Mobs of honeybees suffocate hornets to death

Earlier today, I published a post on how Japanese honeybees defend themselves from hornets with a mass defence that relies on heat and carbon dioxide. This article was originally written two years ago, and describes the slightly different tactic of Cyprian honeybees.

When Oriental hornets attack, Cyprian honeybees mob them in a huge ball that targets the breathing apparatus in the hornet's abdomen. The hornets can't breathe without expanding their abdomens and with sheer numbers, the bees strangle the hornets to death.

Hornets are giant wasps that pack a powerful sting. To most people, they can be a painful nuisance, but to honeybees, they're killing machines. Hornets greatly outsize and overpower honeybees and a few individuals can decimate entire colonies.

Asian honeybees have developed a remarkable defence called 'heat-balling' against their local hornet, Vespa velutina. A giant ball of bees piles onto the predator, weighing it down while vibrating their wing muscles. The frenetic activity greatly increases the temperature inside the ball to about 46C - hot enough to cook the hornet alive, but five degrees under the bees' maximum tolerated temperature.

Cyprian honeybees face a different predator, the Oriental hornet Vespa orientalis and unlike its wimpier cousin V.velutina, this species can take the heat. The Oriental hornet lives in hot, dry climates ranging from Central Asia to the Mediterrenean and it tolerates temperatures just as high as honeybees.

Heat-balling shouldn't work on them. And yet, Cyprian bees still encase Oriental hornets in large balls. Surprisingly, the strategy works - despite their heat tolerance, the hornets still die. The bees' stings are useless against the hornet's tough cuticle and they barely use them. What could they be doing instead?

Alexandros Papachristoforou and colleagues from the Aristotle University of Thessaloniki noticed that when the bees mob the hornet, they go for its back end first. That seems like an unlikely spot for it's where the powerful sting sits.

But it's also the location of the hornets' Achilles heel - it's breathing apparatus. By mobbing its abdomen, the bees effectively strangle the hornet to death. (Have a look at this video of a this tactic at work - the hornet's hapless head sticks out of a ball of bees).

The hornets' entire abdomen is the equivalent of a human lung and they breathe by actively pumping air through small holes located there, called spiracles. They exhale by contracting muscles that pull the abdominal segments together, forcing air out through the spiracles. When these muscles relax, the segments spread out, the abdomen expands and air is sucked in.

When the abdomen is fully contracted, the spiracle is covered by a plate called a tergite. That stops the hornet from losing too much air, but it also gives the honeybees a window. Papchristoforou theorised that by piling onto the hornet, the bees prevented it from expanding its abdomen. The tergites prevent air from entering the spiracle and the hornet chokes.

When Papachristoforou covered these tergites manually, he found that the hornets' respiration fell by 87%. To prove that this was what the bees were doing, he inserted plastic blocks under the tergites of some hornets (right), so they couldn't properly cover the spiracles. Honeybees took twice as long to kill these hornets as individuals who hadn't been similarly protected.

The increased temperature inside the ball isn't enough to finish the hornets off, but the researchers believe that it probably helps to speed things along. The extra heat increases the concentration of carbon dioxide in the hornet's blood, anaesthetising it and making things easier for the surrounding bees.

Nonetheless, the Cyprian bees' main strategy is to suffocate hornets rather than bake them. Papachristoforou has called this strategy 'asphyxia-balling' and it's a fantastic example of an adaptive defence in the face of a super-predator.

Reference: Papachristoforou, Rortais, Zafeiridou, Theophilidis, Garnery, Thrasyvoulou & Arnold. 2007. Smothered to death: Hornets asphyxiated by honeybees. Curr Biol 17: R795-796