Thursday, August 27, 2015

Are Asian or Japanese giant hornets in the United States?


But there is a lot of fear of them. It could be from news reports out of China stating death tolls in the dozens, or natural history videos depicting them massacring thousands of honey bees. They certainly are vicious insects with an aggressive reputation, but plaguing North Carolinians (or other Americans)? It's not happening.

Here's a little background on these wasps. The family Vespidae has a large diversity of forms and social structures and is found throughout the world. Many species are solitary, preferring to work alone to provide nests and food for their young - food typically being other insects (as in the Eumeninae; see image below left) or pollen (as in the Masarinae). The familiar paper wasps make up the subfamily Polistinae (see image below right), and are social but usually have small to moderate-sized colonies. Many build open nests out of paper made by gathering pulp from dried wood. They are often seen under the eaves of homes in our area and, while they can be aggressive, are often tolerant of people. They are also beneficial predators of many garden pests.

Potter wasps (left) and paper wasps (right) are two familiar groups in the family Vespidae. Note how members of this family fold their wings lengthwise at rest. Photos by Matt Bertone.

The subfamily Vespinae encompasses species with the largest colonies (numbering in the hundreds to over one thousand individuals) and contains the subject of this article. Some are small wasps that typically build paper nests in the ground, including the well-known yellowjackets (Vespula spp.; see image below). Others, generally called hornets, build large paper nests in the open or inside dead trees, attics or other hollow structures. Hornets and yellow-jackets are highly aggressive toward perceived threats and, thus, have a bad reputation. Among the vespines, the largest wasps are in the genus Vespa. This genus has 23 species native to Europe, the Middle East, NE Africa, and Asia (most diverse in SE Asia); there are no native Vespa in the Western Hemisphere. Like many other members of the subfamily, the adult wasps will hunt and dismember prey (even scavenging carrion) to feed to their young, while taking sugary meals of ripe fruit or oozing tree sap for their own energy.

Yellowjackets (like this Vespula maculifrons du Buysson) are small to medium wasps that typically create nests under ground. Note the notched eye, characteristic of Vespidae. Photo by Matt Bertone.

The largest are Asian giant hornets (Vespa mandarinia Smith; see image below) also called yak-killer hornets and including a subspecies called the Japanese giant hornet (V. mandarinia japonica). Workers average about 3.5 cm, but queens can be a whopping 5.5 cm with a 7 cm wingspan! These wasps, as alluded to above, are very aggressive and pack large quantities of potent venom. While the LD50 of V. mandarinia is 4.0 mg/kg (on par with the toxicity of some venomous snakes), it is actually less toxic by volume than our native southeastern yellowjacket, Vespula squamosa (Drury), with an LD50 of 3.5 mg/kg. However, the volume of venom injected and some necrotic and hemorrhagic properties of the venom make this hornet dangerous. The biology of Vespa mandarinia, especially related to their attacks on honey bees, was outlined thoroughly by Matsuura and Sakagami (1973).

The Asian giant hornet (Vespa mandarinia) is a large wasp found in many parts of middle and southern mainland Asia and nearby islands. This specimen measures just over 3.5 cm (not including the stinger), and in life would have a vibrant yellow/orange head and stripes on a dark brown or black background. Note that the eyes are very far from the back of the head, almost twice the distance as the width of each eye - a characteristic of this species. Photo by Matt Bertone.

So what are people seeing when they think one of these monster killer hornets is on the loose? There are two main suspects found in the Eastern US that are often confused for Asian giant hornets. The first is actually a close relative, the European hornet (Vespa crabro L.; see images below). This species was introduced from Europe/Asia into the US sometime in the 1800s and first recorded in NY. It has since spread across much of the eastern half of the US. These wasps tend to nest in tree cavities and prefer to be away from humans, but sometimes are found in other situations, even in attics or walls of homes. They can be aggressive and have painful stings, but are not usually an issue to homeowners.

In life, European hornets (Vespa crabro) have a mix of yellow, black and reddish-brown colors.
European hornets (Vespa crabro) have a much darker head with larger eyes than Asian giant hornets. They also have wider yellow stripes with black markings on their abdomen. This specimen is about 3.0 cm in length. Photo by Matt Bertone.

Another candidate, and a wasp that is even larger than any hornet we have here, is the Eastern cicada killer (Sphecius speciosus (Drury)). These wasps are distantly related to hornets. They are in fact a type of solitary hunting wasp in the family Crabronidae (formerly a part of Sphecidae, which contains familiar wasps like mud daubers). As their name implies, cicada killer wasps hunt cicadas. In mid-summer, males guard plots of ground suitable for tunnels and may "attack" any intruder, especially other males. However, they cannot sting. It's the females that find this prey and paralyze it, not for her own food but for her young (larvae) to consume. Once a female has paralyzed her cicada, she buries it in the ground and lays an egg that will hatch into a larva that will consume the prey. Although they have an impressive stinger and have venom, they are not aggressive and prefer to avoid conflict - after all there is no one to do the work if they die! If you can get them to sting (which would take a lot) they apparently cause very mild pain, especially for their size.

Eastern cicada killers (Sphecius speciosus) prefer to be hunting cicadas or digging in the ground than swarming or attacking. They are solitary by definition, but sometimes nest in large aggregations, somewhat like an apartment building. Note the large, unnotched eyes and wings that are not folded lengthwise at rest.
The Eastern cicada killer wasp (Sphecius speciosus) is an impressive insect. It has to be, though: females must attack and wrestle down cicadas to paralyze and bury for their young. Note the large, round eyes with no notch and elongate, pointed abdomen. Photo by Matt Bertone.

In conclusion, although you may see a large and scary wasp out and about, the likelihood of it being an invasive Asian giant hornet is zero. The two giant wasps we have are impressive, but not very surprising or dangerous (basically harmless in the case of cicada killers). Finally, for comparison, here is a photo of all three next to each other:

Three giant wasps, the first of which you will not see in the US: A) Asian giant hornet (Vespa mandarinia), B) European hornet (Vespa crabro), C) cicada killer (Sphecius speciosus). Photo by Matt Bertone.


One final thought: despite lacking "killer" Asian giant hornets, every year dozens of people in the US die from wasp stings (including those from honey bees, Apis mellifera L.). These are largely attributed to allergic reactions, but in some rare cases that involve extremely large numbers of stings, the venom itself can have lethal effects. It is always good to be careful around any stinging insect, especially those that live in large colonies that might be vigorously defended. People who are allergic should always have an epinephrine autoinjector ready when working around such insects. If you have a large wasp nest that appears too difficult to control, it is best to hire a professional to take care of the nest. For more information on control of these wasps, see this note on Dolichovespula and Vespula, and this note on Vespa crabro.

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