To the casual observer, the millions of swarming locusts that descended on West Africa last year were like something straight out of a science fiction novel. Several mile-wide bands of the voracious insects ate their way through the region’s crop lands, threatening to cause food shortages and loss of income for local farmers.
Now, Gregory Sword, an ecologist with the Agricultural Research Service, has an explanation for why some insects, like the desert locust, Schistocerca gregaria, gather in mind-boggling numbers and move together across the landscape.
In the current issue of the journal Nature, Sword describes how the Mormon cricket–a species of katydid known to periodically overrun agricultural fields in the Northern Plains of the United States–relies on the protection afforded by thousands, if not millions, of its fellow crickets to reduce the risk of attack by predators.
Researchers have speculated that insects moving in bands derive some benefits from traveling en masse. But no one has previously attempted to quantify those advantages, mostly because of the inherent difficulty in tracking how individual insects move within a band of millions.
Sword, who works at the ARS Northern Plains Agricultural Research Laboratory in Sidney, Mont., teamed with colleagues Patrick Lorch of the University of North Carolina at Chapel Hill and Darryl Gwynne of the University of Toronto at Mississauga to use radio transmitters to monitor the movements of individual Mormon crickets during a study last year near Dinosaur National Monument in Colorado.
The researchers discovered that for the crickets, there’s safety in large numbers. Those insects which were part of a large moving band were much less likely to be eaten. In fact, 50 to 60 percent of the Mormon crickets that were separated from a migratory band were killed within two days by predators such as birds and rodents, while none of those staying with the band were eaten. Radio transmitters belonging to those unfortunate, lone insects were found either chewed or still glued to a partially-eaten cricket corpse.