Asian honey bees paste pellets of animal poo on to their nests to repel attacks by giant killer hornets, scientists have revealed.
The attacks can involve dozens of the heavily armoured hornets and lead to the “mass slaughter” of thousands of bees, the researchers said, after which the hornets carry off the bee larvae to feed their own offspring. But in a continuing evolutionary arms race, the bees have developed defence mechanisms such as hissing at them or mobbing the hornets to suffocate them.
However, the newly discovered strategy using faeces shocked the scientists because bees are famously hygienic in order to prevent disease in their hives. The team do not yet know exactly why the poo pellets are particularly repulsive.
The giant hornets studied by the team are a very close relative of the “murder hornets” that have alarmed beekeepers in the US and Canada over the past year, while other giant hornets are regularly found and destroyed in the UK. Bees in Europe and North America have not evolved the defences, making them “sitting ducks”, the scientists said.
Prof Heather Mattila at Wellesley College in the US, who led the study, said: “I was shocked [by the use of faeces] because bees have such a good reputation for being clean. They have hot, wet, permanent homes that are a great place for disease to grow and are filled with babies and food.”
She said the arrival of giant hornets in North America in 2019 was very concerning. “The first time I heard about it, I was so worried I couldn’t sleep. I thought, ‘These bees are going to get massacred.’ They just don’t have the exposure to these hornet species and, as a result, they are sitting ducks.”
Matt Shardlow, at Buglife, said the bees’ use of faecal pellets was “an amazingly sophisticated defence mechanism” showing that surviving predation is a complex task. He said there was no guarantee that native bees could develop defences to hornets, even over thousands of years: “It is a clear illustration of why we must try to slow the spate of invasive species we are instigating.”
The research, published in the journal Plos One, began when Prof Gard Otis, at the University of Guelph, Canada, and one of the study team asked a beekeeper in Vietnam what the spots around the hive entrances were. The reply: buffalo dung.
The team found that the bees also collected faeces from livestock. “We spent a lot of time hanging out in nearby farms. I sat with pigs and a chicken coop and finally it paid off.” They then conducted experiments that showed the bees only spotted their hives after visits from predatory hornets.
They found these hornets spent less than half the time at nest entrances with dung spotting, compared with clean hives, and 94% less time trying to chew their way in. A final experiment discovered that hives daubed with the secretions that giant hornets use to flag nests for attack were quickly plastered with poo.
After contacting other bee experts, the researchers learned that the spotting behaviour is widespread throughout Vietnam and reported in China, Thailand, Bhutan, and Nepal. Some species of stingless bee were known to collect animal faeces and incorporate it into their nests, but this was the first report for honey bees.
Why the dung repels the hornets is as yet unknown, but other insects such as the caterpillar of the tobacco hornworm moth are known to cover themselves in poo to deter predators. The researchers said the deterrence may simply be because it is faeces, and seen as unclean, or it may be due to substances present in the animals’ food. Another possibility is that the poo pellets are olfactory camouflage for the nest, masking the scents the hornets use to mark target nests.
Mattila said the discovery may also be the first clear example of honey bee tool use. “It gets down to semantics, but the fact that these bees are collecting something from the environment, holding it, manipulating it and changing the character of the thing that they’re applying it to, makes it a tool by virtually every definition.”
During the research, the honey bees were not the only ones at risk from the hornets, which pack about seven times as much venom in a single sting as a honeybee. “I got stung by one and it was the most excruciating sting in my life,” said Otis.