by Whit Gibbons

October 1, 2006

Who would win in a contest between Japanese giant hornets and honeybees? Both can sting, but neither stings the other. The hornet crushes the honeybee with its mandibles. The honeybee kills the hornet through overheating. The winner depends on the homeland of the bees.

The giant hornet is the only species of hornet known in which individuals gather together and then attack other social bees or wasps en masse, fascinating behavior displayed by an extraordinary predator. But one of its prey species doesn't take too well to being bullied.

The hornet's foraging strategy sounds like a four-phase military exercise. The first step in the giant hornet's feeding sequence is raiding. Hornets fly solo missions in search of nests of other social bees and wasps. Upon finding a bee's nest, the lone hornet crushes individual bees in its jaws. The dead bees are taken to the hornet nest to feed larvae. The solitary raider returns to the bee colony a few times to take additional bees.

Then comes phase two, recruitment. The hornet rubs secretions from a special gland onto the area surrounding the honeybee nest. The secretion is a pheromone, a chemical compound used in communicating specific messages to its own or other species of animals. Detecting a pheromone is like having a sixth sense for chemical awareness. And the chemicals deliver information. In this case the hornet's pheromone is a signal for other giant hornets to amass and attack.

Nestmates of the giant hornet flying in the area congregate as they sense the pheromone. Then they attack. The well-named slaughter phase is under way. As many as 40 European honeybees are killed per minute, and an attacking force of 20 to 30 hornets can kill 30,000 bees in three hours. When the hornets achieve such an impressive victory, they enter phase four: occupation of the bee nest. For more than a week the hornets carry bee larvae and pupae to their own nest as food for the hornet larvae.

If you like to see the underdog win occasionally, stay tuned. The attack-and-conquer approach in Japan works in favor of the marauding giant hornets when the prey is a colony of European honeybees. This is the same species found in America, where they were also introduced from Europe.

European honeybees seem to be virtually defenseless against mass hornet attacks, in part because they are oblivious to the impending onslaught. That is, they are unable to detect the hornet pheromone and so are unaware of the presence or plan of the hornets. But not all bees in Japan are the introduced European honeybee.

Native Japanese honeybees can detect the hornet pheromone, and they understand the message being sent, something akin to deciphering an enemy code. The bees modify their behavior by increasing the number of defenders at the nest. The first hornet to attack is greeted by a swarm of more than 500 bees, which form a large ball around the intruder. The ball of bees may stay intact for up to 20 minutes, with the inner temperature of the ball reaching 116 degrees F, which is lethal to the hornet. The bees can withstand temperatures up to 122 degrees, although some bees in the center die from hornet bites.

Giant hornet attacks occur in autumn when a surplus of food is needed to feed developing hornet larvae. Sometimes the Japanese honeybees do not defeat the hornet, if the surprise attack is carried out quickly, before the bees can mobilize. Or if the bees have a small colony. But even then, most adult bees usually escape and forfeit only the nest.

Giant hornet attacks and the differential response of nonnative and native honeybees are a fascinating example of coevolution, in which two species evolve in response to each other. The hornets have evolved a mechanism for acquiring large quantities of baby food in a short time. Meanwhile, the native Japanese honeybee evolved a counterstrategy, developing an effective defense against the predator. The introduced European honeybee did not evolve in a system requiring such a response. Hence, as a species they are essentially helpless against the warlike tactics of the giant hornet.

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