The poor European honeybee, deliberately imported to Japan about 120 years ago and very popular with beekeepers there, still hasn’t figured out how to deal with vicious surprise attacks by masses of giant Japanese hornets. A foraging hornet marks the target nest with a pheromone--a chemical attractant that draws its own nest mates--and the slaughter begins. A single hornet, 3 times the size and 20 times the weight of a worker bee, can kill up to 40 bees a minute with its powerful jaws; a group of 20 hornets can wipe out an entire colony of 30,000 bees in about three hours. Stinging is not effective against the attack of the giant hornet, says entomologist Masato Ono of Tamagawa University in Tokyo. European honeybees try to defend their colony by stinging, but they are soon annihilated. More effective, Ono has discovered, is a unique strategy adopted by the Japanese honeybee, which has evolved alongside the hornets: death by baking.
When Japanese honeybees detect hornet pheromone, about 100 worker bees flock to the nest entrance. As the hornets approach, the bees retreat back into the nest, drawing the hornets inside, where over a thousand worker bees are lying in ambush. About 500 of them quickly engulf one of the invaders in a dense ball. Vibrating wing muscles inside their thoraxes, the bees raise their body temperature, and the temperature inside the bee ball quickly rises to 116 degrees. Bees tolerate temperatures of up to 122 degrees, but hornets perish at 114. After baking for about 15 minutes, the hornet dies. If the bees (which don’t eat the hornets) succeed in killing the first recruiter hornets, they stave off a swarming attack. If they fail, the bees abandon their nest and stream off to build a new one.
Ono thinks the bees’ defense strategy evolved along with the mass attack strategy of the hornets. This is an excellent example of prey- predator coevolution, he says. During fall, after their usual meals of beetle or moth larvae have become scarce, the hornets still need to procure large amounts of protein for young queens and reproductive males being reared in their nests. The only protein source is the nests of other, lesser wasps and bees, even though the attack is risky, Ono explains. This predaceous pressure has led to the evolution of an effective defensive strategy in the Japanese honeybee.