With so many more significant stories in the news recently, you may have only vaguely noticed stories about murder hornets. Is this yet another thing we should worry about? It turns out, not as much as the name would imply. While they might look very frightening, they aren’t going around killing people. However, they may be a problem for our already threatened bee population.
The insects referred to as murder hornets in the media are officially called Asian giant hornets, or Vespa mandarinia. They are found all over Asia and far eastern Russia, but most commonly in Japan, where they are called giant sparrow bees. In 2008 the name murder hornet appeared in some Japanese news stories. That name was picked up by a New York Times article and has spread ever since.
The name probably evolved because of their formidable appearance. They can be up to two inches long with a wingspan of up to three inches and a stinger a quarter of an inch long. The thorax is dark brown and the abdomen has contrasting stripes of dark brown to black alternating with yellow to orange bands. The queens are larger than the males, also known as drones or workers. The males do not have a stinger. They can be distinguished from the common hornets found in the United States, Vespa crabro, not only by their size, and also by their different color patterns. A dark brown anterior abdomen and a yellow posterior abdomen with dark brown spots is found on our native hornets.
In September of 2019, a nest of Asian giant hornets was found on Vancouver Island, Canada. That December, there were four confirmed sightings in the state of Washington, with one dead specimen found. The nest in Vancouver was immediately destroyed by the Canadian Ministry of Agriculture. There were two more sightings, one each in Vancouver and Washington as of May 29. The United States Department of Agriculture (USDA) is telling Washington residents to report any sightings. DNA tests showed that the nest in Vancouver and the hornet in Washington came from two different colonies.
While the sting from an Asian giant hornet can be very painful, it is very rarely fatal. Only about forty people a year die from these hornet stings in all of Asia, always from multiple stings. In contrast, sixty to eighty people die from bee stings in this country alone. These hornets are not aggressive and only sting if their nest is threatened. What makes their sting so painful is that a peptide in the venom, mastoparan, stimulates the enzyme phospholipase, which degrades tissue. It also contains mandaratoxin, a neurotoxin. It takes a high volume for this venom to be fatal. It has a lethal dose measurement (LD50) of 4 mg/kg, while the venom of our southern yellowjacket has an LD50 of 3.5 mg/kg. In Asia, people who have died had an average of fifty-nine stings, while most have survived with an average twenty-eight stings.
These hornets have a life cycle similar to other hornet species. Over the winter, queens hibernate and all the male hornets in the nest die. In the spring, fertilized queens will leave the old nest and start looking for a new spot to build. Asian giant hornets almost always build their nest underground, often in abandoned rodent tunnels. They also like to nest under tree roots, or in the bottom of a hollow of a dead tree. In contrast, our native hornets build their nests well above ground, in tree branches or under roof eaves. After the queen establishes a spot, unfertilized females build the cells in the nest and raise the workers. Then the workers go out and get tree sap to feed to the queens. There is a hierarchy among queens and the alpha queens get fed first. Around July, the unfertilized females stop leaving the nest and die, and the workers continue to go out and get food. By August the nest is usually at its peak of about one hundred workers.
From August through October, the workers switch to hunting other insects for food, as a source of protein: bees, other hornets, and mantises. A worker will find a beehive and release a pheromone to attract other workers. Japanese bees have evolved a defense mechanism. They will let the hornet in, then many bees will surround it, beating their wings. This generates heat and CO2, which suffocates the hornet. European honeybees that have been introduced into Japan do not have this defense. A single bee will attack the hornet outside the hive, but because of the difference in size, it does not have a chance. The Asian giant hornet will take apart the bee, only carrying its protein-rich thorax back to its nest. If more than one hornet attacks a hive, they go into what is called a slaughter phase. They will keep killing the bees instead of returning to their nest, until all the bees are dead. Then they enter the occupation phase, where they go inside the hive and prey on the larvae. They can kill a whole hive in one to two days. It is not known how bees in this country would react since they have not encountered Asian giant hornets before.
In the fall the fertilized queens produce both male and female larvae and care for them. From October to November males and new queens leave the nest and mate. This is their most active time. Their colors get more intense and queens grow an average of twenty percent larger. Workers then change their food source from proteins to carbohydrates.
There are two main ways to get rid of these hornets. One way is to burn or apply pesticide to the nests at night when they are asleep. The other way is to set bait traps. These traps have a sweet solution to attract the hornets, but the hornets can’t exit the trap. These are often used by beekeepers to help prevent loss of their colonies.
There is no need for people to worry about being attacked and killed by these insects. The USDA is vigilant about not letting them become a new invasive species. However if they do take hold in North America, they could be a threat to our bee population, which is already threatened by nicotinamides and colony collapse disorder.