert!
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