Mobs of honeybees kill hornets by asphyxio-balling

Giant hornets are the honeybee's arch enemy. They enter nests, kill the bees and take them home to feed their young. Before leaving the nest, the pioneer foraging hornet secretes a hormone which attracts its nestmates. Other hornets then congregate at the nest, and attack it en masse. In this way, several dozen hornets can wipe out a colony of tens of thousands of bees in a matter of hours. 

Bees normally fend off predators by stinging them. But this doesn't work with hornets, because a bee's stinger cannot penetrate a hornet's tough exoskeleton. Instead, the bees have adopted another strategy to kill foraging hornets.

The strategy is called thermo-balling. The bees detect the hornet's marking pheromone, and quickly increase the numbers of defenders at the entrance to the nest. Then, up to 500 bees engulf the hornet in a ball. Vibrations produced by the mob of bees increases the temperature inside the ball to 47 degrees Celcius, which is lethal to the hornet.

The offense of the hornet and the defence of the bee co-evolved. However, those species of hornet that live in warmer climes (such as the oriental hornet, Vespa orientalis), can tolerate relatively high temperatures, and are therefore resistant to thermo-balling. But new research, published yesterday in Current Biology, shows that at least one bee species can kill oriental hornets in a different way.

Alexandros Papachristoforou of the Aristotle University in Thessaloniki, have found that the Cyprian honeybee (Apis mellifera cypria) uses an alternative strategy to kill invading hornets. Like thermo-balling, it involves the co-operative actions of dozens or hundred of bees, which mob the invader and smother it to death. 

Like other insects, hornets breathe through small holes in the exoskeleton called spiracles. When closed, the spiracles are covered by exoskeletal plates called tergites. Breathing requires movements of the abdomen; relaxation of a set of abdominal muscles leads to inhalation, and contraction of the same muscles leads to exhalation.

Papachristoforou's team had observed Cyprian honeybees killing oriental hornets by engulfing them, but knew that the bees weren't heating the hornets to death. They also noticed that the bees aim for the abdomen when mobbing hornets, and so wondered if the mobbing might be affecting the hornets' ability to breathe.

First, they measured hornets' respiration rates after covering either 2 or 4 of the tergites (and thus preventing the spiracles from opening).They found that this led, respectively, to a reduction in respiration by about 33 and 90%. Then, the tergites were held open using small pieces of plastic. When gangs of  honeybees were set upon the hornets, the bees took twice as long as they normally would to kill the hornets.

From the results obtained, it seemed clear that the bees suffocate oriental hornets when engulfing them. Hence, Papachristoforou and his colleagues named the behaviour asphyxio-balling. (Watch a film clip of the process at YouTube.) When an oriental hornet is asphyxio-balled by a mob of Cyprian bees, its abdomen is crushed. The weight of the honeybees prevents the abdominal movements necessary for breathing. As a result, the tergites covering the spiracles cannot open and close, and the hornet suffocates.

References:

Papachristoforou, A., et al. (2007). Smothered to death: Hornets asphyxiated by honeybees Curr. Biol. 17: R795-R796. [Abstract]

Ono, M., et al. (1995). Unusual thermal defence by a honeybee against mass attack by hornets. Nature 377: 334-336. [Abstract]