November 21, 2016

The Asian giant hornet is so large, it is sometimes mistaken for a small bird in flight. This big predator is equipped with piercing jaws, a quarter-inch-long (half-centimeter-long) stinger loaded with deadly venom (poison), and an aggressive disposition. It keeps beekeepers up at night and is responsible for the deaths of dozens of people each year.

The Asian giant hornet is the largest of its kind. Credit: Yasunori Koide (licensed under CC BY-SA 4.0)

The Asian giant hornet (Vespa mandarinia) is the largest hornet in the world. Workers grow up to 1.75 inches (4.5 centimeters) long, and queens can grow even larger. They are found throughout Japan and Southeast Asia. Asian giant hornets make their nests in the ground or in hollow logs in wooded regions, but they also often venture into urban areas looking for food. These flying insects are extremely aggressive and quick to sting. Asian giant hornet venom is not more dangerous than the venom of other hornets or wasps, but these giants deliver more venom when they sting. And, because of the hornets’ violent swarming behavior, victims are often stung many times at once by many different hornets. Asian giant hornet venom destroys flesh and red blood cells and, if it is delivered in a large enough dose, can lead to cardiac arrest or kidney failure. In Japan alone, Asian giant hornets kill about 40 people and injure some 1,500 others each year.

Asian giant hornets prey on such large insects as beetles and praying mantises. After the victim is stung and killed, adult hornet workers take the insect carcasses back to the nests. There, the dinner is ground up and fed to larvae (young), which in turn produce a nutritious secretion for the workers. Also, adult hornets sometimes feed on tree sap or rotting fruit.

One remarkable behavior of the Asian giant hornet is its tendency to swarm and attack beehives to eat the honey. A scout hornet will locate a beehive and then lead groups of other giant hornets in an attack. The hornets then break into the hive and, using their powerful mandibles, sever the heads of the much smaller bees. Bees cannot sting through the hornets’ thick armor, so they are helpless against such an attack. An Asian giant hornet can kill a honeybee in seconds, so a few dozen hornets can completely destroy an entire hive of thousands of bees in a few hours.

Some kinds of Asian honeybees have developed a defense, however, to stop a hornet invasion before it happens. When an Asian giant hornet scout enters the hive, the bees swarm to it. The hornet may kill the first few defenders, but the bees quickly cover and immobilize (prevent from moving) the invader. Once the bees have trapped the scout, they begin to shiver their wings and bodies, generating heat. The trapped hornet eventually overheats and dies. This strategy costs the lives of many honeybees, both from being killed by the hornet and from overheating themselves. But their sacrifice can save the hive. The hornet scout doesn’t live to reveal the location of the hive, thus preventing an invasion.

Some environmentalists fear that the Asian giant hornet will invade other continents, particularly because of the uncertain effects of global warming. Isolated sightings have already been reported in the United States and Europe. Many of these sightings may be cases of mistaken identity, however, as these areas have their own large species (kinds) of wasps and hornets. Outside the Asian giant hornet’s native range, honeybees have not evolved (developed over time) the swarming behavior to defend against scouts. Non-Asian bees are at great risk, then, from Asian giant hornets. Many bee populations beyond Asia are already severely threatened by changes in land-use patterns and a mysterious disorder that causes bee colonies to collapse. In addition, a class of insecticides called neonicotinoids has already crippled honeybee populations. Invasions of Asian giant hornets could prove the end of honeybees in general.