The Asian giant hornet (Vespa mandarinia) is the largest of the true hornets, with a body length of nearly 2 inches (5 cm) and a wingspan of 3 inches (7.5 cm). Like other true hornets, it has a venom-injecting stinger and releases attack pheromones when it feels threatened, so other Asian giant hornets join in. This species also has considerable variation, especially noticeable in the head and abdomen, as seen here. (Images by Allan Smith-Pardo, USDA-APHIS-PPQ.)

By Leslie Mertz, Ph.D.

Leslie Mertz, Ph.D.

Hornets are interesting for many reasons, not the least of which are their size and sting. One of the biggest of the 22 species of true hornets in the genus Vespa—the Asian giant hornet (Vespa mandarinia), frequently referred to in the news of late as the “murder hornet”—has a body that is nearly 2 inches long (5 centimeters), a wingspan of 3 inches (7.5 cm), and a stinger that is capable of injecting a large dose of venom. On top of that, a stinging hornet also emits a pheromone that calls in reinforcements, so before long, others can join in the attack. For people who are allergic, this can be deadly.

Fortunately, hornets have little interest in humans, typically only going on the offensive when they are provoked, such as when someone accidentally leans on one or starts flailing arms at one that lands nearby. Hornets do, however, become troublemakers when they invade a new area. A particular issue is their diet, because they will attack, kill, and eat other insects, including honey bees and numerous other species of bees and wasps, as well as beetles, dragonflies, mantids, and even large moths. This can disrupt a local ecosystem and also cause problems for farmers who rely on pollinators for their crops.

Allan Smith-Pardo, Ph.D.

Because they can pose ecological, agricultural, and health threats, the ability to identify a new invading Vespa species is important. That is easier said than done, because the 22 species have a considerable amount of variation within them. In addition, the scientific nomenclature has been inconsistent, with some species having several synonyms, adding to the confusion. To sort it all out, a new key to the genus Vespa has just been published in the journal Insect Systematics and Diversity. The study’s lead author is Allan H. Smith-Pardo, Ph.D., an entomologist with the United States Department of Agriculture Animal and Plant Health Inspection Service (APHIS); co-authors were James Carpenter, Ph.D., of the American Museum of Natural History’s Division of Invertebrate Zoology and Lynn Kimsey, Ph.D., who is director of the Bohart Museum of Entomology and professor of entomology at the University of California–Davis.

The three scientists spent many months reading through scientific literature about the 22 species and combing through hundreds of hornet museum collections to get a handle on the wide-ranging variation within the species, determine which outward physical features (morphological characters) they could use to separate one species from another, and put all the information together in a heavily illustrated key that a user can follow to identify a specimen correctly. “It was a long process,” Smith-Pardo says. He notes that nearly all of the images in the key are stacked, which means that each image is actually a composite of 20-50 individual photographs, thus allowing every aspect of the hornet to be completely in focus.

The European hornet (Vespa crabro) is the only Vespa species to have colonized the United States. It was introduced to the eastern U.S. in the 1800s and has now spread west to the Rocky Mountains. It nests in tree hollows, wall voids, beehives, outhouses, and other above-ground sites. These photos show some of the variation within this species. (Images by Allan Smith-Pardo, USDA-APHIS-PPQ.)

Smith-Pardo anticipates the key being especially helpful to federal and state officials working to ensure no additional invasive hornets arrive in the United States. Currently, the U.S. has only one established species of Vespa: the European hornet (Vespa crabro), which was introduced by European settlers to the East Coast in the 1800s and has since spread west to the Rocky Mountains. In late 2019, however, a sighting of the Asian giant hornet was reported to Washington state through its invasive species app. And several years ago while Smith-Pardo was working in San Francisco as an entomologist, U.S. Customs and Border Protection agents seized a live Asian giant hornet nest in an incoming express-mail package. “In some parts of Asia this species is considered a food item and a delicacy, so somebody was trying to bring in this whole nest for food without knowing how much damage that can cause,” he recalls. “That would have been a big problem.”

Smith-Pardo believes that this key will also help beekeepers, farmers, ecologists, and others spot newly arriving invasive hornet species: “The most important thing when dealing with invasive species is to recognize them in time so you can take the proper action when it’s easier to control them.”

Smith-Pardo and his group at APHIS’s Identification Technology Program have hundreds of full-body and close-up images of Vespa species online. In addition, Smith-Pardo says, they are building a publicly available, online adjunct to the newly published key that uses menus of distinguishing characteristics, as well as illustrations and photographs. They hope to have the online key up and running in 2021.

Leslie Mertz, Ph.D., writes about science and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.