Image courtesy A. Ugajin et al, PLoS ONE

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A bee swarm forms a hot defensive bee ball around a giant hornet hanging from a wire in an experiment.

Image courtesy A. Ugajin et al, PLoS ONE

"Hot Bee Balls" Cook Enemy Hornets—But How Do Bees Endure the Heat?

Genetic boost may tell bees when to stop attack, study finds.

In a battle with Asian giant hornets, Japanese honeybees turn up the heat—quite literally—by swarming around the hornets and cooking them to death.

Now scientists have found a genetic switch in the honeybees' brains that turns on during the attacks. Though the gene activity doesn't seem to trigger the behavior, it might provide leads in explaining the insects' ability to withstand the heat of so called hot defensive bee balls, a new study says.

The bees' strange defensive tactic evolved because their venomous stingers are too small to pierce the thick exoskeletons of the giant hornets—insects which can grow about two inches (five centimeters) long. The quivering of muscle fibers from so many bees creates real heat that kills off the predators.

European honeybee hives, common in the U.S. and Europe, are sitting ducks against the hornets' merciless raids, scientists say.

"These hornets go and rob honeybee colonies and the [European] bees have no chance. They can't sting through them. These hornets can bring down entire colonies," said Wulfi Gronenberg, a neuroscientist at the University of Arizona who was not involved in the new study.

Bee-Ball Genetics Probed

Researchers already knew hundreds of buzzing bees can warm the centers of "hot defensive bee balls" to about 117 degrees Fahrenheit (47 degrees Celsius) for an hour without dying, according to the study. But scientists weren't certain how the insects' bodies pull off the stunt.

Lacking the Japanese bees' cooking skills, a group of Japanese scientists looked into honeybees' genetic code and brain structure.

They started by heating honeybees up to bee-ball temperatures and looking for unusual genetic activity in the bees' brains. The team found that at least one immediate early gene—a kind of gene that polices the function of many other genes—became much more active within the brains of Japanese honeybees compared with those of European honeybees.

Specifically, the team traced the boosted genetic activity to a region of the honeybee's brain called the mushroom body. (Get a genetics overview.)

"It's a pretty large area involved in learning; memory; and processing light, taste, smell and all kinds of other sensory cues," Gronenberg said.

In a second experiment, the scientists staged a hornet attack on Japanese honeybees and examined the bees' brains and genetics. Those bees, too, showed increased activity in the same regions of the mushroom body.

Bees' Genetic Kick May Stop Attack?

The gene kicks on only minutes after a hot, defensive swarm has begun, so it's too early to tell what it does, Gronenburg said. But it's almost certain that the genetic boost plays an important role in bee-ball behavior.

"This cannot be the cause for forming the bee ball. The authors were careful not to interpret this too much," he said.

"But it may be involved in perceiving the heat. It could be a sort of timer, a signal that tells the honeybees when to stop [a bee-ball attack]."

Hot bee-ball study appeared March 14 in the journal PLoS ONE.