Balls of fire: bees carefully cook invaders to death.

Honeybees that defend their colonies by killing wasps with body heat come within 5[degrees]C of cooking themselves in the process, according to a study in China.

At least two species of honeybees there, the native Apis cerana and the introduced European honeybee, Apis mellifera, engulf a wasp in a living ball of defenders and heat the predator to death. A new study of heat balling has described a margin of safety for the defending bees, says Tan Ken of Yunnan Agricultural University in Kunming, China.

He and his team also report in an upcoming issue of Naturwissenschaften that the native bees have heat-balling tricks that the European bees don't. That makes sense, the researchers say, since the Asian bees have long shared their range with the attacker wasp Vespa velutina, but the European bees became widespread in Asia only some 50 years ago and so have had much less time to adapt to the wasp.

The attacker wasps are "gigantic," says Thomas Seeley of Cornell University, who studies bee behavior. Of all social insects, the species has the largest workers, with wingspans that can stretch 5 centimeters. The wasps build large versions of the papery nests of hornets found in North America, and they specialize in breaking into other social-insect nests and carrying off larvae as food for young wasps.

"I've seen a single wasp overwhelm a colony of 6,000 bees" of a species that doesn't make heat balls, says Seeley. The invader wasp stands at the nest's entrance as one guard bee after another comes out to defend its home. "The wasp cuts the guard into pieces ... and waits for the next one," says Seeley. When all the defenders are dead, "the wasps strip-mine out the larvae," he reports.

However, a few honeybee species can defend themselves by surrounding an invader. Researchers used to think that the few-dozen bees were trying to sting the wasp, says Seeley. Thermal cameras, however, revealed the balls' soaring heat.

To see what margin of safety the bees have, Tan and his research colleagues presented tethered wasps to six colonies each of native Asian bees and European bees. At each nest, worker bees engulfed the wasp immediately. Within 5 minutes, the center of a typical bee ball had reached 45[degrees]C.

To check the bees' and wasps' tolerance for heat, researchers then caged each of the species in incubators and systematically cranked up the temperature. The wasps died at 45.7[degrees]C, but the Asian honeybees survived heat to 50.7[degrees]C and the European bees made it to 51.8[degrees]C.

The native Asian bees, ancient adversaries of the wasps, mobilized half again as many defenders into a heat ball as the European bees did, the researchers report. Furthermore, Asian bees not mobbing the wasp were more likely to take shelter during an attack than bystander European bees were.

Heat balling is the flip side of bees nursing larvae in a nest, says Seeley. To keep the youngsters at the right temperature in cool weather, honeybees space themselves around the nursery and shiver their powerful flight muscles to generate heat. Seeley notes, however, that the nursemaids don't raise the temperature above 36[degrees]C, so the brood stays safe.