How Do Honey Bees Fight Giant Hornets? Bee Balls of Heat

How a giant hornet raid on a hive unfolds

The northern giant hornet (Vespa mandarinia) raids beehives mainly in late summer and autumn, when its colony is raising hundreds of new queens and males that need protein. The larvae and pupae inside a honey bee nest are an ideal food source.

A raid begins with a scout. A single hornet hovers near the entrance and snatches bees one at a time. If the colony looks worth attacking, the scout rubs a scent from a gland on its abdomen onto or near the hive, which guides nestmates to the spot. A group of a few to about 50 hornets then arrives for the slaughter phase, killing defending bees with their jaws. When resistance stops, the hornets occupy the hive and carry off the brood for days.

The key moment is the scout’s first visit. If the bees can kill the scout before it marks the hive and recruits others, the mass attack never happens.

The heat ball: how Japanese honey bees cook a hornet

The Japanese honey bee (Apis cerana japonica) is a subspecies of the eastern honey bee, which lives across much of southern and eastern Asia and evolved alongside giant hornets. In 1995 Japanese researchers reported in the journal Nature how it defends itself.

Japanese honey bees can detect the scent marks of a giant hornet and respond by gathering more guards at the nest entrance. When a hornet comes close, hundreds of workers, sometimes more than 500, rush out and engulf it, forming a dense ball with the hornet trapped in the middle. The bees then shiver their flight muscles without moving their wings, the same trick they use to keep the nest warm in winter.

Thermal cameras showed that the core of the ball heats up to about 46–47 °C (115–117 °F). The hornet cannot escape, because it is packed in by bees on every side.

Why the bees survive and the hornet dies

Heat alone is only part of the story. A 2009 study found that giant hornets can survive about 47 °C (117 °F) for 10 minutes in a laboratory, and that the temperature inside real bee balls peaked at about 45.9 °C (114.6 °F). The missing factor was carbon dioxide. As the bees work their muscles, carbon dioxide inside the ball rises to about 3.6%, and at that level the hornet’s lethal temperature falls to about 45–46 °C.

The bees are more tolerant and can survive about 50–51 °C (122–124 °F) under the same conditions. That small margin of a few degrees is what makes the defense work. Every hornet tested in the 2009 study died in less than 10 minutes. Later work suggests that some bees in the ball also sting the hornet, adding venom to the heat. A few bees may die, but the colony saves itself from a mass attack that could kill thousands. Honey bee workers often die after stinging mammals, a sacrifice explained in Do bees die after stinging?

Other ways Asian honey bees defend their hives

Heat balls are not the only weapon. When a hornet hovers near the entrance, guards of the eastern honey bee shake their abdomens from side to side and beat their wings more loudly as the hornet gets closer. Researchers have described this as an “I see you” signal, and hornets often give up and leave. Like the waggle dance, it is one of many signals in the bees’ rich communication.

Japanese honey bees also smear chewed plant material around the entrance after a hornet scouts the nest. In Vietnam, eastern honey bees collect spots of animal dung and place them around the entrance after attacks by the southern giant hornet (Vespa soror). A 2020 study found that these spots made hornets less likely to land and chew at the entrance, and the authors described it as the first known case of tool use by honey bees. The same behavior has been reported in China, Thailand, Bhutan and Nepal.

Why western honey bees lose to giant hornets

The western honey bee (Apis mellifera) evolved in Europe, Africa and western Asia, where there are no giant hornets. When beekeepers brought it to Japan, it turned out to be almost defenseless. Guards fly out to sting the hornets one by one, but a giant hornet is several times larger than a bee and heavily armored, and its jaws can kill bees quickly.

According to Animal Diversity Web, an attack by 20–30 giant hornets usually kills 5,000–25,000 western honey bees within one to six hours. Western honey bees sometimes try to ball an attacker, but they recruit fewer bees and produce less heat, so the tactic rarely stops a giant hornet. In Japan, beekeepers who keep western honey bees must protect their hives every autumn, and giant hornets are thought to be one reason why escaped western honey bee swarms rarely survive long in the wild there.

Western honey bees that fight back

Some western honey bee populations do have a defense against smaller hornets. On Cyprus, the local honey bee forms tight balls around the Oriental hornet (Vespa orientalis). A 2007 study found that these hornets died mainly because the bees blocked their breathing movements, suffocating them, rather than from heat.

In Europe, western honey bees face the yellow-legged Asian hornet, a smaller species that hunts bees in the air near hives. The bees sometimes ball these hornets, but their main response is to stop foraging while hornets are hawking outside, which can weaken colonies before winter.

What this means for beekeepers outside Asia

The giant hornets that reached the Pacific Northwest in 2019 alarmed beekeepers because North America’s managed bees are western honey bees with no evolved defense. Officials destroyed four nests in Washington State and declared the species eradicated from the US in December 2024, as told in Are murder hornets in the US?