A mob of Cyprian honeybees will form a ball around an intruding hornet to suffocate this arch-enemy to death (Image: Emmanouil Filippou)

Video: A mob of Cyprian honeybees form a ball around a hornet to suffocate this arch-enemy (Credit: Papachristoforou et al.)

Certain honeybees may suffocate an enemy insect to death, new research suggests.

Cyprian honeybees will swarm together around a threatening hornet, forming a tightly-packed ball that kills the would-be-invader, but exactly how this happened was unknown. Now – by a process of elimination – a study suggests that the honeybees kill by depriving their enemy of oxygen.


Hornets are particularly nasty predators and will chop off the head of a bee before carrying it back to their nest to eat. Often greedy hornets will go so far as to invade the honeybee hive to obtain food such as larvae. “They can slaughter whole colonies in a few hours,” says Alexandros Papachristoforou of the Aristotle University of Thessaloniki in Greece.

After a particularly dry summer the bees can become so weak from lack of nutrition they become unable to defend themselves against an attack. In such years, some beekeepers loose 30% of their colonies to hornet invasions, says Papachristoforou

Honeybee roast

Beekeepers have long observed that when bees guarding a hive are in good health they will instantly form a lemon-sized ball of about 150-300 bees to engulf any hornet that ventures too close to the entrance. But the bees cannot penetrate the hornet’s tough armour with their stings, so exactly how the insects killed the hornets remained a mystery.

In the mid-1990s, researchers used thermal imaging equipment and showed that a hornet caught in a ball of Japanese honeybees gets roasted to death by the bees’ body heat.

So Papachristoforou and his colleagues assumed that the Cyprian honeybee Apis mellifera cypria similarly killed its arch-enemy the Oriental hornet Vespa orientalis.

However, when the team poked a thermal probe into a ball of Cyprian bees holding a hornet, they found that the temperature (44° C) was lower than that measured inside balls of Japanese honeybees (49° C).

Oxygen deprivation

Moreover, the temperature inside the ball of Cyprian honeybees was too cool to kill their particular hornet predator, which dies at 50° C.

This led Papachristoforou and his teammates to conclude that Cyprian honeybees use another method to murder their enemy: suffocation. The ball of bees forms in a matter of seconds and traps the wasp inside, depriving it of oxygen for about an hour until it dies.

“To a human being it all looks the same,” comments Stan Schneider, who studies bee behaviour at the University of North Carolina in Charlotte, North Carolina, US. “But it’s interesting that there is this other way that the balling behaviour kills the hornets.”

Exactly how the bees coordinate to form the ball remains unclear, although Papachristoforou suspects the insects might use chemical signals known as alarm pheromones.

Journal reference: Current Biology (vol 17, R795-R796)