Murder wasps – time to panic? Asian giant hornet, Vespa mandarinia

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Last week an article in the New York Times set off a firestorm of interest in a fierce predatory hornet that was recently discovered in the Pacific Northwest. By the way, hornets are a type of social wasps that live in colonies with a caste system of queens, workers, and drones. In September of 2019, in the town of Nanaimo on Vancouver Island, a colony of Asian giant hornets (AGH) was discovered and destroyed. Three months later in Blaine, Washington, the first confirmed detection of AGH in the United states was made. Several more potential sightings and suspected attacks on honey bee colonies were reported in Washington State during the latter part of 2019. How AGH arrived in North America from infested nations in Asia including Japan, China, Taiwan, Laos, Thailand, Cambodia, Myanmar, Vietnam, Nepal, India and Sri Lanka, remains a mystery. However, as with recent introductions of other invaders such as spotted lanternfly, brown marmorated stink bug, and emerald ash borer we met in previous episodes, it is probably a safe bet that they sailed across the seas as stowaways in a shipment of goods headed for a port on the Pacific coast. One disquieting aspect of these recent discoveries is evidence suggesting that hornets found in Canada were genetically distinct from those discovered in the US. This opens the door to the possibility that at least two independent introductions have occurred. Not surprising, but not good news.

Let’s take a quick look at four other species of stingers that might be confused with the Asian giant hornet. The Asian giant hornet is large with queens about two inches long. A large yellow head at the front leads a yellow abdomen encircled with several dark bands with even margins. Queens of the next largest hornet, the European hornet, measure just shy of an inch and a half. Heads are chestnut or amber and the abdomen has a few dark bands with strange dots at the posterior margin. The Baldfaced hornet is a native with striking colors of white bands and markings on a black background. Queens are about three quarters of an inch long. Several species of native and introduced yellowjackets sport bold alternating bands of black on yellow. They range in size from about half to three quarters of an inch. The last wasp in this gallery is our native Cicada killer. Commonly seen in summer, these hunters of annual cicadas have a shiny black abdomen with yellow or creamy patches partially encircling the sides. They are quite large at an inch and a half or more.  Image of Asian giant hornet by Allan Smith-Pardo of USDA APHIS PPQ and Bugwood.org; other images and video by Michael J. Raupp.

People have asked why they should be concerned about this invader. Two good reasons exist. First, even though these are not “generally aggressive” towards humans, like other social insects including paper wasps, yellowjackets, and honey bees, they will forcefully defend their colony if the colony is threatened. And their sting really packs a wallop. Descriptions of pain associated with the sting liken it to being stabbed with a hot nail, not something I have experienced but it sure sounds painful. The toxicity of AGH’s venom is actually somewhat less than that of the honey bee, however, due to their very large size, among the largest of all hornets on the planet, the volume of venom delivered per sting is quite large. Many deaths associated with stings from hornets, bees, and wasps result from severe allergic reactions called anaphylaxis. However, reports of human deaths from China and Japan have been linked to the toxic properties of the venom itself when people received multiple stings by many AGHs. The venom of AGH is a witches brew of chemicals, including cytolytic toxins that destroy cell membranes, and neurotoxins that attack the nervous system. One account from China claimed that 41 people had been killed by AGH stings with another 1,600 needing medical attention. To put this into perspective a bit, here in the US the CDC reports on average 62 deaths each year due to stings from hornets, wasps, and bees. And for the major gender demographic of Bug of the Week, the men, about 80% of these deaths are males. Figure that one out.

The second, and perhaps more important reason for concern, is the potential effect AGH can have on the already beleaguered honey bee industry and non-commercial beekeepers. Already imperiled by invasive mites and diseases, climate change, and pesticides, honey bees now face a new enemy. AGH has been described as a specialist predator on social bees including our domestic European honey bee, Apis mellifera. In the typical seasonal progression of business in a colony of AGH, many different types of insects and other arthropods are captured by foraging workers, dismembered, and chewed into balls of flesh. These meat balls are taken back to the colony as food for the queen and developing hornet larvae. In a remarkable treatise on the biology of AGH, Matsuura and his colleague Sakagami described a unique and diabolical attack levied on honey bee colonies that begins in late summer and early autumn. Phase 1 of the attack, called the hunting phase, occurs when individual hornets lurk near a hive, capture and kill singular honey bees, macerate them into a ball, and take them back to their colony to feed the brood. Phase 2, graphically named the slaughter phase, happens when several hornets focus their attention on one beehive. As guard honey bees mount a counterattack to the pillaging hornets, the attackers grapple with the defenders, decapitate them with powerful jaws, and discard their victims in front of the hive. The slaughtered bees are not taken back to the hornet’s nest as a source of food. They are simply discarded on the ground. The slaughter phase can last several hours and decimate a hive. One report of an attack by 30 hornets resulted in the death of 25,000 honey bees. Phase 3 of the attack is called the occupation phase. After the slaughter is complete, with most honey bee defenders dead and most other workers and foragers having abandoned the hive, the killing of adult honey bees ceases. Attention turns to bee brood (immatures) developing in their cells. Asian giant hornets post guards at the entrance of the beehive ready to attack humans or other hornets not part of the same clan that approach the hive. Inside the hive, bee larvae and pupae are pulled from their chambers and transported back to the hornets’ nest to feed developing hornet larvae. This nightmare for honey bees leaves little wonder for why these marauders are called murder hornets.

Should the discovery of AGH on the west coast send shockwaves across our nation? Not at this point in time. The infestation on the west coast at the time of this writing consists of one colony in British Columbia that was completely destroyed last year. While the discovery of hornets and suspected colony raids on beehives nearby portend other colonies of hornets, this season awaits the discovery and confirmation of more established colonies. The critical issues now are to survey, detect, and delimit the extent of this introduction and to act swiftly to eradicate colonies of AGH before this invader becomes well established and spreads. This effort is already underway by agencies and scientists in the United States and Canada. Citizen scientists are being enlisted to help find the hornet and report its location to officials. Past history has proven that very few invaders that arrive on our shores actually become established and achieve significant pest status. Even if AGH were to establish in the Pacific Northwest, it would likely take years if not decades to become broadly distributed across our nation. However, as we have seen with other pests, like emerald ash borer and brown marmorated stink bug, spread of an invasive species can be greatly accelerated by inadvertent human assistance, including interstate transport of materials that may harbor hornets, or arrival of new impregnated queens from Asia that can establish colonies near ports of entry into our country.   

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What should the general public and beekeepers do at this time? Learn to identify AGH and distinguish it from other hornets and wasps. Washington State University and USDA have excellent resources to learn about the biology and management of Asian giant hornet (see the links below). This week’s YouTube video and images should help with identification.

I have received several inquiries over the last week regarding insects thought to be Asian giant hornet. These have all turned out to be European hornets, which are quite common here in the DMV. Check out the image with this story to become an expert at telling these two rascals apart and click on this link to see how European hornets roll.

Please keep your eyes open and if you believe you have discovered AGH, snap a photo with your cell phone and send it to your state department of agriculture or local university extension service. Beekeepers, now is the time to learn about this new threat to your colonies and to learn what steps can be taken should it arrive in your area.  So, is it time to panic? Nah, not here in the DMV for sure; for most of us it is time to become informed about this invader. For beekeepers throughout our country, particularly those in the northwestern United Sates, it is time to be vigilant and learn about the biology and management of this pest.  

Links to other web sites with great information and video of Asian giant hornets include the following:

USDA New Pest Response Guidelines: Vespa mandarinia, Asian giant hornet 

WSU scientists enlist citizens in hunt for giant, bee-killing hornet 

SIZING UP THE ASIAN GIANT HORNET 

Acknowledgements

We thank Kathryn Fink and Celeste Headlee for providing the inspiration for this episode.  We also thank Allan Smith-Pardo of USDA APHIS PPQ and Bugwood.org for the amazing images of Asian giant hornet. “A Bionomic Sketch of the Giant Hornet, Vespa mandarinia, a Serious Pest for Japanese Apiculture” by Makoto Matsuura and Shoichi F. Sakagami, “Purification and properties of a presynaptically acting neurotoxin, mandaratoxin, from hornet (Vespa mandarinia)” by Takashi Abe , N. Kawai, and A. Niwa, “Cardioactive effects of hornet venom, Vespa mandarinia” by T. Abe, and “Giant hornet (Vespa mandarinia) venomous phospholipases” by Takashi Abe, Masato Sugita, Tsuyoshi Fujikura, Jiro Hiyoshi, and Michinori Akasu, provided valuable insights for this week’s episode.

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