Researchers discover honeybees stick animal dung around the entrances to their hives to protect against the predators
The Asian giant hornet, native to southeastern Asia, is a serious honeybee predator.
Recent sightings of what is also called the murder hornet in British Columbia and Washington state have caused alarm.
The hornet is about 45 millimetres long, has a wing span around 75 mm, and a stinger of about six mm. They can deliver about seven times as much venom in a single sting as a honeybee.
The hornets raid honeybee nests, kill the bees and carry off the larvae and pupae to feed their own developing brood.
Now, recent research by scientists at the University of Guelph, Ont., has revealed a unique behaviour advanced by the Asian honeybees to ward off the predator. The bees stick spots of animal dung around the entrance to their hives. In response, the hornets spend less time and do less chewing at a hive and are less likely to launch mass attacks on the honeybees living there.
Gard Otis, professor of behavioural ecology and apiculture at the School of Environmental Studies, has studied honeybees in Vietnam for decades.
During his encounters with Vietnamese beekeepers, it became clear that at least 50 percent of the hives he saw had spots around the entrances.
One farmer confirmed to him it was buffalo dung as he had watched the bees collect the material and then place it on the hive. It was a first-time example of bees using a tool, the dung, as a defence against hornet attacks.
With US$25,000 in funding from National Geographic, Otis put together a research team and began the study in 2013.
They gathered dung from water buffalo, chickens, pigs and cows and then placed it in mounds near an apiary. By day’s end, some 150 bees had visited the piles, collecting more odoriferous manure from the pigs and chickens. Individual bees were marked for identification at their hives. Minutes later, the marked bees were recorded on video as they applied the material to the nest entrances.
They set up an experiment and allowed the hornets to visit certain hives and not others.
On the hives where researchers did not allow hornets to enter, they were chased away by team members wielding plastic bags on sticks. This strategy allowed for a control and an experiment in which they could count all the spots at the end of each day.
“The spots appeared in pretty large numbers on the hives the hornets visited and none where they did not,” said Otis.
The video data showed the hornets spent less than half as much time at nest entrances with moderate to heavy dung spotting as they did at hives with few spots. As well, they spent only one-10th as much time chewing at the hive entrances to get at the bees’ brood.
Exactly what deterred the hornets was not clear, but it was surmised it was odour. In addition, the dung may have masked odours emitted by the bees.
“In Asia, there are some 23 species of hornets and of those six to eight are good bee predators,” Otis said. “There are two species (Vespa sorer and Vespa Mandarina) that are giant hornets. When they find a rich source of food like a hive of bees, the scout that finds it will return to her nest, alert her nest mates and recruit them to return and attack the bees.”
How this plays out with Canadian honeybees and the possible threat of Asian giant hornets is a key issue facing Canadian beekeepers.
Honeybees (Apis mellifera) have not developed natural defence tactics against the hornets. Consequently, North American beekeepers will have to rely on locating and destroying Asian giant hornet nests to protect their bees.
Otis conducted the research with lead author Heather Matilla, now with Wellesley College in Massachusetts.
Co-authors were U of G grad students Hanh Pham and Olivia Knight, as well as Ngoc Pham and Lien Nguyen in Vietnam.
The research paper was recently published in the journal PLOS ONE. All the field research was conducted in Vietnam.