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Honey Bee Defense Leaves Hornets Breathless

17 September 2007 (All day)
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Emmanouil Filippou

Buzz kill.
Cyprian honey bees swarm an Oriental hornet to weigh down its abdomen and prevent it from breathing.

Faced with a giant marauding hornet, Cyprian honey bees swarm the insect, killing it by literally squeezing its breath away, according to a new study. The behavior takes advantage of the hornet's vulnerable mode of respiration and represents a unique form of defense among insects.

Bees can defend themselves with more than their stingers. Certain subspecies, such as Japanese honey bees, guard against hornets by "thermoballing." Tens to hundreds of bees surround the predator and vibrate their muscles, heating the hornet to a deadly 45°C.

That trick won't work on Oriental hornets (Vespa orientalis), which are native to Cyprus and other warm locales, such as Saudi Arabia, and thus tolerate heat. Yet Cyprian bees (Apis mellifera cypria) still manage to kill the insects by mobbing them. The phenomenon led entomologist Alexandros Papachristoforou of Aristotle University in Thessaloniki, Greece, and his colleagues to wonder if the bee swarm might somehow suffocate the hornet.

Hornets breathe through holes in their abdomens called spiracles. They contract their abdominal muscles to exhale and relax the muscles to inhale. As the insects exhale, the plates of their exoskeleton gradually cover the spiracles, and hornets can only open the plates back up to uncover the holes when their abdomens are free to move. Papachristoforou's team wondered if the sheer weight of the bee ball might keep the plates closed, preventing the hornets from sucking in fresh air. To find out, the researchers fit hornets with plastic blocks designed to wedge open the spiracle covers, allowing some air to move back and forth no matter how much insect weight was on the hornets' backs. It took the bees an hour and a half--nearly 50% longer--to kill hornets whose spiracle covers were wedged open versus control animals without the wedges, the team reports in the 18 September issue of Current Biology.

It's a good example of an evolving arms race between predator and prey, says Papachristoforou. Cyprus is isolated, he notes, meaning that the Cyprian bees have likely not faced many invasions of new predators. That's allowed them to evolve tailored defenses that exploit the weaknesses of the native hornets, Papachristoforou says.

The results may prompt researchers to take a closer look at how other bee subspecies deal with their own native hornet predators, says Randall Hepburn, an entomologist at Rhodes University, in Grahamstown, South Africa.

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