To protect their nests from the wrath of giant hornets, honeybees in Vietnam spread the dung of buffalo and other animals around the entrances to their hives.

According to a new paper, published Wednesday in the journal PLOS One, the scent of animal dung prevents deadly raids by teams of hungry hornets.

Though recognized by local beekeepers, the defense mechanism was, until recently, unknown to scientists.

While working as director of a beekeeping development project in Vietnam, researcher Gard Otis noticed many of the hives he visited were marked by dark spots.

"I had had experience with honey bees in Malaysia and Indonesia, and less experience in the Philippines, Thailand and Cambodia, and I had never seen hive spots before," Otis, co-author of the new study, told UPI in an email.

When Otis, an expert on honeybee behavior and a professor of ecology at the University of Guelph in Canada, asked the local beekeepers what the spots were, all of them replied "hornets."

"When I asked them what the material was, I got a variety of answers, but one of our most knowledgeable beekeepers answered, 'buffalo dung!'" Otis said. "When I asked him how he know that, he looked at me like I was stupid and said, 'Because I watched them collect it.'"

When Otis failed to turn up any mention of dung collection in the scientific literature on bees, he set out to study the phenomenon -- with funding from the National Geographic Society's Committee for Research and Exploration and assistance from several other researchers.

In North America, bees face a variety of threats and predators: the parasitic mite Varroa, spiders, bears, skunks. Bears and spiders are threats in Asia, too, but Asian honeybees are also preyed upon by at least six different hornet species.

When giant hornets find a vulnerable hive, they recruit nest members to carry out an organized raid, killing the colony's bees and stealing away the larvae and pupae to feed their own developing brood.

At first, Otis and his researchers were intimidated by the prospects of studying such a ruthless predator -- giant hornets are rumored to deliver one of the most painful stings on the planet.

However, they soon realized the insects weren't particularly aggressive when foraging away from their nests. The researchers were able to swat the hornets without repercussions.

"Once we figured that out, we set up one of our first field experiments. We washed the fronts of the 136 small hives in our primary apiary that was visited daily by several giant hornets," Otis said. "Then, we had some members of our team shake a white balloon on a stick at Vespa soror hornets that arrived at some of those hives, the 'control' hives, but allowed the hornets access to other."

At the end of each day, researchers photographed the fronts of the different hives, documenting the degree of spotting. Researchers conducted a followup experiment to gauge the efficacy of the defense mechanism, filming the behavior of individual hornets as they flew from hive to hive.

Lead study author Heather Mattila, who completed her doctoral degree at the University of Guelph in 2006 and is now a biology professor at Wellesley College, led analysis of the photographs and video captured in the Vietnamese apiary.

"We were able to determine that bees apply spots in response to V. soror visits," Otis said. "Using the same method, we showed that the bees did not apply spots when visited by the other common hornet species, like Vespa velutina."

From the video footage, Mattila and her students determined the hornets spent more time hovering in front of hives that were only lightly spotted than those that were heavily spotted. The hornets were also more likely to land on hives that were lightly spotted.

"The dung repelled the hornets -- we are not sure how, but it works!" Otis said.

Though Otis and his research partners only documented the phenomenon in Vietnam, hive spotting has been observed in southern China, Thailand and Nepal.Otis said he and his colleagues are anxious to conduct more experiments.

Many questions about the phenomenon remain: like how bees coordinate their dung-spreading response and what specific chemicals, or pheromones, interfere with a hornet's ability to scout for vulnerable hives.

"I also believe that experiments should be conducted to determine our ability to deter hornet visitation and attacks through applications of smelly substances -- clove oil, lemon grass oil, etc. -- to hive fronts," Otis said.

While beekeepers in Europe and North America are busy protecting their hives from parasites and disease, invasive species remain a looming threat. Some giant hornet species have made appearances in Europe, and early this year, scientists confirmed the presence of so-called "murder hornets" in the Pacific Northwest.

If giant hornets gain a foothold outside of Asia, American and European honeybee colonies will likely need help protecting their hives, the researchers said.

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