Dr. Wilson suspects that the 1518 outbreaks occurred after the Spanish colonists had introduced new species of sap-suckers to Hispaniola. He notes that the first shipment of plantains arrived there from the Canary Islands in 1516. After the native tropical fire ants began protecting them, the sap-suckers exploded.

"The Spanish would not have made the connection," Dr. Wilson said. "They'd be paying attention to the ants' stinging them every time they were handling a plant."

Dr. Wilson described his findings in the Jan. 6 issue of Nature. He also examined major outbreak in the Lesser Antilles in the 1760's. Those ants were clearly different. For one thing, they did not bite. Dr. Wilson proposes that the culprit was the big-headed ant, Pheidole megacephala. A native of Africa, the ant is currently causing agricultural damage in other parts of the tropics where it has been accidentally introduced. The Lesser Antilles may have been one of the first places the ant invaded and wreaked havoc.

Invasive species represent major threats to farmers and endangered native species. If Dr. Wilson is right, they had major effects in the New World from the earliest days of European exploration. "Almost as soon as the first Europeans got there, it started," he said.

For now, however, Dr. Wilson cautions that he has identified just the most likely suspects of the ant plagues.