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Enemy hornets suffocate within honeybees' ball: heat may boost vulnerability to carbon dioxide increase.

Call it death by a thousand breaths. When hundreds of honeybees envelop a giant, predatory hornet in a ball, the bees aren't just putting on the heat, as researchers had thought. Carbon dioxide levels spike along with temperature, fingering suffocation as the hornet's cause of death, scientists report in an upcoming Naturwissenschaften.

Bees inside the bail can apparently cope with the smothering heat and low oxygen levels, but the high temperature appears to make giant hornets, Vespa mandarinia japonica, less tolerant of cranked-up carbon dioxide levels. The concentration of C[O.sub.2] in the bee ball--enclosed air increases to about 3.6 percent after the ball forms, dropping sharply to lower levels five minutes later, report Michio Sugahara and Fumio Sakamoto, both of Kyoto Gakuen University in Japan.

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The researchers taped anesthetized giant hornets to gas detectors and thermometer probes to measure C[O.sub.2] concentrations and temperatures inside bee balls. When the probes touched open bee nests, the bees formed balls around the hornets. All 24 test hornets died within 10 minutes of bee ball formation, the team reports. Hornets bore no sign of stings, pointing to smothering as the cause of death.

The spike in C[O.sub.2] might be just a metabolic by-product of the frenetic activity of the bees. But, says Stan Schneider of the University of North Carolina at Charlotte, bees might regulate this "panting," perhaps in response to odor or behavioral cues from the giant hornets. "The specificity of the behavior suggests a very long coevolution in this predatorprey relationship," Schneider says.

Although ball forming is unusual among bee species, coordinated defensive behavior is not, says entomologist P. Kirk Visscher of the University of California, Riverside. Giant honeybees form rippling waves en masse, startling predators (SN: 10/11/08, p. 10). There are even bees that mount a collective attack by yanking individual hairs on the enemy's body. "It's not like being stung by a swarm, but it is still pretty annoying," Visscher says.

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Author:Ehrenberg, Rachel
Publication:Science News
Article Type:Brief article
Geographic Code:1USA
Date:Aug 1, 2009
Words:335
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