Tuesday, May 12, 2020

What PMPs need to know about the Asian giant hornet

The large yellow-orange head and dark eyes and dark thorax
distinguish the Asian giant hornet from similar large wasps.
Photo courtesy Washington State Department of Agriculture.
If ever there was an insect pest designed to generate fear and panic, it would be the Asian giant hornet, Vespula mandarinia. The largest wasp in the world, with a sting once described like a hot nail being punched through the skin, we should give this wasp credit. It is one scary dude (or more accurately dude-ess).

Adding to the hornet's fearsome reputation is it's impact on honey bees. One of the favored foods for the Asian giant hornet is the brood and workers of social wasps and bees, including honey bees. In the fall hornets start actively searching for bee and wasp nests.  Once a nest is discovered, the hornets overpower the inhabitants, bite off their heads and consume the brood and honey [in the case of honey bees].

Commercial honey bee apiaries are especially vulnerable to hornet attack because of the close spacing of hives.  An apiary can quickly turn into a scene of pillage and destruction as wasps move from hive to hive.

So that's the essence of the bad news that you can read in most media accounts of the hornet. Here are a few things every PMP should know about this hornet as you talk with your customers. 


The largest wasp in the world, the Asian giant hornet is
1 to 1 ½ inches long with a ¼ inch-long stinger.  Photo
courtesy Washington State Department of Agriculture.
The Asian giant hornet was first detected in September, 2019 on Vancouver Island in British Columbia, about 200 miles northwest of Seattle, WA.  In December, three months later, a dead wasp was found in the small town of Blaine, Washington (100 miles north of Seattle) and reported to the Washington Department of Agriculture.  It was confirmed as the first detection of Vespa mandarinia in the U.S.  As of last February according to the USDA, so far a total of six sightings have been confirmed from British Columbia and Washington State (September through December).

You need to know. So far this wasp is only found in Washington State and British Columbia.  There is practically zero chance that anyone outside northwest Washington and southern BC will encounter this wasp this year.  Most of your experience with this wasp will be explaining to customers why they are not the first house in their state to get Asian giant hornets, aka "murder hornets" (more on identification tips below).


Asian giant wasps from Japan (A, B), India (C.) and Washington state
(D) show some color variability, but all have the distinctive yellow-orange
head with dark eyes. Photo credits: Yasunori Koide, Wikimedia Commons
(A); Alpsdake, Wikimedia Commons (B); Chief Red Earth,
https://indiabiodiversity.org (C); Sven-Erik Spichiger, Washington State
Department of Agriculture (D). Composite image from USDA
APHIS response guidelines 2020.
Researchers are still unsure how the Asian giant hornet made its way to North America, but suspect that mated, overwintering queens may have been transported in soil-containing plant pots shipped from Asia. A similar route of entry was responsible for introducing another Asian hornet to France a few years ago.  Recent genetic analyses suggest that the wasp was introduced on at least two occasions very recently.

You need to know. Human transport of the wasp has occurred but it is likely a rare event.  It is more difficult to transport a social insect, because it must be transported either as a mated queen or as an intact colony.  Individual hornet workers do not survive long if separated from their colony.  In my opinion, this means that, given reasonable precautions, we shouldn't expect rapid spread of this wasp throughout the states.  On the other hand, it is estimated that the wasps would be capable of surviving in plant hardiness zones 6 and above, which means that about 2/3 of the U.S. may ultimately be capable of supporting this hornet.


Like our smaller, ground-nesting yellowjacket wasps, Asian giant hornets build underground nests that survive for one growing season.  Queens leave the nest with the coming of cold weather and overwinter in protected hiding places until mid-spring when they emerge and hunt by themselves. Small colonies are formed and by early- to mid-summer worker wasps are produced and cooperative nest building proceeds. Not until fall do these wasps go on their campaigns of slaughter and occupation of bee hives.

You need to know.  Most people will face little risk of stings from Asian giant hornets.  These insects are aggressively territorial only when their nest is disturbed. Nests are usually found in wooded areas and only occupied by guard hornets from mid-summer through the fall. If their biology turns out to be similar to our native yellowjacket wasps, most nest encounters (and stings) will occur after the nest grows in size in late summer and fall (September and October). There is a more limited time frame in the fall (October and November) when honey bee hives are at risk from attack by wasps.  Fall will be the time to be most concerned about stings and beehive attacks from these wasps.


This may be the most useful information in this post, as most of a PMP's role will be reassuring the public that any big insects they see are NOT Asian giant hornets.

Since this is the world's largest wasp, the first thing is to measure its length.  Workers range from 20 to 40 mm-long (up to 1 ½ inches) and queens up to 45 mm (2 inches).  While coloration patterns can differ, the most distinctive and prominent feature is the yellow head that contrasts with dark eyes and thorax. See this useful chart published by the Washington State Department of Agriculture and this training slideset developed by USDA APHIS.

North American social wasps similar to Vespa mandarinia. (clockwise from left) A. European giant hornet (Vespa crabro); B. Bald-faced hornet (Dolichovespula maculata); C. Western cicada killer (Sphecius grandis); D. Eastern cicada killer (Sphecius speciosus); E. Pacific cicada killer (Sphecius convallis). From New Pest Response Guidelines for Vespa mandarinia, USDA APHIS, 2-2020.  

The closest relative of the Asian giant hornet is Vespa crabro, the European giant hornet. In the south and west, cicada killer wasps are the most common giant hornet look-alikes, reaching up to 1 ½ inches in length, but are generally more slender and lacking the large yellow-orange head and contrasting black eyes.

You need to know. There are lots of big insects that people will mistake for Asian giant hornets.  The chance of encountering one of these invasive hornets outside of Washington state is about zero right now. But be ready to put a name on the insects your customers will bring to you.

Hornet vs. Wasp

In this post I've used the terms hornet and wasp almost interchangeably. That is because hornets are a kind of wasp.  The term wasp refers generally to any member of the insect order Hymenoptera that is not a bee or an ant.  Most of the stinging wasps we think of as pests belong to the wasp family Vespidae.  The term hornet refers to vespid wasps in the genus Vespa.  In the U.S. we have only two species of hornet, the European giant hornet and (now) the Asian giant hornet. Despite its common name, the baldfaced hornet in the genus Dolichovespula is considered a type of yellowjacket wasp, so is not technically a hornet. 

You need to know. There are only two true hornets in the U.S., however, the smaller yellowjacket wasps and Polistes paper wasps are also social and will aggressively defend their nests like hornets and some bees.  Any of these species can be considered pests when their nests are built in areas where people travel or live.


Japanese hornet-hunters wear special protective gear to excavate a nest.  These special suits are designed to be slippery so the hornet cannot hold on; and are made of tough fabric that keeps stingers from penetrating. Note the use of smoke to calm the hornets while removing brood for human consumption. Photo credit Nonaka, 2008. From USDA APHIS 2020.
A variety of approaches have been taken to control the Asian giant hornet, but none seem capable of eradicating the pest at this time.  The USDA APHIS recently published response guidelines for the Asian giant hornet which includes a summary of different control measures with pros and cons of their use. Pest management professionals who encounter this hornet should be aware that traditional bee protective suits are not adequate protection for the one-quarter inch stingers carried by this hornet. In Japan special suits are sold ($700-$900) to protect people exterminating or digging up hornet colonies for food.

You need to know. Killing individual wasps through baits or sprays will not control this species. Locating and exterminating the nest is what is being attempted in Washington state right now. Anyone who encounters a suspected Asian giant hornet should contact their state department of agriculture or a university entomologist.  Do not attempt to remove or kill a suspected Asian giant hornet nest without adequate personal protective equipment.