Instead, they say, more than 500 bees engulf each hornet in a ball and raise their body temperatures to levels so high that the hornet dies in about 20 minutes. Some defenders die in the struggle against an enemy that is four times the length and 20 times larger than an individual bee. But their bodies are pushed out of the ball and they are replaced by others as the bees turn up the heat of their attack.

With the hornet entrapped, the bees vibrate and quickly raise the temperature of the ball to 116 degrees Fahrenheit, above the laboratory-measured lethal temperature range for the hornet of 111 to 114.8 degrees, the report said. Tests show that the bees, which display unagitated body temperatures of 95 degrees or less, can survive heat of up to 122 degrees.

Earlier studies by the same researchers showed that Japanese honeybees use the same heat-to-kill method against a smaller hornet, although it involved fewer bees to do the job and did not appear part of as intricate a defense strategy as that against the giant hornets.

Dr. Gene E. Robinson, an entomologist at the University of Illinois at Urbana-Champaign who specializes in bees, said the Japanese honeybee behavior was extraordinary and deserved more study. "Group defense among insects is common, but thermal group defense is unique," he said. Dr. Robinson said researchers should look to see if elevations in body temperature are associated with other defensive behavior in insects, which could be the origin of a response that eventually led to using heat in a coordinated attack.

Defense against mass attack by the giant hornets is mainly observed in the autumn, when nests of hornets produce hundreds of new queens and males that require large amounts of protein. Researchers say this food pressure may force the hornets into intensive foraging leading to the gang attacks.