Asian Giant Hornet: The invasion unfolds
First spotted in North America in 2019, increased sightings raise concern that this hornet may be here to stay.
On October 29, 2020, the cameras rolled as the first Asian giant hornet nest identified in the U.S. was carefully extracted from a hollowed-out tree by members of the Washington State Department of Agriculture (WSDA). The team painstakingly sampled the anesthetized hornets and pieces of papery brood comb from the tree cavity, which was found just outside Blaine, WA.
But the story of Asian giant hornet dispersal begins much earlier. In a time almost unthinkable now, before “murder hornets” made worldwide headlines, before COVID-19 swept across the globe, even before Obama left the White House, this pest was on Sven-Erik Spichiger’s radar.
“Forty-two people died in southern China from attacks by Asian giant hornet in 2013,” says Spichiger, as he recalls watching news reports on CNN. “Any time you’re an entomologist and something makes international news, you look into the species that did it.”
He quickly realized that this hornet was also a significant threat to honey bees, and that the habitat in many parts of North America was suitable for it to live in. He started to anticipate an eventual invasion, and says it’s “no surprise” that it ended up here. Six years later, he was proven right when beekeepers in Nanaimo, British Columbia, found the first nest in North America.
With guidance from BC’s provincial apiculturist, Paul vanWestendorp, and another government entomologist, Conrad Berube, the Nanaimo beekeepers tracked down and eradicated the nest with nothing more than household equipment (for beekeepers) and a bit of bravery. [See “Giant Alien Insect Invasion Averted,” by Conrad Berube, February 2020 ABJ.] The lead eradicator was stung several times in the process and was said to be in bed the next day with flu-like symptoms.
That nest was found just meters from a pedestrian walkway in Robin’s Park, but the extraction event was covered only briefly by local news stations. The beekeepers’ YouTube video, posted a few months later, attracted only a few thousand views.
In September and October 2019, further sightings were reported in Blaine, WA. In November, one was confirmed in White Rock, BC. In December, another Asian giant hornet was identified in Blaine feeding at a hummingbird feeder, and government entomologists in BC and WA knew they would be in for a busy 2020.
“I always expected we might encounter this,” says Spichiger in a presentation for the Washington State Beekeeping Association, “but never in my wildest dreams did I think we would encounter this in December, of any year.”
That’s because the Asian giant hornet’s life cycle is similar to that of bumble bees: They establish nests headed by a queen with a few hundred workers, and in the late summer they begin rearing reproductive individuals. Mated queens overwinter alone, emerging in the spring to found new nests. By winter, workers and drones should have been long dead and queens in a dormant slumber, so the December sighting was a surprise for everyone.
For reasons that are still unclear, it is not until late summer and early fall that the Asian giant hornet begins to ravage beehives. These brutal attacks, which are why beekeepers in northern Washington and southern BC were on high alert last fall, are divided into three phases: hunting, slaughter, and occupation.
In the hunting phase, hornet workers visit apiaries and catch honey bees one by one, preparing them into meat balls and bringing them back to the nest. In the slaughter phase, the hornets somehow choose a specific hive upon which to launch a concerted attack, during which a handful of hornets linger at the hive entrance, killing any bee that emerges, tossing their bodies on the ground. A hive can be decimated within hours or days by this method.
The slaughtering hornets stay at their post until the job is done or until dusk. They become hungry, and their nestmates come to their aid with provisions. Ironically, the hornets may starve during a prolonged slaughter, despite the masses of dead bees around them and a hive full of honey just out of reach. Estimates from Japan state that 20-30 hornets can typically kill 5,000-25,000 honey bees within 1-6 hours.1
Finally, once most of the bees have been killed, the hornets occupy the hive and are quick to defend their conquest. They preferentially extract pupae, then larvae, then the dead adult bees, bringing their meatballs back to the nest to feed their own brood. Some beekeepers in Blaine have, unfortunately, witnessed the aftermath of these attacks first-hand. Perhaps “murder hornet” is an appropriate name after all.
Early on, it was clear that this was a hardy, ferocious insect and we should expect multiple nests to eventually be discovered. Some skeptics (optimists?) suggested that the 2019 sightings could have all originated from the same mother nest, but this postulation was lacking knowledge of Pacific Northwest geography. Nanaimo is about 80 kilometers (~50 miles), as the hornet flies, from White Rock, and is separated by the mighty Strait of Georgia.
The early sightings in Whatcom County and Vancouver’s Lower Mainland suggest there were at least three nests in 2019, not including the one that was destroyed in Nanaimo. In 2020, sightings and sampling indicate there are six or seven nests — that we know about.
Dr. Gard Otis, a retired professor at the University of Guelph School of Environmental Sciences, stresses that although multiple nests have been established, it still isn’t clear if the population of hornets will be self-sustaining. “Vespa mandarinia [Asian giant hornet] queens usually mate with just one male,” says Otis, “so immigrant queens arriving in North America individually do not have a lot of genetic diversity.”
This is important because, since North America is an entirely new region, it is unlikely for the hornet to be perfectly adapted to our conditions. Yes, the climate is within its range of tolerance, but there will probably be at least some new challenges to which the hornet will need to adapt. And having low genetic diversity in the initial population means it would be less likely for the hornet to thrive.
“We need to know how genetically diverse the hornets collected in 2020 are,” says Otis, as this would tell us how much inbreeding is going on in the landed population. “We have to assume that the hornets that have arrived in North America were not perfectly pre-adapted to their new environment and that they are experiencing a possibly severe genetic bottleneck.”
Without knowing how diverse the landed queens are, how far queens and drones may travel from their nest to mate, and how serious the consequences of inbreeding are for colony fitness, Otis says he’s “not convinced” that the North American population will be self-sustaining. But he adds, “We do not want this species to become established, and should take efforts to prevent that. We needed to do everything we could this year to try to delineate their distribution and to try to reduce the population.”
Although the early detection events were covered by local news outlets, for most of us the news really broke with the New York Times article in May — the second one they published on the subject — naming this invader as the ….