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Hornet on a stickScientists taped giant hornets to probes to measure carbon dioxide concentrations and temperature inside bee balls. The results show that a spike in carbon dioxide, along with an increase in heat, makes honeybees' enemies vulnerable. M. Sugahara, F. Sakamoto

Call it death by a thousand breaths. When hundreds of honeybees envelop a giant, enemy 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 online and in an upcoming Naturwissenschaften.

Bees inside the ball 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. The concentration of CO2 in the air increases to 3.6 percent after the bee 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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Bee ball attackAll of the 24 hornets that stayed in bee balls (above) for 10 minutes died. Hornets removed after four minutes were alive but in critical condition. M. Sugahara, F. Sakamoto

The researchers taped anesthetized giant hornets to gas detectors and thermometer probes to measure carbon dioxide 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 CO2 might be just a metabolic byproduct of the frenetic activity of balling 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 predator-prey 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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