Giant hornets, which were detected in North America for the first time last year, can devastate colonies of the most common type of honey bee. But in Asia, where the fearsome predators are native, honey bees mount a vigorous defense with intimidating behaviors and coordinated counterstrikes. Now, researchers have discovered that Asian bees employ another, surprising defense strategy: To repel hornets, they daub their hives with the feces of other animals, which some scientists consider a form of tool use.
“For a bee to do that is incredible,” says Susan Cobey, a bee biologist at Washington State University, Pullman, who was not involved. “It just floored me.” It’s hardly the behavior you’d expect from a social insect famous for hygiene, says Heather Mattila, a behavioral ecologist at Wellesley College, who led the new research. “The thought of honey bees walking around in feces is just shocking,” she says. Bees work hard to keep their hives clean and to remove parasites and pathogens from their colonies.
Giant hornets hunt many insects, but the European honey bee (Apis mellifera) is easy picking. The hornets, which can be up to 4.5 centimeters long, hover above a hive and bite off the heads of the bees with their strong jaws, taking the meaty bodies back to their own colony. One species—Vespa mandarinia, aka the murder hornet—marks a colony for invasion by rubbing the hive with its scent glands. A pack of hornets then picks off defending bees and chews on the hive to enlarge the entrance. If hornets penetrate the hive, the bees flee, leaving behind their food and larvae, which the hornets feed to their own young.
Asian honey bees (A. cerana), which coevolved with numerous species of giant hornet, don’t give in easily, however. Unlike highly bred European honey bees, which slowly cruise in and out of the hive, speedy Asian honey bees swirl and zigzag to avoid capture. When guard bees spot a hornet, they hiss a warning and try to confuse the predator by shimmering their wings. The rest retreat inside. If a hornet manages to get in, masses of bees surround it. They buzz their wings to nearly double their body temperature, creating a “heat ball” that can kill a hornet.
The research has its roots in observations by co-author Gard Otis, now retired from Guelph University. While working in Vietnam he noticed that some A. cerana hives had small spots dotted around the entrance, something not seen in North America. Beekeepers told him the bees added spots whenever giant hornets arrived; one noted he’d seen his bees collecting bits of water buffalo dung. To find out more, Mattila organized a research trip. “Every day we were making major progress and discoveries,” she recalls.
Mattila sat in pigsties and chicken coops to confirm that honey bees were landing in poop. Then the researchers collected the dung of various kinds of animals from neighboring farms. They placed samples around the hives, captured bees on the dung, and painted their backs to confirm that those bees were bringing it back to the hives. They saw bees return to the same spot over and over, grasping the material and balling it up. “It’s like a flea market; they’re sorting through the top of it, pulling at it,” she says. “There was a real purpose in the search.”
The team confirmed bees were adding the spots by cleaning the outside of some hives and videotaping the bees redaubing feces. But were the bees doing this because of hornets? To find out, the researchers cleaned more colonies and guarded some against hornets, waving them away with plastic bags tied to long sticks. The bees in the colonies harassed by hornets added more dung than those in protected colonies, the researchers report today in PLOS ONE. By dabbing the hives with scents from different species of hornet, they discovered the bees applied the dung in response to hornets that invade their hives (V. soror), but not a species that only hunts foragers (V. velutina).
Finally, the team studied an invasion in detail by enlarging a hive entrance, which allowed the hornets to take it over within 1 hour. Several invaders were heat-balled by the Asian bees. The scientists also videotaped about 300 attacks and found that the hornets approached heavily spotted hives hesitantly, touching down briefly and then flitting away. If the colony was only lightly spotted, they landed and began to chew at the entrance.
Mattila suspects there’s something in the dung that repels the hornet. The bees might also use the dung to mask the scent that hornets use to mark a hive for a mass attack. But that seems less likely, Mattila says, because the hornets mark the hive in various places and the bees concentrate the dung at the entrance.
The murder hornet was recently detected in Washington, where entomologists are trying to eradicate it. The new research won’t help protect colonies immediately. “The last thing we want to do is spray colonies with watery feces and hope it helps with murder hornets,” Mattila says. But if researchers can identify the chemical that repels the hornets, they might be able to help defend European honey bees.
The researchers argue the bees are using the dung as a tool, because they’re gathering a foreign material and manipulating it for a specific purpose. “It’s a broader definition of tool than I had originally thought, but it makes sense to me,” says Tom Seeley, a behavioral biologist at Cornell University who was not involved in the work. (He notes that bees gather tree resin to disinfect their hives, which would also qualify as a tool.) Cobey calls the new paper a milestone. “I’ve spent most of my career working with honey bees and they continue to amaze me, but this is over the top.”