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What you need to know about invasive giant hornets

This article was originally published in May 2020. Due to an increased awareness of this insect, we've updated the links to more information and are republishing it to answer some commonly asked questions.  So far in 2021, there has been only one confirmed detection of the invasive giant hornet. All have been found in Washington state.— Editor

Very large hornet with yellow head, black midsection, and yellow and black striped lower body in a person's hand. The hornet is as long as the person's second finger from the tip to the knuckle.
Vespa mandarinia is a very large, distinctive insect.

You may have heard about “murder hornets” arriving in the U.S. In December 2019, a large hornet was found in Washington State and confirmed to be Vespa mandarinia. This hornet was also discovered earlier in two places in British Columbia, including a nest that was eradicated. The nest is now thought to be the source of the hornets found in Washington.

Why all this concern over one colony of hornets being found in North America? First, this is a very large insect. Vespa mandarinia is the world’s largest hornet, measuring between 1½ to 2 inches in length.

They also impact honey bees. While these hornets eat many different insects, when they can find them, they prefer to eat honey bees and are capable of destroying entire colonies.

Do you need to be concerned? Curious to learn more? Read on to get some answers to questions about these wasps.

Where did this invasive hornet come from?

Vespa mandarinia is native to Asia including eastern and southeastern Asia in temperate and subtropical areas. No one knows how this hornet arrived in North America, although it probably was through international commerce, like a cargo ship.

Introduction of this and other agricultural and forest pests is a constant threat due to global commerce with little oversight to intercept pests before they become invasive species like the Emerald Ash Borer. To prevent problems like this in the future, our best defense is to support monitoring by APHIS (Animal and Plant Health Inspection Service) to stop pests before they arrive.

Is there a threat that this hornet will make its way into Minnesota at some point?

While possible, it is unlikely that we would see the hornets here any time soon. If they make their way to Minnesota, our winters are likely too cold for V. mandarinia to survive here. They originate from temperate and subtropical areas with similar climates to coastal areas and areas south of Minnesota.

Why are people calling them "murder" hornets?

The name comes from their status as insect predators. A group of 30 V. mandarinia can kill a colony of 30,000 honey bees in under four hours. The attacks occur in three stages: hunting, slaughter, and occupation.

These hornets scout honey bee or wasp colonies and mark them with a scent to alert their other nest members. Next, the hornets kill the adults in the colony. Lastly, the hornets occupy the colony and harvest the larvae and pupae to bring back to their colony to feed their own young.

Are these hornets a major concern for beekeepers due to their impact on honey bee colonies?

V. mandarinia is a predator of honey bees. However, they are currently a minor concern in the U.S., since they are at such low numbers. Other threats such as parasites, pathogens, pesticides, and lack of nutrition are still the primary threats to honey bee health.

Since honey bees are like livestock, beekeepers can take steps to reduce the impact on their colonies, like setting traps. However, managing these hornets can be difficult and not always effective. For more information about the impacts on honey bees see the Washington State Department of Agriculture website.

Large hornet viewed from the top with wings spread out to each side.
This hornet can be very destructive to honey bees.

How will the hornets affect native bees?

Since the wasps often attack large social insect colonies, they could attack bumble bees or other social wasps like yellowjackets. The impact on wild social insects will be hard to determine and manage.

We don't know the impact the hornets could have on other pollinators, but it is unlikely that they will affect solitary bee populations.

Will the hornets affect farmers that rely on pollination services to produce their crop?

The effect on farmers will depend on if this hornet establishes and spreads and how it affects native bee and honey bee populations. Currently, it is a minor concern as there have been few sightings in a small area.

If the hornet does spread and reduce honey bee populations, our native and mostly solitary bees would be able to provide most of the pollination services needed by farmers in Minnesota. The impact on larger agricultural systems that require colonies of honey bees to be transported for pollination is unclear.

What is Minnesota doing about this invasive hornet?

The Minnesota Department of Agriculture (MDA) is responsible for invasive terrestrial species in Minnesota. Right now, they are aware of the problem in Washington State and are taking a wait-and-see attitude.

Because the threat is not imminent in Minnesota, they are not conducting any surveys to detect it at this time. However, should that change, they will take the appropriate action for early detection. If this invasive hornet is identified in Minnesota, MDA will take all steps to eradicate it. If you feel you have found an invasive hornet, contact MDA at arrest.the.pest@state.mn.us or 1-888-545-6684.

If you are in Washington State or British Columbia, please report any sightings.

What does this hornet look like?

Sizing up the Asian giant hornet. Chart shows 4 rows of hornets that may be confused with the Asian giant hornet. From top left to right: row 1: cicada killer, great golden digger wasp, European hornet. row 2: elm sawfly, Asian giant hornet (4cm), western yellowjacket. row 3: European paper wasp, western honey bee. row 4: bald-faced hornet, pigeon tremex, yellow head bumble bee.
There are native Minnesota insects that might be confused for an invasive Asian giant hornet.
  • It is large: about 1 ½ to 2 inches in length.
  • It has a large orange or yellow head with conspicuous black eyes.
  • It has a dark-colored thorax (the middle section of the insect), a black and yellow striped abdomen, and a smooth body with very little hair.

There are some insects in Minnesota that may be confused with this invasive hornet. The most common insects are shown in a comparative diagram below.

What should I do if I see a large wasp?

Minnesota is home to a wide variety of wasps of varying sizes, shapes, and colors. Hornets and wasps are closely related and look similar, with hornets being larger in size. Many of these wasps are important in controlling populations of pest insects. Some are pollinators. These wasps pose no threats and should be left alone.

Due to alarm about Vespa mandarinia, some people are killing anything they see that looks like a hornet to them, including bumble bees, some of which are threatened with extinction. If killing wasps in Minnesota, you are more likely to kill a beneficial insect than stop this invasive hornet, so please let the wasps live.

If after looking at the above picture you still believe it is an invasive hornet, take a picture of it and either email MDA at arrest.the.pest@state.mn.us or call and leave a message at 1-888-545-6684.

Should those of us that spend a lot of time outside be concerned about this insect?

The media coverage is a bit alarmist as they pose little threat to people in most parts of the U.S. The chance of encountering a nest in Minnesota is extremely low. The only place they would be expected to be seen is the area within about 50 miles of Vancouver B.C.

Sadly, these hornets do cause several dozen deaths every year globally. Most of the deaths occur when people receive over 40 stings, which is likely to only occur if you stumble on a nest. Overall, the risk of death is extremely low, with more people dying from attacks from pet dogs. However, being aware of your surroundings and leaving hornets' nests alone is always prudent.

For more information:

Elaine Evans and Katie Lee, Extension educators in apiculture and Jeffrey Hahn, Extension entomologist

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