This spring has certainly been one for the history books, with a deadly viral pandemic, social justice issues, and a looming presidential election with extremely polarized parties. For the first time since March, I have an opportunity to work from my office and provide some commentary on insect goings-on in the world. You may have noticed a blip in the Covid-19 media coverage a few weeks back as a new threat caught the nation’s attention – the “murder hornets” that arrived in the Pacific Northwest. Usually I try to trust media sources to a reasonable extent, but the NY Times really did some poor journalism with their report on these hornets. I would like to provide some facts on these hornets, then use the topic to springboard onto discussions of their closest native relatives.
The Asian giant hornet, Vespa mandarinia, that got all the attention is indeed a very impressive insect. The largest of all social wasps, queens can measure 1.5-2 inches in length and workers are usually a bit over an inch. That’s a big wasp, and big wasps need big food sources. Like most hornets, V. mandarinia primarily feeds their larvae meat, and V. mandarinia seems to have a bit of a specialization for raiding the nests of other social insects, especially honey bees, for the large collections of larvae that they can feed to their own young. Their hunts against honeybees are certainly worthy of National Geographic documentaries; a scout hornet marks the honey bee colony with a pheromone, then returns to its nest to recruit reinforcements. A hunting party of around 15 hornets or so can massacre a few tens of thousands of honey bee workers in a matter of hours, then haul the honey bee larvae and pupae back to feed to their own larvae.
The toxicity of V. mandarinia venom is not particularly impressive, with an LD50 of 4.1 mg/kg (Schmidt et al. 1986). By comparison, honey bee venom has an LD50 of 2.8 mg/kg and harvester ants – the insect with the deadliest venom – have venom with an LD50 of 0.12 mg/kg. The LD50 is the measure of how much of a toxin per kg of animal body weight is required to kill 50% of a test population, so the lower the LD50, the more toxic something is. However, dose size matters, and a huge hornet can inject quite a bit more venom than a small honey bee or a tiny ant. Therefore, multiple stings can pose a health risk through direct toxicity, and any stings pose a risk of causing anaphylactic shock – just as is the case with honey bees and native social wasps.
So the main risk with V. mandarinia is that it will be bad for honey bees if it establishes and spreads, and will pose a minor public health risk. It bears mentioning that honey bees are also not native to North America, and that they are an agricultural species (like cows, pigs, and chickens) rather than an essential component of our ecosystems. Yes, there would be some economic consequences if the hornets destroyed too many honey bee colonies, but in the grand scheme of things life would go on mostly undisturbed. So one side of the risk calculation is the likely damage this invader would cause if it establishes and spreads, and the answer seems to be “minor”. The other side of the risk calculation is how likely are we to end up with giant hornets up to our elbows? For that to happen, the hornet would have to establish, spread, and reproduce to a population density that actually causes appreciable harm. Invasion biologists often refer to a “rule of tens” which states that roughly one in ten species that are introduced establish. Of those that establish, roughly one in ten spreads. Of those that spread, roughly one in ten is ecologically dominant enough to warrant the designation of “invasive species.” This hornet has all kinds of natural barriers against all three of those steps, from lack of suitable prey and habitat to new pathogens to new predators, so there’s no reason it is naturally predisposed to be the one-in-a-thousand introductions that turns into a major invasive species. On top of that, it’s big and conspicuous, so tracking their movements and eradicating nests is legitimately feasible. With proper monitoring in the affected areas of Washington and British Columbia, it is likely that we can find and eliminate any nests before they produce new queens this summer and the problem will fizzle away on its own. Unless something drastically changes with the situation, we do not need to worry about V. mandarinia in the Midwest.
As promised, I wanted to discuss the wasps that have been here for a while, too. North America does not have any native hornets, and the only hornet that IS well established is the European hornet, Vespa crabro, which is found in the eastern and southern parts of the country with little to no presence in the Midwest (see the range maps from GBIF, iNaturalist, and BugGuide). But we do have a diverse community of yellowjackets, which are likely to be the insects responsible for most stinging. At this time of year, queens that matured and mated last fall are busily building nests and raising their first batch of workers. While working in the field last week, I was amazed by how many I encountered; one house in Hayward WI had two Dolichovespula arenaria nests, a D. maculata nest, and a Vespula alascensis nest within 15 feet of each other under a deck, plus an underground V. consobrina nest under the driveway. Coexisting with these yellowjackets is not always easy; they will sting if they feel their nest is threatened, and walking on a deck directly above their nest is usually a recipe for stings. However, yellowjackets are important predators of a number of pest insects, especially flies, caterpillars, smaller yellowjackets, bees, and winged ants, as well as pollinators of a wide range of native flowers. Therefore, I would recommend leaving nests to develop undisturbed if they are in a location that will not cause problems, such as a high roof peak, tall tree, or a low-traffic area that is easy to avoid throughout the summer, but to relocate or eliminate the very young nests that may be under construction in less favorable locations.