A suspected “inbreeding event” may suggest that the number of Asian giant hornets in Washington remains small, state Department of Agriculture entomologist Chris Looney said March 4.

A nest eradicated last September was “tiny” and included male hornets, a surprising finding for that time of year, Looney said. Researchers suspect, but haven’t confirmed, they were diploid males, the offspring of closely related queens and mates.

Inbreeding is a feature of small populations. Diploid males are usually sterile and inactive, not contributing to the colony’s growth.

“They don’t seem to forage. There’s no evidence they expand the colony,” Looney said. “That would actually explain why this nest is so tiny.

“And that turns out to be really good for us,” he said. “It’s the kind of thing we’d like to see more of in Washington state because it does slow the ability of the hornets to maybe spread.”

Looney talked about past and ongoing research into Asian giant hornets during online presentation hosted by Washington Invasive Species Council.

Asian giant hornets were first detected in North America in late 2019 and are popularly known by the sensational nickname “murder hornets.” They have been found in only British Columbia and Whatcom County in northwest Washingon.

The state agriculture department eradicated one nest in 2020 and three in 2021. The nests were within a few miles of each other and near the Canadian border. No nest has been found in Canada.

The hornets somehow made their way from Asia, likely Japan and South Korea. Scientists believe the hornets could thrive in the climate and habitat of Western Washington and Western Oregon.

Only one of the four destroyed nests showed clear signs of inbreeding. Looney said in an interview that scientists have too little information to understand the significance of the one nest that suggested inbreeding.

“We don’t know whether to be thrilled about it or to write it off,” he said.

Scientists also are unsure what to make of the fact that the four nests found in the U.S. were much smaller than three nests recently found in Japan.

Small nests could suggest the hornet population will grow slowly in Washington, but too few nests have been found in the U.S. or Japan to draw conclusions, Looney said.

“It gives me some hope that maybe our hornets are not able to grow quite as fast,” he said. “Is that a function of weather? Is it a function of inbreeding? Are they just getting their hornet legs under them? We’ll wait and see.”

Researchers in Asia report that giant hornets nest in the ground, but that has not been the case in Washington. All four nests were in alder trees.

“It was totally unexpected,” Looney said. “We’re not sure what explains this strange nesting behavior.”

The ground in northwest Washington may be too wet in the spring for ground nests, or it could be that Asian giant hornets have taken to alder trees in the Northwest, he said.

“Maybe it’s the luck of the draw. The first nest got stuck in an alder tree and subsequent queens associate alders with nesting sites. We just don’t know,” Looney said.

Scientists are pursing several lines of research, including finding a bait for traps. “What we’d love to have is something that hornets can’t resist,” Looney said. “What we want is the food that hornets want more than anything else.”

Asian giant hornets are not out this time of year. Queens won’t emerge until the spring.

“If you’re a vespa mandarinia in Whatcom County right now, you are holed up somewhere in the ground or maybe a pile of straw or compost, hoping not to be trod upon or dug up by raccoons,” Looney said.