Asian Giant Hornets and Honeybees
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Asian giant hornets (Vespa mandarinia) are truly the stuff of nightmares. They are the world’s largest hornet and measure a whopping 1.75 inches in length (queens reach over two inches) with a wingspan of three inches and stingers a quarter of an inch long. They can fly up to 25 mph and regularly kill people in their native East Asia. While a single sting generally is not fatal, multiple stings can be lethal even among those who are not allergic.
Asian giant hornets get their nickname of “murder hornets,” not as much for their lethality to humans as the damage they cause to other bee and hornet species. Understandably, sightings of Asian giant hornets in North America’s Pacific Northwest (southern British Columbia and northern Washington State) in late 2019 were greeted with deep concern that the species was becoming invasive.
Giant hornets have mandibles shaped like spiked shark fins. Their technique is to decapitate the bees defending the hive and carry away the bee brood to feed their young. When attacking a beehive, the scout hornet will excrete a pheromone marker that attracts other hornets. As many as 50 will attack at once, and can wipe out a hive in less than two hours.
Analysis of the collected Pacific Northwest specimens of giant hornets determined they had two points of origin: Japan in the case of the ones found in British Columbia and South Korea for the Washington State population. This implies that two separate introductions of the hornet occurred within fifty miles of each other within a few months.
As one concerned researcher put it, if murder hornets get a foothold in North America, it will be “massacre after massacre after massacre” of pollinators. Biologists responded aggressively to the invasion of Asian giant hornets, knowing they had a short window of time to beat them back before they spread. Any delay meant the hornets could establish themselves with such a deep presence that “all hope for eradication could be lost.”
Because of this uncompromising approach to eradication, no confirmed sightings have occurred since November 2022 as of this writing. Biologists are cautiously optimistic the threat has been eliminated … for now.
If Asian giant hornets are such a threat to honeybees, why are the Asian honeybee populations so stable? Asian honeybees (Apis cerana) differ from Western honeybees in a number of ways, not least their coping techniques for giant hornets. When a scout hornet investigates an Asian honeybee hive, the bees will lure it inside and then swarm it in a frantic ball of activity. The inability to handle high temperatures is the giant hornet’s Achilles heel. Crucially, the heat tolerance of Asian honeybees is a tiny bit higher (122F) than that of the giant hornet (113F). After mobbing the scout wasp, Asian bees flex their wing muscles and increase the temperature inside their tiny swarm until the hornet is literally roasted alive. In this manner, they prevent the scout from attracting other hornets.
Additionally, Asian honeybees emit a harsh warning signal known as an “antipredator pipe,” which lets other bees in the hive know of impending danger. Asian honeybees also participate in a strategy called “dung spotting” in which they collect animal feces and apply it to the entrances of their colony to repel and confuse hornets. Finally, since Asian honeybees cannot ward off continual attacks by giant hornets, a last-resort survival mechanism is to simply “abscond” and abandon the hive at a moment’s notice.
But Western honeybees in Europe and North America have no such defenses. When the hives of Asian honeybees are attacked by giant hornets, the bees retreat inside for safety, where they wait out the danger. When the hives of western honeybees are attacked, they come out of their hive to fend off the hornets. This is why they are vulnerable to the superior strength and aggression of giant hornets.
To help protect their hives, many Japanese beekeepers install metal entrance plates to the hives. The plates leave a gap of one-quarter inch (7 mm), which is too narrow for the hornets to pass through. Keep in mind this works only with Asian honeybees because their defense mechanism is to retreat inside the hive, rather than defend it from outside.
Other enterprising methods used by Japanese beekeepers include using rodent glue sheets. If hornets discover a honeybee hive, beekeepers will capture a hornet using a net and then injure it by stomping on it. The dying hornet releases a pheromone that attracts other hornets. Beekeepers place the dying hornet on a glue sheet, which is then placed on top of the beehive. The incoming hornets are stuck on the glue, and they, in turn, release pheromones, which attract yet more hornets. This affects only the hornets homing in to attack the hive rather than killing the hornets at their source. In other words, it is solely a beehive defense mechanism. The glue sheets are generally left in place overnight and then removed. While a few honeybees are also caught, it prevents the hive from being attacked by the hornets.
When it came to preventing the spread of giant hornets in the Pacific Northwest, the hornets’ own biology may have worked against them. A queen overwinters while carrying fertilized eggs inside her. In spring, she lays eggs that will become workers. It’s not until late summer or early fall that she starts laying eggs that will become queens or males. These will become the founders of new colonies, so if the hornet nests can be eradicated before these queens and males emerge, any spreading is curtailed. This, apparently, has been a successful strategy.
Interestingly, solitary North American bee species likely are not under threat by invasive giant hornets. The hornets are after bee brood — lots of it — and solitary bees may not be worth the bother except in species that cluster.
If Asian giant hornets do indeed get a permanent foothold in North America, the best defense mechanism for vulnerable beehives is likely to be mechanical, in which the beehive entrances are literally caged in a mesh too small to allow the wasps to pass through, but spacious enough to allow the honeybees to emerge from their hives in large numbers for defense.
All insect species have redeeming qualities in their native range. Asian giant hornets have a role in their own ecosystem. But as an invasive species, the damage they could incur is incalculable. The brief experience of dealing with Asian giant hornets has put entomologists and beekeepers on high alert. Should another infestation occur, it is likely to be noticed immediately and handled with similar aggression.
Sources
https://academic.oup.com/aesa/article/113/6/468/5901552
https://www.nationalgeographic.com/animals/article/asian-giant-hornets-arrive-united-states
https://www.cnn.com/2021/11/09/asia/bees-warning-noise-murder-hornets-scn/index.html
https://www.nytimes.com/2020/05/02/us/asian-giant-hornet-washington.html
