European hornets (Vespa crabro) are widespread across the eastern United States. Native to Europe and Asia, they were introduced to North America in the mid-1800s. This hornet is the largest species in the U.S. and is particularly common in Georgia. It is also known as the brown or giant hornet. In 2019, the northern giant hornet (Vespa mandarinia) was discovered in Washington State, and in 2023, the yellow-legged hornet (Vespa velutina) was discovered in Georgia around Savannah (Fig. 6). They are not widespread as of now from the areas where they were initially discovered.
Description
Adult European hornet workers are approximately 1 inch long and have black and yellow stripes on their abdomens (Fig. 1). The black strips have three downward extensions, and some appear as teardrops. The queens of European hornets grow to 1.3 inches in length and are reddish-brown with yellow stripes. The heads of European hornets are hairy and reddish-brown, although the redness fades on the face, becoming yellowish-brown. European hornets primarily build nests in hollow trees or on bark, using fiber they collect from the tree bark (Fig. 2). Nests are typically found in woodlots, but they occasionally construct nests in barns and sheds, as well as in attics and wall voids of houses near wooded areas. They prefer dark areas for building nests. From the outside, European hornet nests resemble gray paper coverings made from decayed wood fibers. The nest is smaller in size in the spring or early summer, but eventually increases in size as the season progresses. There are approximately 6-8 combs within a finished nest in the fall. European hornets are also active at night and are attracted to lights.
Life history
Upon emerging from their overwintering sites in spring, queens build nests. Inside these newly constructed nests, queens lay eggs and raise workers. Once a colony has enough workers, they take over the task of nest-building. A queen can lay approximately 500 eggs, dedicating her entire time to this task, thus increasing the worker population within the colony. All workers are females (sisters), but they are sterile. Workers are responsible for foraging, caring for the young, continuing nest-building activities, and defending the nest from intruders. There are about 800-1000 workers in a nest. European hornets are predators that feed on other insects, such as flies and wasps. They do not aggressively attack bees. By the fall, each colony produces a group of reproductively active males and females, which become kings and queens, respectively. They mate in the fall, and the mated queens overwinter, while males and workers die by the end of the year as temperatures dip below 33°F. The queens overwinter in natural sites, such as under bark and tree hollows, or man-made structures like attics. Every year, the mated queen builds a new nest in spring and does not reuse nests from previous years.
Damage
Workers of European hornets collect sap from the wounds they create in the tree bark (Fig. 3). They girdle the stem by removing the bark (Fig. 4), which causes the branch or twig beyond that point to die. Occasionally, they chew and strip the bark from the tree (Fig. 5) and use some of the fiber for nest construction. The damage is more severe in the fall when the number of workers reaches its maximum.
Plant host
Attacks from European hornets are reported on several shrub or tree species, such as boxwood (Buxus sempervirens), birch (Betula spp.), dogwood (Cornus spp.), ligustrum (Ligustrum vulgare), lilac (Syringa vulgaris), rhododendron (Rhododendron spp.), poplar (Populus spp.), viburnum (Viburnum lepidotulum), and willow (Salix spp.).
Management
The best strategy for managing European hornets is to eliminate the nest. Locating a nest in a tree can sometimes be challenging. Since hornets can also build nests inside the wall voids, those areas should also be inspected. Once the nest is found, it is advisable to seek professional help for its removal. Do not close the entrance of the nest when you find one. Stings from European hornets can be painful and may cause severe allergic reactions; in some cases, one may need to seek medical attention. The stinger (ovipositor) is not left behind after a sting, allowing them to sting multiple times. Generally, they are not aggressive but may attack if they perceive a threat to their nest.
European hornets can be managed by spraying insecticide directly onto the nest. Many aerosol products, referred to as “Wasp and Hornet,” are available on the market. Because these aerosols can project insecticide approximately 15 feet away, the applicator may not need to be very close to the nest for effective spraying. The best time to apply insecticide is in the evening, just before sunset. Re-application may be required if the number of workers is not reduced with the first application.
Bifenthrin can be applied to the trunk to reduce the feeding activity of worker insects. It is effective only if there is noticeable feeding activity on the bark or sap. Please read the insecticide label thoroughly before applying it. A dust formulation of insecticide can be applied at the entrance of the nest, as it carries residues into the colony and affects other workers. For more information, contact your county agent.
References
Day E and Dellinger TA. 2023. European Hornet. 2911-1422 (ENTO-576NP). https://www.pubs.ext.vt.edu/2911/2911-1422/2911-1422.html
Anonymous. 2024. European Hornet. PennState Extension. https://extension.psu.edu/european-hornet
Shaw, F.R., and J. Weidhaus, Jr. 1956. Distribution and habits of the giant hornet in North America. Journal of Economic Entomology, 49: 275.
Smith-Pardo, A. H., J. M. Carpenter, and L. Kimsey. 2020. The diversity of hornets in the genus Vespa (Hymenoptera: Vespidae; Vespinae), their importance and interceptions in the Untied States. Insect Systematics and Diversity, 4(3): 2; 1–27. doi: 10.1093/isd/ixaa006
