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Asian giant hornets (Vespa mandarinia japonica) drinking sap from tree bark in Japan. Alpsdake/Wikipedia, CC BY-SA

What are Asian giant hornets, and are they really that dangerous? 5 questions answered

Editor’s note: In late 2019 two Asian giant hornets – a species not known to occur in North America – were found in northwest Washington state, and a hornet colony was found and eliminated in British Columbia. On Oct. 24, 2020, entomologists found a nest of the hornets in Blaine, Washington, on the U.S.-Canadian border, which they destroyed two days later. Now scientists are trying to determine whether more of these large predatory insects are present in the region. Entomologist Akito Kawahara explains how these insects behave and why headlines referring to “murder hornets” are misleading.

1. How common are these hornets in Asia, and how much alarm do they cause?

The Asian giant hornet (Vespa mandarinia) is fairly common in many parts of Asia, where it is called the “giant hornet” or “giant sparrow wasp.” Growing up in Japan, I saw them relatively frequently in the mountains outside of Tokyo.

These insects are large and distinctive, with a characteristic orange head and black-banded orange body. Like any other social wasp, they will defend their nest if the colony is disrupted. But in most cases they will not do anything if people aren’t aggressive toward them.

Giant hornets have longer stingers than a honeybee’s, and hornets do not break off their stingers when they sting. Because hornet stingers can puncture thick clothing, people should avoid hornets and their nests whenever possible.

Giant hornets frequently are attracted to tree sap: I was stung by one when I was looking for butterflies on trees. The sting is painful, but the swelling and pain in most cases subside in a few days.

Just as with honeybee stings, an allergic reaction, or anaphylaxis, can occasionally put people in the hospital. In very rare cases, severe reactions can become fatal. But wasp and hornet stings killed fewer than 13 people a year in 2017 and 2018 in Japan – less than 0.00001% of the national population – in a country where many people spend time in the woods.

If you are allergic to bee and wasp stings, it is best to avoid getting close to these insects and their nests, wear white clothing outdoors (they are attracted to dark colors) and avoid carrying open-top sweet drinks such as sodas in the woods.

Barricade and warning sign in front of giant hornet nest under a tree stump in Deba, Ritto City, Japan. Greg Peterson/Flickr, CC BY-NC-SA

2. Are you surprised that the hornets have appeared in North America?

To some degree, yes. Most likely, one or two fertile queen hornets entered Canada via shipping packaging and created the colony that was discovered in 2019.

It’s easy for invasive species to travel this way. More than 19,000 cargo containers arrive daily at U.S. ports, and inspectors can only do random searches of shipping containers. One estimate suggests that just 2% of shipments are searched for evidence of invasive species. Many harmful organisms such as plant pests are intercepted, but some do get through.

It’s very unlikely that an entire colony of hornets was transferred to North America. Colonies of this hornet are often large, and the hornets would be visible and potentially aggressive if their nest were disturbed.

A genetic test indicated that one of the hornets found in Washington was not related to the Canadian colony.

Four wasp and hornet species that are often confused with the giant hornet. Upper left: European hornet (Vespa crabro). Upper right: common aerial yellowjacket (Dolichovespula arenaria). Lower left: European paper wasp (Polistes dominula). Lower right: baldfaced hornet (Dolichovespula maculata). gailhampshire (upper left), Gilles Gonthier (upper right), Judy Gallagher (bottom images), all via Flickr, CC BY

3. What kind of conditions do these insects need to survive?

Giant hornets are fairly common in mountainous regions of Asia, but they’re not often seen in large cities or highly urbanized areas. They usually nest at the base of large trees and inside dead logs. The fact that they can’t tolerate extremely hot or cold temperatures makes it unlikely that they would spread to very hot or cold areas of North America.

An active colony was recently discovered in October 2020 in Blaine, Washington. It is possible that a few colonies of hornets may have survived there and could spread. However, it is unlikely that this would happen quickly, as foraging ranges of Vespa are only about 2,300 feet (700 meters) from their nest.

The key to prevent spread is surveillance. Anyone in the Pacific Northwest should be alert for giant hornets while they are outdoors this fall and next spring. They have been seen on our continent only in the Pacific Northwest, and the chance that they will appear in the northeastern or southeastern U.S. is very small. Entomologists received thousands of reports in 2020 from states outside the Pacific Northwest, but these all were misidentifications of other wasp species.

4. If more hornets are found, could they threaten honeybees and other pollinators?

Possibly. Some media posts have described destruction of honeybee nests by what could have been giant hornets, but honeybees are not these insects’ only prey. The hornets feed on different kinds of insects, and bring captured dead prey back to their hive to feed to their young.

In Japan, beekeepers surround their hives with wire screen nets to protect them from hornets. North American beekeepers can replicate these with wire netting from local hardware stores.

Many honeybees in Asia have the ability to protect their hive from intruding giant hornets by scorching them. They wait for a hornet to enter their nest, then mob it by surrounding it completely with their bodies. Each honeybee vibrates its wings, and the combined warming of honeybee bodies raises the temperature in the center of the cluster to 122 degrees F (50 degrees C), killing the hornet. Carbon dioxide levels in the nest also increase during this process, which contributes to the hornet’s death.

Japanese honeybees swarm a giant hornet, killing it with their body heat.

5. Are news stories about ‘murder hornets’ overreacting?

Yes, very much so. In parts of Japan, people consider these hornets beneficial because they remove pests, such as harmful caterpillars, from crops. The hornets’ bodies also contain nutrients, and have been used as ingredients in Japanese food and some strong liquors. Some people believe the hornets’ essence has medicinal benefits.

People who live in Vancouver, Seattle or nearby should certainly take note of what these insects look like. They are 2 inches long or more, with a 3-inch wingspan, and have distinctly orange heads and broad-striped orange and black-banded abdomens. That’s quite different from typical North American hornets, which have yellow or white bodies with black marks. Remember that the chance of seeing a giant hornet anywhere outside of the Pacific Northwest is very unlikely.

In the unlikely case that you see a giant hornet in Washington state, do not try to remove nests yourself or spray hornets with pesticides. Cutting down trees to prevent nesting sites is also unnecessary and can affect many other kinds of native wildlife, including beneficial insects that are needed for pollination and decomposition. Many native insects are declining globally, and it’s important to make sure these insects are not affected.

Instead, take a photo from a distance and report it to the Washington State Department of Agriculture. Photos are essential to verify that identifications are accurate.

Consider also uploading your images to iNaturalist, which is one of the primary sources for information on tracking wildlife. The images are archived and carry data, such as location, time of observance and the insect’s morphological features, that scientists can use for research.

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This is an updated version of an article originally published on May 11, 2020.

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