What does this mean for beekeepers?
By: Gard W. Otis
The Yellow-legged Hornet, Vespa velutina, has been discovered in the state of Georgia! This social wasp, native to Asia, preys extensively on honey bees and other pollinators. Its arrival in North America, while not wholly unexpected, is a cause for alarm for beekeepers and agriculture in general. What is this wasp? Why is it a concern? What can we do to control it? And how concerned should we be at this time?
Figure 1. Yellow-legged Hornet, Vespa velutina, at rest. Photo taken in France, courtesy of Patrick Le Mao
Figure 1. A Yellow-legged Hornet hovers in front of a hive, awaiting incoming forager bees. Photos taken in France, courtesy of Quentin Rome / Muséum national d’Histoire naturelle, Paris, France.
V. velutina is a large wasp, approximately 0.7–1.0 inch (1.7–2.5 cm) in length and with distinctive yellow legs (Fig. 1). It is a social insect, with colonies composed of numerous workers and a slightly larger queen (Pérez-de-Heredia et al., 2017). The species has been studied extensively, both within its native range and in the regions of western Europe, South Korea and Japan it has invaded. Its colonies, founded by a single mated queen, become more populous as Summer progresses. By late Summer, its large, round papery nests, usually concealed by leaves in tree-tops, contain hundreds of worker hornets. To feed their larvae, they capture a wide variety of insects, with the Western Honey Bee (Apis mellifera) being a favored prey species, in part because stationary honey bee colonies provide food consistently for weeks on end (Fig. 1; Roy, 2023). Laurino et al. (2020) provided a review of the biology of the species and its invasion of Europe, where programs to control the species cost millions of dollars annually.
Figure 2. Distribution of V. veluntina (a) in Asia where it occurs naturally and has been introduced to South Korea and Japan, and (b) in Europe where it has been introduced. The red dot indicates where the first hornets were discovered in 2004. (Maps from (a) iNaturalist.org and (b) INPN (2023), used with permission of Quentin Rome / MNHN
Vespa velutina naturally inhabits a region that extends from northern China south through Indonesia and Southeast Asia, and westward along the foothills of the Himalayas to Afghanistan (Fig. 2a). Over that range, it occurs in 13 different color forms (formerly referred to as subspecies). It is likely that the hornets in Georgia are of the northern mainland form, V. velutina nigrothorax, that has also invaded Europe (Fig. 2b) and northeastern Asia. V. v. nigrothorax has often been inappropriately referred to in much of the literature written about it as the “Asian hornet”, a misnomer given that all 22 species of hornets inhabit some part of Asia!
The life cycle of the Yellow-legged Hornet is generally the same as that of all species of hornets (i.e., wasps in the genus Vespa) in temperate climates (Laurino et al., 2020). A mated queen emerges from her wintering site and becomes active in Spring when the weather becomes suitable. After a period of feeding on floral nectar and tree sap, followed by dispersal estimated to exceed 30 miles (50 km) in some instances, she individually constructs a small comb enclosed within a papery envelope made from plant fibers she chews and mixes with her saliva. The queen then rears the first generation of hornets by herself in an “embryo nest.” When those first workers have completed their development and emerge as adults, they take over most colony activities (nest construction, brood care, foraging and colony defense) while the queen continues to lay most, if not all, eggs. Towards Fall, the colony rears new queens and males. Those new queens mate with males, search for a wintering site that is usually in soil, leaf litter or rotten wood, and become quiescent for several months while temperatures are too cool for activity. It is while they are in Winter diapause that the queens can be accidentally transported long distances as stowaways among cargo on ships and, less frequently, on planes. This is almost certainly how Yellow-legged Hornets arrived in Georgia: the sites where the first individuals were discovered are within 12 miles of the Port of Savannah, one of the largest shipping ports in North America that receives cargo from everywhere in the world.
One important difference between the Yellow-legged Hornet and other species is the relatively large number of males with which young queens mate. The majority of hornet species that have been studied, mate with one or sometimes two males. In contrast, V. velutina queens mate with three males on average (range from one to 5+). The invasion of Europe by this species is believed to have originated from a single queen, mated with four males, that was accidentally imported to southwestern France (Fig. 2b). The added genetic variability provided by mating with several males has undoubtedly contributed to its successful invasions (reviewed by Otis et al., 2023).
Vespa velutina colonies can become very large. Quentin Rome and his colleagues (2015) collected nests in southwestern France from June to November and quantified their populations of immature and adult hornets. The colonies, generally initiated in March, had an average of 440 worker hornets by late October and early November. Some of those nests remained very small, but the most populous colony had 1,740 workers (News reports that colonies may contain 6,000 or more workers are erroneous). Mature colonies in Fall contained an average of 190 new queens (Rome et al., 2015), but because young queens only remain in their nest for about two weeks, that number must represent only a portion of all queens reared. I estimate that the average number reared is approximately 300–400 per colony. The greatest number of new queens collected inside one single nest was 463.
In an odd quirk of biology, hornet larvae serve as reservoirs of food within their colony. Larvae convert the proteins in chewed-up insect prey collected by worker hornets into amino-acid rich sugars. They then feed those secretions to young queens, resulting in them increasing in weight by 20–40% within one to two weeks of emergence as adults due to deposition of fat that fuels their survival during several months of Winter diapause. Young males also require the nutrients provided by larvae in order to mature sexually. Because hornets do not store honey in their nests, the sugary secretions from larvae also sustain their colonies during periods of inclement weather when foraging is not possible (reviewed by Otis et al. 2023).
How does the Yellow-legged Hornet affect honey bees and other pollinators?
The greatest concern related to the establishment of Yellow-legged Hornets in North America is their effects on honey bees and other pollinators. These hornets capture a wide variety of flying insects, a high percentage of which are pollinators, to feed to their larvae. Honey bees in particular are heavily preyed upon. Hornets hover near a hive entrance, facing outwards, so they can pick off incoming foragers (Fig. 1). Once a hive has been located, a hornet can return repeatedly to prey upon the essentially defenseless honey bees. Dr. Yves Le Conte, former head of the French honey bee research unit in Avignon, told me that by late Summer, when honey bees colonies should “prepare good Winter bees and collect honey for Winter survival, the hornets put pressure on the bees.” Hornets seem to focus on weaker colonies and in some cases, once they have killed most of the bees in a hive, they may enter and eat both the brood and the honey. In addition to the constant attrition of foraging bees, the presence of multiple hornets at the entrance of a colony causes “foraging paralysis” in extreme cases, flight activity is completely suppressed. The reduced foraging results in low honey stores and Winter starvation, with losses of 30–50% often reported (Yves Le Conte, pers. comm.; Requier et al., 2019). This hornet is a very significant threat to honey bees!
What has happened in Georgia in 2023?
From the information I have been able to obtain, Yellow-legged Hornets were first observed on or before August 1st by Sarah Beth Waller (personal communication), the beekeeper at the Savannah Bee Company on the eastern edge of Savannah where they were feeding on fallen pears. She posted a photo to iNaturalist on August 7th that yielded a tentative identification of the hornet as Vespa velutina (iNaturalist, 2023). Also in early August, a beekeeper somewhere in Savannah (at a date and location that have yet to be revealed by the Georgia Department of Agriculture) collected two hornets as they visited his hives. They were identified first by a University of Georgia entomologist and subsequently confirmed by the USDA on August 9th (GDA, 2023a). Thanks to news reports on August 15th about this discovery, a homeowner in the vicinity of the Savannah Bee Company reported a large nest 75 feet up in a pine tree. That nest, destroyed on the evening of August 23rd, proved to be exceptionally large (GDA 2023a, b). Keith Delaplane (University of Georgia) informed me that hornet traps of an unknown type have been deployed by the GDA from Port Wentworth to the mouth of the Savannah River, a distance of 18 miles. One person I interviewed was of the understanding that the first two locations where the hornets were observed are approximately eight miles apart, which, if true, would suggest that additional colonies may be present (hornets usually forage within a mile of their nest). Hornets have been sent for genetic analysis to determine if they originated in Europe or Asia. The situation in Savannah is fluid and changing rapidly. By the time this article is published, much more will be known and divulged. It may take a year or more before we know whether this incipient infestation has been eradicated.
What can be done to reduce the threat posed by the Yellow-legged Hornet?
Traps of many designs have been developed and tested in Europe. They have proven moderately effective for monitoring the presence of the hornet, but do not catch sufficient hornets to appreciably affect established colonies. Moreover, they have been criticized for their extensive by-catch of non-target insects. If the traps that have been deployed in Georgia capture additional hornets, finding and destroying their nests before new queens disperse into the environment presents the greatest probability of successful eradication. As an example, in spite of repeated discovery of Yellow-legged Hornet colonies in southern England, coordinated efforts of agricultural personnel, beekeepers and scientists have so far been successful at preventing the wasp’s establishment in the United Kingdom (Jones et al., 2020). On the Mediterranean island of Mallorca, the combination of Spring trapping of gynes before they establish nests, baiting for foragers, and triangulating and destroying colonies before they produce new queens, eliminated the initial infestation. There have been some successes in using very light-weight radio transmitters to track Vespa hornets to their nests (Kennedy et al., 2018; Looney et al. 2023). After a few years, however, once a population of hornets has become established, the proportion of nests that can be found and destroyed has proven in several European countries, to be insufficient to control the invasion.
Monitoring by state departments of agriculture (e.g., see the online form set up for reporting in Georgia; GDA 2023a) or through community-scientist platforms such as iNaturalist can provide accurate reporting of exotic hornets and other pests. Beekeepers can play a huge role in early detection of non-native hornets because several species that have the potential to become established in North America prey extensively on honey bees. Any wasp that seems unusual can be photographed or caught in a jar, then compared with similar species (USDA-APHIS, 2023). It should be reported if you suspect it is V. velutina or another non-native species.
Figure 3. Regions of North America with climate well suited to Yellow-legged Hornet survival. Other regions with lower climate suitability not shown. Adapted from Villemany et al. (2011).
Where is the Yellow-legged Hornet likely to survive in North America?
GIS technology coupled with climate data for localities throughout the world that are available online have revolutionized our ability to predict potential distributions of species outside of their native ranges. In the case of the Yellow-legged Hornet, climate variables from where the species has been documented in Asia as well as in its introduced range in France, were analyzed along with climate data for sites throughout the rest of the world. A map showing the probability of suitable climate for hornet survival was created (Villemant et al., 2011). I superimposed the North American portion of that map onto a map of the United States, then modified it to show the approximate regions in North America where Vespa velutina would almost certainly be able to survive and reproduce (Fig. 3).
A large area of southeastern USA is predicted to have climate suitable for V. velutina. That region extends from the southern tip of Texas north into Oklahoma, then eastwards to somewhere between Baltimore and New York (Fig. 3.). Hornet survival may even be possible as far north as Boston and southern Ontario (Villemant et al., 2011). On the west coast, there is a zone in the rain shadow of the coastal mountains, from Vancouver, Canada, south into California, that seems prone to invasion. The hornet is likely to do better where the minimum temperature in Winter is not too cold, relative humidity is fairly high and the maximum temperature in Summer is not too hot.
Figure 4. The Northern Japanese Hornet, Vespa mandarinia, a species that invaded the Pacific Northwest in 2019. Photo taken by Aline Horikawa near Kyoto, Japan, and used with her permission.
What about other exotic species of hornets?
Who can forget the “invasion by murder hornets” in 2019–2021? The Northern Giant Hornet (formerly known as the Asian Giant Hornet), Vespa mandarinia, is a huge social hornet (Fig. 4; also, refer to the image within YLH lookalikes” in USDA-APHIS, 2023) that is infamous for its ability to rapidly slaughter entire colonies of Apis mellifera. Its discovery in Fall of 2019 led to an extensive monitoring and eradication effort that is on-going in Washington state and British Columbia. Following a peak of 35 sightings and specimens in 2020, there were only 10 reports in 2021 (Looney et al., 2023). Since then, there have been no Northern Giant Hornets detected in North America, and there is optimism that the introduction of this species has failed. It is not clear what combination of factors is responsible for the decline in its population: relatively unsuitable climate, low genetic diversity, destruction of nests (one in British Columbia, four in Washington state; Looney et al. 2023) or other factors. It is known that relatively few species that reach foreign lands are successful in establishing permanent populations. We may simply have gotten lucky with the recent Northern Giant Hornet introduction.
Several groups of researchers have modeled the potential distribution of V. mandarinia. All of them yielded a strong probability of it surviving in the Pacific Northwest where it was initially discovered as well as a large region of eastern North America; however, these models fail to agree on the regions in the east that are most prone to invasion. Because this species focuses its predation in late Summer and Fall on social insects, including honey bees, beekeepers are again the group most likely to encounter it. If you see any wasp attacking and killing honey bees, you should report it immediately.
Figure 5. The Oriental Hornet, Vespa orientalis, a demonstrated invader in northern Italy, southern France, Sardinia, southern Spain, eastern Europe and Chile. Photo taken by Nicola Addelfio near Palermo, Sicily, Italy, and used with her permission.
The Oriental Hornet (Vespa orientalis: Fig 5.), naturally inhabits the Mediterranean, Middle Eastern and western Asian countries and is the only hornet species adapted to hot, arid climates. This striking reddish brown hornet is an agricultural pest that attacks honey bee colonies and damages fleshy fruits such as grapes. It has already been detected in at least 10 countries outside of its native range (reviewed by Otis et al., 2023), and has successfully colonized southern Spain and Chile. Young queens often Winter in groups (Eran Levin, pers. comm.), a behavior which may have helped it to overcome genetic bottlenecks. Species distribution modeling suggests it has a high probability of successfully colonizing the Gulf Coast region of the United States as well as central California.
I would be remiss not to mention the European Hornet, also a large wasp that could be mistaken for one of the other species mentioned (USDA-APHIS, 2023). It was accidentally introduced to New York City nearly 180 years ago and has become relatively common in much of Eastern North America, but because it rarely captures honey bees and has relatively small colonies, it has not proven to be a concern for beekeeping.
Conclusion
Several hornet species cause extensive damage to honey bee colonies in other parts of the world. Unlike Varroa mites and small hive beetles, hornets do not inhabit bee colonies and would be very unlikely to be transported with hives when they are moved for pollination. However, they do have the potential to arrive at any port and subsequently be transported by trucks and trains anywhere in North America! The establishment of any of the exotic hornets discussed before would cause extensive disruption to beekeeping due to their predation on bees and secondary effects on Winter colony survival. The propensity of these hornets to attack honey bees makes beekeepers the most likely people to encounter them, as demonstrated recently with the detection of the Yellow-legged Hornet in Georgia. Readers are encouraged to learn how to identify them: review the figures in this article and “lookalikes” in USDA-APHIS (2023). Stay vigilant—and let’s hope that we remain “hornet-free” for many years to come.
Acknowledgments
I am indebted to Lien T.P. Nguyen of Vietnam and Heather Mattila of Wellesley College, MA, my hornet research collaborators, for the knowledge and experiences they have shared with me over the past decade. Staff at the University of Georgia, K. Cecelia Sequira of USDA-APHIS, and Sarah Beth Waller shared important information about the Yellow-legged Hornet detection in Georgia up until 27 August, 2023. I must also recognize the dozens of wasp biologists worldwide who have willingly answered my many naive questions about hornets.
Gard W. Otis
School of Environmental Sciences, University of Guelph, Ontario, Canada
Institute of Bee Health, University of Bern and Agroscope, Bern, Switzerland
Selected References
GDA (2023a). Georgia Department of Agriculture press release (Aug. 25, 2023). https://agr.georgia.gov/pr/yellow-legged-hornet-nest-eradicated-savannah-area
GDA (2023b) Georgia Department of Agriculture press conference (Aug. 25, 2023). https://www.facebook.com/watch/live/?ref=watch_permalink&v=639982398259487
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