The Asian Giant Hornet

One of the marvels of evolution is the Asian giant hornet, a predatory wasp especially common in Japan. It’s hard to imagine a more frightening insect. The world’s largest hornet, it is as long as your thumb, with a two-inch body bedecked with menacing orange and black stripes. It’s armed with fearsome jaws to clasp and kill its insect prey, and also a quarter-inch stinger that proves lethal to several dozen Asians a year. And with a 3-inch wingspan, it can fly 25 miles per hour (far faster than you can run), and can cover 60 miles in a single day.

This hornet is not only ferocious, but voracious. Its young larval grubs are fat, insatiable eating machines, who insistently rap their heads against the hive to signal their hunger for meat. To satisfy their relentless demands for food, adult hornets raid the nests of social bees and wasps.

One of the hornet’s prime victims is the introduced European honeybee. The raid on a honeybee nest involves a merciless mass slaughter that has few parallels in nature. It starts when a lone hornet scout finds a nest. With its abdomen, the scout marks the nest for doom, placing a drop of pheromone near the entrance of the bee colony. Alerted by this mark, the scout’s nestmates descend on the spot, a group of twenty or thirty hornets arrayed against a colony of up to thirty thousand honeybees.

But it’s no contest. Wading into the hive with jaws slashing, the hornets decapitate the bees one by one. With each hornet making bee heads roll at a rate of forty per minute, the battle is over in a few hours: every bee is dead, and body parts litter the hive. Then the hornets stock their larder. Over the next week, they systematically ravage the nest, eating honey and carrying the helpless bee grubs back to their own nests, where they are promptly deposited into the gaping mouths of the hornets’ own ravenous offspring.

This is nature red in tooth and claw, as the poet Tennyson described. The hornets are fearsome hunting machines, and the introduced bees are defenseless. But there are bees that can fight off the giant hornet: honeybees that are native to Japan. And their defense is stunning— another marvel of adaptive behavior. When the hornet scout first arrives at their hive, the honeybees near the entrance rush into the hive, calling nestmates to arms while luring the hornet inside. In the meantime, hundreds of worker bees assemble inside the entrance. Once the hornet is inside, it is mobbed and covered by a tight ball of bees. Vibrating their abdomens, the bees quickly raise the temperature inside the ball to about 47 degrees C. Bees can survive this temperature, but the hornet cannot. In twenty minutes the hornet scout is cooked to death, and—usually—the nest is saved. I can’t think of another case (save the Spanish Inquisition) in which animals kill their enemies by roasting them.

There are several evolutionary lessons in this twisted tale. The most obvious is that the hornet is marvelously adapted to kill—it looks as though it were designed for mass slaughter. Moreover, many traits work together to make the wasp a killing machine. They include body form (large size, stings, deadly jaws, big wings), chemicals (marking pheromones and deadly venom in the sting), and behavior (rapid flight, coordinated attacks on bee nests, and the larval “I am hungry” behavior that prompts the hornet attacks). And then there is the defense of the native honeybees—the coordinated swarming and subsequent roasting of their enemy—certainly an evolved response to repeated attacks by hornets. (Remember, this behavior is genetically encoded in a brain smaller than a pencil point.)

On the other hand, the recently introduced European honeybees are virtually defenseless against the hornet. This is exactly what we would expect, for those bees evolved in an area lacking giant predatory hornets, and therefore natural selection did not build a defense. We can predict, though, that if the hornets are sufficiently strong predators, the European bees will either die out (unless they are reintroduced), or will find their own evolutionary response to the hornets—and not necessarily the same one as the native bees.

European honeybees are defenseless against this predator, but Asian honeybees have evolved an amazing counter attack.