Getting to the Roots of Flying Famine

Huge swarms of desert locusts have devastated crops in Africa, Asia, and Europe since biblical times, but no mortals have been able to predict when they will strike. Now a new computer model, reported in today's Proceedings of the National Academy of Sciences, appears to mimic how solitary locusts gang up and begin to swarm. The new findings could help pest control officials locate nascent locust swarms before they get off the ground.

Desert locusts live most years as ordinary grasshoppers, munching scattered stands of vegetation and keeping to themselves. But when too many hatch in a particular region, the solitary brown insects morph into a mob of gregarious, black-and-yellow bugs that consume everything in sight--and then cross continents en masse in search of more food. Because locusts become more sociable when they casually bump each other while eating, scientists have tried to pinpoint emerging swarms by using satellite maps of vegetation. But that attempt has largely failed.

To better understand what turns lonely locusts into a marauding scourge, integrative biologist Stephen Simpson and his colleagues at the University of Oxford, United Kingdom, created a computer model that probed how the pattern of vegetation influenced the social life of 20 locusts. They found that the solitary insects grew more gregarious as the vegetation became more patchy and, thus, forced them into closer quarters.

To test the predictions of the computer model, the Oxford team let 20 real locusts roam for 8 hours in a small square arena dotted with tiny pots of wheat. Patches of food caused the real locusts to become more sociable, just as the model predicted. "If the vegetation is more clumped ... they'll end up on the same food plant, which means they'll contact each other and cause each other to change" into social insects, Simpson says. The Oxford team has preliminary results from field experiments in Morocco that show that locusts in the desert behave like those in the lab.

The results may apply to more than locusts, says population biologist Alan Hastings of University of California, Davis. "It throws into question some [modeling] approaches that [compile an] average over small spatial scales," he says. Recognizing areas with patchy vegetation could also help pest control agencies to identify where locust swarms might originate.