Yellow-Legged Hornet

Classification

Common name: Yellow-legged hornet

Scientific name: Vespa velutina Lepeletier, 1836

Order: Hymenoptera (bees, wasps, and related insects)

Family: Vespidae (yellowjackets, hornets, and paper wasps)

Distribution

Yellow-legged hornets are native to Southeast Asia. They were accidentally introduced into France, where they were first discovered in 2004. In the decades since, they have spread throughout France and northern Spain and gained a foothold in the United Kingdom, Germany, Italy, and elsewhere. A single worker was discovered in Georgia (USA) in August 2023, the first time this species has been reported from North America.

Description

Yellow-legged hornet queens vary in length from 0.78–1.26 inches (20–32 mm). The first workers to emerge in the summer are around 0.6 inches (15 mm) in length, but as the season progresses, they become similar in length to and difficult to distinguish from queens. Multiple color morphs exist across Southeast Asia (Figure 2), which has resulted in issues classifying various morphs as different species and subspecies. However, only one color morph (previously called Vespa velutina nigrithorax) has been introduced outside of Asia, which makes identification of yellow-legged hornets in Europe and North America easier. This color morph is primarily black and yellow, with a black head and yellow face, black thorax, black legs with yellow tarsi, and an abdomen with the first three segments black and the remaining segments yellow (Figures 1, 3).

Look-Alike Species

Bald-faced hornets are a native species that occurs throughout Pennsylvania and the United States more generally. Workers and queens are slightly smaller than yellow-legged hornet workers and queens (0.55 inch and 0.78 inch, respectively), although queen bald-faced hornets are about the same size as yellow-legged hornet workers. Bald-faced hornets are black and white in color rather than black and yellow, and the legs are entirely black rather than black and yellow.

Bald-faced hornets construct exposed, round paper nests that are superficially similar to the nests made by yellow-legged hornets. Bald-faced hornets are extremely common and abundant, so wasps should be observed before assuming an egg-shaped paper nest belongs to yellow-legged hornets.

European hornets were introduced into North America in the 1850s and are now widespread and common in Pennsylvania and most of eastern North America. They are somewhat larger than yellow-legged hornets (1 inch and 1.4 inches for workers and queens, respectively) and differently colored: the head is reddish-brown and yellow, the thorax is reddish-brown and black, and the abdomen is black anteriorly and yellow posteriorly with rows of black teardrops.

European hornets construct paper nests in aerial cavities, usually in hollow trees but sometimes in the wall voids of buildings. They do not make exposed, egg-shaped nests.

Cicada killers can be distinguished from yellow-legged hornets based on size, coloration, and behavior. Cicada killers are approximately 2 inches long and significantly larger than yellow-legged hornets. The abdomens of cicada killers are black and yellow anteriorly and completely black posteriorly. Cicada killers nest in the ground and are solitary, so only each female digs her own nest (although they may nest communally, with many nests in a small area that has the right soil texture).

Yellowjackets are black and yellow or black and white wasps in the genera Vespula and Dolichovespula. Excluding bald-faced hornets, which are treated separately above, Pennsylvania has 11-12 species, almost all of which are significantly smaller than yellow-legged hornets (workers up to 0.5 inch, queens up to 0.75 inch). Only queen southern yellowjackets approach the size of yellow-legged hornets. While many species are black and yellow, like yellow-legged hornets, the abdomens are typically striped with black and yellow on each segment. Most yellowjackets build paper nests in the ground, but a few species construct aerial, egg-shaped nests that are superficially similar to bald-faced hornet and yellow-legged hornet nests.

Northern giant hornets do not occur in eastern North America and have only been found in Washington State and adjacent British Columbia. The two species can also be distinguished by a number of features, including size and color pattern: northern giant hornets have completely yellow heads and brown and yellow striped abdomens.

Life History

Like other social wasps in temperate areas, only fertilized queens survive the winter. These queens overwinter in protected places and emerge in the spring to find new nests. While most social wasps construct a single nest during the year, yellow-legged hornets make two. After a queen emerges, she constructs an "embryo" nest, in which she rears the first set of workers. After these workers emerge, they construct a "primary" nest around the embryo nest, which is irregular in structure. Then, during the summer, the workers construct a new "secondary" nest, which the workers and queen move to (Figure 5). This secondary nest is usually constructed higher off the ground compared to the primary nest, often in a tall tree and is egg-shaped. They are similar in size and shape to bald-faced hornet nests, so they may be confused with them.

Mature colonies in the late summer and fall have an average of 400 workers but can have a maximum of nearly 2,000 workers. Once a colony matures, it starts producing new queens and males, which leave the nest to mate. The new queens overwinter, and the old nest dies off with the onset of cold weather.

Impact on Bees and Other Pollinators

Yellow-legged hornets are predators that preferentially feed on bees (especially honey bees), wasps, and flies. This is because they hunt areas that concentrate these kinds of prey, such as outside of bee and wasp nests (a behavior called "bee hawking" or "hawking," Figure 6), flowers that attract pollinators, and carcasses that attract flies. Prey makeup varies depending on what is available. In Europe, honey bees made up nearly 70% of yellow-legged hornet prey in urbanized areas where honey bees were common, but 30% of the overall prey in agricultural areas that had more insect diversity.

In their native range in Southeast Asia, yellow-legged hornets often hunt eastern honey bees (Apis cerana). Because eastern honey bees have co-evolved with yellow-legged hornets, they have a number of behavioral adaptations to counteract hornet attacks. The most famous is the ability of worker bees to form a ball around the hornet, buzz their wing muscles to create heat and raise CO₂ levels so that the invading hornet is killed. This form of defense works because the hornets die at temperatures above 115°F, while honey bees can survive temperatures up to 122°F. Other behavioral adaptations include flying into the nest quickly when hornets are present and producing wing shimmers, which distract the hornets from attacking and warm the flight muscles in case the workers have to form a bee ball.

Western honey bees (Apis mellifera), which are the domesticated species commonly farmed for honey, have not co-evolved with yellow-legged hornets and so do not engage in any of these defensive behaviors. In consequence, western honey bee colonies can be decimated by yellow-legged hornets. One study in Europe reported that 30% of commercial honey bee colonies were weakened by hornet attacks, and 5% were killed outright.

If yellow-legged hornets become established in North America, they could have a major impact on the bee-keeping industry, which has already faced issues after the introduction of various pathogens and parasites. However, the impact on broader agriculture is unclear. Honey bees are often used to pollinate certain crops, but recent research has shown that supplemental pollination often isn't needed (e.g., in pumpkins) or there are better pollinators available (e.g., mason bees in apples, bumble bees in tomatoes). In fact, honey bees, which are not native to North America, have a negative impact on native pollinators and decrease pollination of native wildflowers, so decreasing the number of feral honey bees in the landscape may be beneficial. However, if honey bees are not available as prey items, yellow-legged hornets will likely predate native pollinators in their place. A study in Spain showed that the presence of yellow-legged hornets negatively impacted the visitation rates of honey bees (which are native there), bumble bees, other small wasps and bees, and hover flies to apple mint, which decreased the pollination rates of mint flowers. How this might impact agriculture and native plants is difficult to predict, but there will likely be negative consequences.

Potential Distribution

As of this writing (August 2023), a single worker has been found in Georgia. Authorities are searching for a potential nest where the worker came from, but none have yet been found. While the chances of a single worker being transported through trade and then found once it escapes are small, there is precedent, as a yellow-legged hornet worker was found in Ireland in 2021, but no others have been found since.

If yellow-legged hornets become established, models predict that they will be able to survive throughout most of eastern North America and that climate change will increase the amount of suitable habitat through time, thus making it easier for the hornets to spread.

Medical Importance

Yellow-legged hornet stings are similar in pain to other wasps and hornets. People who are allergic to bee and wasp stings, receive multiple stings, or experience adverse reactions after being stung should seek immediate medical attention. Otherwise, yellow-legged hornet stings should be similar to other wasps and bees and resolve on their own after a few hours.

Because the secondary nests are usually found high up in trees, the risk of accidentally disturbing a nest is relatively low despite the large size of mature colonies. This is supported by a study in France, which found that there was no increase in the number of bee and hornet stings in areas where yellow-legged hornets occurred.

References

Akre, R. D., A. Greene, J. F. MacDonald, P. J. Landolt, and H. G. Davis. 1980. The Yellowjackets of American North of Mexico. U.S. Department of Agriculture, Agriculture Handbook No. 552. 102 pp. (via Utah State University)

Barbet-Massin, M., Q. Rome, F. Muller, A. Perrard, C. Villemant, and F. Jiguet. 2013. Climate change increases the risk of invasion by the yellow-legged hornet. Biological Conservation 157: 4–10.

Dillane, E., R. Hayden, A. O'Hanlon, F. Butler, and S. Harrison. 2022. The first recorded occurrence of the Asian hornet (Vespa velutina) in Ireland, genetic evidence for a continued single invasion across Europe. Journal of Hymenoptera Research 93: 131–138. doi: 10.3897/jhr.93.91209

Hera, O., de la, M. L. Alonso, and R. M. Alonso. 2023. Behaviour of Vespa velutina nigrithorax (Hymenoptera: Vespidae) under controlled environmental conditions. Insects 14(1): 59; 1–14.

Herrera, C., A. Marqués, M. and M. Leza. 2019. Analysis of the secondary nest of the yellow-legged hornet found in the Balearic Islands reveals its high adaptability to Mediterranean isolated ecosystems. pp. 375–380 In C. R. Veitch, M. N. Clout, A. R. Martin, J. C. Russell, and C. J. West. (eds.) Island invasives: Scaling up to meet the challenge, Occasional Paper SSC 62. IUCN, Gland, Switzerland.

Lioy, S., C. Bergamino, and M. Porporato. 2022. The invasive hornet Vespa velutina: distribution, impacts and management options. CABI Reviews 

McGrady, C. M., R. Troyer, and S. J. Fleischer. 2020. Wild bee visitation rates exceed pollination thresholds in commercial Cucurbita agroecosystems. Journal of Economic Entomology 113(2): 562–574.

Page, M. L., and N. M. Williams. 2023. Honey bee introductions displace native bees and decrease pollination of a native wildflower. Ecology 104(2): e3939.

Page, M. L., and N. M. Williams. 2023. Evidence of exploitative competition between honey bees and native bees in two California landscapes. Journal of Animal Ecology. Early access version.

Pérez-de-Heredia, I., E. Darrouzet, A. Goldarazena, P. Romón, and J.-C. Iturrondobeitia. 2017. Differentiating between gynes and workers in the invasive hornet Vespa velutina (Hymenoptera,Vespidae) in Europe. Journal of Hymenoptera Research 60: 119–133. doi: 10.3897/jhr.60.13505

Quaresma, A., D. Henriques, J. Godinho, X. Maside, L. Bortolotti, and M. A. Pinto. 2022. Invasion genetics of the Asian hornet Vespa velutina nigrithorax in Southern Europe. Biological Invasions 24: 1479–1494.

Robinet, C., C. Suppo, E. Darrouzet. 2016. Rapid spread of the invasive yellow-legged hornet in France: The role of human-mediated dispersal and the effects of control measures. Journal of Applied Ecology 54: 205–215.

Rome, Q., F. J. Muller, A. Touret-Alby, E. Darrouzet, A. Perrard, and C. Villemant. 2015. Caste differentiation and seasonal changes in Vespa velutina (Hym.: Vespidae) colonies in its introduced range. Journal of Applied Entomology 139(10): 771–782. doi: 10.1111/jen.12210

Rojas-Nossa, S. V., and M. Calviño-Cancela. 2020. The invasive hornet Vespa velutina affects pollination of a wild plant through changes in abundance and behaviour of floral visitors. Biological Invasions 22: 2609–2618.

Scaccabarozzi, D., L. Guzzetti, E. Pioltelli, M. Brundrett, A. Aromatisi, G. Polverino, M. Vallejo-Marin, S. Cozzolino, and Z.-X. Ren. 2023. Introduced honeybees (Apis mellifera) in orchid pollination: surrogate pollinators or pollen wasters? Preprint.

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 United States. Insect Systematics and Diversity, 4(3): 2; 1–27. doi: 10.1093/isd/ixaa006

Turchi, L., and B. Derijard. 2018. Options for the biological and physical control of Vespa velutina nigrithorax (Hym.: Vespidae) in Europe: A review. Journal of Applied Entomology 142: 553–562.

Villemant, C., M. Barbet-Massin, A. Perrard, F. Muller., O. Gargominy, F. Jiguet, and Q. Rome. 2011. Predicting the invasion risk by the alien bee-hawking Yellow-legged hornet Vespa velutina nigrithorax across Europe and other continents with niche models. Biological Conservation 144(9): 2142–2150.

Villemant, C., F. Muller, Q. Rome, A. Perrard, M. Barbet-Massin, and F. Jiguet. 2014.Estimating the potential range expansion and environmental impact of the invasive bee-hawking hornet, Vespa velutina nigrithorax. pp. 269–287. In Devillers, J. (ed.) In Silico Bees. Taylor & Francis.