Bees surround the invading hornet and roast it alive Takahashi/Wikipedia Commons

When hornets attack, bees know what to do. A few hundred workers can swarm into balls around hornets and roast them alive with their body heat. The formation of such “hot defensive bee balls” was first described in 1995 in Japanese honeybees. Now we know the defence is something of a kamikaze mission for the bees involved.

When hornets attack a hive to carry off bees to eat, a group of worker bees quickly surround the intruder. The bees vibrate their wing muscles to generate temperatures of about 46oC for more than 30 minutes, enough to kill the hornets. It’s crucial they deploy the balls quickly, otherwise the hornet releases pheromones that attracts reinforcements.

Entomologist Atsushi Ugajin at Tamagawa University near Tokyo began wondering about the costs to the honeybees. He wondered if heat exposure in the balls might reduce their life expectancy.


To find out, he and his colleagues marked about 350 Japanese honeybee workers with colours to record their age in days. Then they divided a batch of bees that were 15-20 days old into two groups, one of which was allowed to form hot balls and one of which was kept in the hive at 32o C. Workers typically live for several weeks. The bees that avoided the hot balls were all dead 16 days after the ball, but the ones that took part were all dead within 10 days.

Attack, roast, repeat

But what happens when another hornet inevitably attacks? Hornet often attack hives 30 times a week in the autumn. So Ugajin performed another experiment, exposing the bees to a second hornet attack. It turned out that battle-hardened bees that had joined in the first ball were more likely to help out in a second ball.

Exactly what drives this kamikaze behaviour isn’t clear, says Randolf Menzel, who studies bees at the Free University of Berlin. It would be interesting to find out, for example, if the heat damages their brains and disinhibits them, he says. Whatever the case, it’s good for the colony overall because the heat-exposed bees already have their days numbered.

The hot balls are not the only defence honeybees can deploy. Some species form cooler balls that suffocate hornets while other species dance in waves, producing a shimmer of light that warns hornets to get lost.

Giant hornets have recently been introduced accidentally to Europe. Unfortunately, European honeybees have not developed any of these defensive strategies. So bee keepers are experimenting with all sorts of devices to deter the hornets, including meshes, sticky patches and flashing lights on the hives, says Menzel.

Journal reference: Behavioral Ecology and Sociobiology, DOI: 10.1007/s00265-018-2545-z

Read more: Bumblebees solve the travelling salesman problem on the fly; Bees are first insects shown to understand the concept of zero; Honeybees let out a ‘whoop’ when they bump into each other