Vespa velutina
(Asian hornet)

Vespa velutina (Hymenoptera: Vespidae) is a hornet of Asian origin which is a generalist predator of medium- to large-sized insects, and scavenger of vertebrate carrion. It has large impacts on Diptera and social hymenopterans, and in particular on honey bees (Apis spp.). It has recently been spreading in Asia (it is an invasive species in South Korea and Japan), and the subspecies V. v. nigrithorax has been accidentally introduced to Europe where it was first recorded from southern France in 2005. Since then it has been found in Spain, Portugal, Belgium, Italy, the UK, the Netherlands, Germany, the Channel Islands and the Balearic Islands. This invasive species threatens honey production and native pollinating insects. It may be introduced and transported accidentally with soil associated with plants, garden furniture and pots, timber, vegetables, camping equipment, etc.

Of the 22 Vespa hornet species from Asia, only a few have extended their geographical range to include the Philippines and New Guinea. Only two species are native to Europe: the European hornet, Vespa crabro Linnaeus (1758) and the oriental hornet Vespa orientalis Linnaeus (1771) (Matsuura and Yamane, 1990). V. crabro is found throughout Europe whereas V. orientalis is restricted to Bulgaria, Greece, southern Italy and eastern North Africa (Carpenter and Kojima, 1997).

V. velutina is widespread in Asia, from north-eastern India throughout southern and central China as far as Taiwan and as far south as Indonesia (Archer 1994) and is recorded in the following countries: Afghanistan, Bhutan, China (including Hong Kong), India, Indonesia (except Kalimantan and Irian Jaya), Japan, Laos, Malaysia, Myanmar, Nepal, Pakistan, South Korea, Taiwan, Thailand and Vietnam (Archer, 2012). Its presence in South Korea results from an introduction in 2003 and it has become invasive there (Choi et al., 2012; Jung et al., 2007). It was reported from the nearby Japanese Tsushima Island in 2012 (Kishi and Goka, 2017) and, in 2015, on Kyushu Island (Minoshima et al., (2015).

In 2005 the first confirmed report came from the southwest of France (Haxaire et al., 2006; Villemant et al., 2006a,b), and the species has now spread throughout almost the whole of France (Rome, 2019); in 2010 it was reported for the first time in Spain (López et al., 2011) and in 2011 in Portugal (Grosso-Silva and Maia, 2012). It has also spread to Belgium (Rome et al., 2013; Q. Rome, Museum National d'Histoire Naturelle, Paris, France, personal communication, 2019), Italy (Federazione Apicoltori Italiani, 2013), the UK (in 2016 -- NNSS undated; with further confirmed sightings in 2018 -- DEFRA, 2018), the Netherlands (in 2017 -- Smit et al., 2018), the Channel Islands (States of Guernsey Government, 2016), the Balearic Islands (Leza et al., 2018) and Germany (Q. Rome, Museum National d'Histoire Naturelle, Paris, France, personal communication, 2019).

In Asia, V. velutina is found in climate ranges close to those found in the south of Europe. It was thought that only unusually cold winters could stop its spread further north and/or reduce the population build-up in the southwestern areas of France (Villemant et al. 2006b), but modelling showed that in fact only dry summers could limit expansion in southern Europe (Villemant et al., 2011). The same study showed that the range could potentially include most of the southern part of Europe and possibly other areas around the world. V. velutina can apparently survive long distance transport (as seen in the French incursion with specimens from China) so its potential for introduction and spread in countries with similar climates to the south-western part of France is extremely high. In France, it spread to cover 120,000 km² within 3 years, demonstrating that it can colonize large areas in a very short period of time if the climatic conditions are favourable. In the UK it was considered to be very likely to enter and become established, and likely to spread rapidly with moderate impacts if and when it does arrive (Marris et al., 2011); first sightings were recorded in 2016 (Budge et al., 2017).

V. velutina commonly build two nests in a season (approximately 70% of nests relocated in summer in France -- Rome et al., 2015). Primary nests are often found in or on man-made structures (77% in France -- Franklin et al., 2017), while secondary nests are more likely to be found on natural structures (78% in France -- Franklin et al., 2017), such as in tree canopies well above the ground, or occasionally in undisturbed and sheltered areas in buildings (under stairwells, in abandoned barns, chicken coops or sections of buildings, etc.) or in bramble bushes, and very rarely underground (Martin, 1995; Rome et al., 2009; Franklin et al., 2017). See the Habitat table for the types of habitat in which the species is found.

Reproductive Biology

All the hornets have an annual life cycle. Nests in temperate regions are founded in spring by a single queen after the over-wintering hibernation period. In Vespa velutina, the queen (also known as a foundress) quickly builds a small embryo nest in which to rear the first batch of workers (females) in an enclosed and protected place (a wall cavity, tree hollow, shed, porch etc.). Nests are built from wood removed from dead trees, shrubs, posts, etc. These nests are only small -- they consist of 30-40 cells and are initially the size of a lemon. It appears that all nests are only ever used once and are destroyed by birds or the weather, although the same nest-sites can be used year after year. It takes 30-50 days for the first batch of workers to emerge in sub-tropical and temperate regions (Dong and Wang, 1989; Choi et al., 2012). If the site does not allow for colony expansion, relocation is triggered and the colony relocates and builds a secondary nest up to 200 m from the primary nest (Matsuura and Yamane, 1990)

After this the queen's duties gradually become confined to egg laying as the workers take over the duties of foraging and nest building. As the number of workers increases, the nest undergoes a rapid period of expansion before the queen switches to laying eggs which become sexual males and females (future queens, known as gynes). By the end of summer, the colony can reach a maximum size of more than 1000 adult workers (average in a French study by Rome et al. 2015 was 436 workers present between late October and early November) and hundreds to thousands of sexuals (i.e. potential queens and males) (Choi et al., 2012; Archer, 2012). In France, Villemant et al. (2011) found an average of 350 gynes (future queens) vs 900 males. When the new sexuals start to emerge, the females feed on larval regurgitations to build up their fat bodies, after which they leave the nest and mate, usually up in the tree canopy. The mother queen lives for about one year. After mating the males die. The workers probably die of old age in the tropics and not starvation due to the onset of winter as they do in temperate regions. The fertilised queens undergo a period of dormancy in temperate climates and hibernate during winter. They can hibernate alone or in clusters of 2 to 3 under the bark of trees or under stones. The mortality rate of overwintering queens is not known. Because of the speed of spread of V. velutina in France (around 78 km per year -- Robinet et al., 2017), it is assumed that fertile queens migrate after winter hibernation.

Nests can be initiated at any time of the year in tropical regions but are only built yearly in spring in temperate climates; they are left empty during winter. There is a positive correlation between the nest size and the number of queens raised (Chauzat and Martin 2009). If the queen dies before sexual production, some of the workers will lay unfertilized (haploid) eggs which will develop into males but the colony will usually perish as any new queens do not lay eggs until some weeks after fertilization. Queenless V. velutina nests have been reported (F. Muller, Museum National d'Histoire Naturelle, Paris, France, personal communication, 2013).

Nutrition

Kuo and Yeh (1990) observed that V. velutina was better at catching prey (e.g. flies and honey bees: for further detail on types of prey, see ‘Impact on Biodiversity’ below) as it is much faster and more agile than other hornet species. V. velutina has been reported to limit colony development of European honey bees in Asia by the persistent predation of adult bees (Shah and Shah, 1991). In fact, it is one of the most adept hornets at catching honey bees on the wing: other species land on the hive and grab bees that try and attack the hornet, whereas V. velutina hovers in front of the hive at a distance of 30–40 cm, and swoops down trying repeatedly to catch foragers. Once a bee has been caught it may be taken straight to the nest, or is carried off to a tree branch where the head, wings, legs and abdomen are removed (C. Villemant, Museum National d'Histoire Naturelle, Paris, France, personal communication, 2013) and a meatball made of the flight muscle which is fed to larvae back in the nest. The attempts by hornets to attack honey bee colonies are numerous and frequent, particularly at the end of the season (September to December) when the production of new queens makes high demands on hornet workers (Mollet and Torre 2006). Adult V. velutina do not consume meat themselves (although they may ingest meat juices), but they feed the larvae in the colony on insect prey and meat from mammal and bird carcasses, as well as fish and meat from fishmonger’s and butcher’s stalls. Adult V. velutina feed on sweet carbohydrates, such as nectar, ripe fruit and tree sap, and on special regurgitations from the larvae (Matsuura and Yamane, 1990).

Environmental Requirements

A key characteristic of hornets’ success is their resilience to environmental change and their capacity to overcome difficulties (Chauzat and Martin, 2009; Villemant et al., 2011; Barbet-Massin et al., 2013). Natural population control may occur due to lack of prey, bad weather at key stages of the life cycle, limited nest sites and usurpation (queen fighting) (Martin, 1992).

One of the key factors that allows hornets to be highly successful predators is their ability to thermoregulate their nests to a constant temperature of around 30°C even if ambient temperatures are lower (Spradberry, 1973; Martin, 1990). When temperatures reach maximum levels in summer months, hornets ventilate their nest. Workers regurgitate water from their mandibles onto the nest (sometimes soaking the nest wall), and evaporate that water through the vibration of their wings to cool down the colony (Perrard et al. 2009).

The nests of V. velutina have some natural predators in Aquitaine (France) -- in the period of pre-winter decline of the colony, green woodpeckers (Picus viridis), jays (Garrulus glandarius) and tits (Paridae) are often seen pillaging nests and eating the remaining larvae -- but these are unable to attack large active colonies. There have been rare records of more specialist enemies such as the European honey buzzard (Pernis apivorus) and the European bee-eater (Merops apiaster), but these have a negligible impact on the population (F. Muller, Museum National d'Histoire Naturelle, Paris, France, personal communication, 2013). It is not yet known if there are other hornet predators (Mollet and Torre, 2006).

Like other wasps, V. velutina is susceptible to various diseases and parasites, including Conops vesicularis (a parasitic fly), and various nematodes and entomopathogenic fungi (Turchi and Derijard, 2018).

Population levels are quite low in Asia compared to Europe, most probably due to competition with many other Vespa species for food resources and nesting sites (Matsuura, 1984). In France, extremely high densities of nests have been reported: 8 nests per km² in mixed land usage to 23 nests per km² in an urban area (Franklin et al., 2017).

Molecular techniques to diagnose V. velutina are in development for use in the lab and field (Stainton et al., 2018).

V. velutina is very easy to recognize since it is the only hornet or wasp that has an entirely dark brown-black body with a thin yellow stripe on the dorsal border of the 1^st abdominal tergite, an orange-yellow band towards the end of the abdomen (4th tergite), and yellow legs (C. Villemant, Muséum National d’Histoire Naturelle, Paris, France, personal communication, 2013). Being very dark, individuals stand out as dark spots in front of beehives or their own nests. V. velutina is also smaller than the two other species found in Europe and/or North America (V. crabro and V. orientalis). It also tends to build larger nests. In Java, a nest was found that was 75 cm high, containing around 12,000 brood cells in 11 combs (Vecht, 1957). In France, the largest nest dissected by Rome et al. (2015) had 11 combs and an estimated 13,547 cells.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

So far, countries that have been invaded by wasp species, such as New Zealand, Australia and the islands of Hawaii, have failed to control their populations despite many attempts (Beggs et al., 2011). These have included the introduction of parasitic nematodes, poison baits and entomopathogenic fungi (Martin, 2004; Beggs et al., 2011). Measures to reduce the expansion of V. velutina in Europe are reviewed by Turchi and Derijard (2018) but most efforts are concerned with the physical destruction of individual nests; although it is often difficult to spot these as they can remain hidden until leaf fall in autumn.

Current recommendations in France are aimed at bee keepers and apiarists in order to protect their bee colonies.

In Asia, the Eastern honey bee Apis cerana has developed two defensive behaviours against predation by hornets: a warning system for returning workers by shimmering their wings in unison (Tan et al., 2005) and a thermo-balling system whereby intruders are suffocated and heated to death (up to 45°C) by a ball of bees (Ono, 1995; Tan et al., 2005; Papachristoforou et al., 2007). In Kashmir, a colony of A. cerana can kill 10 hornets a day, whereas the European honey bee manages on average to only kill one (Abrol, 2006; Villemant, 2008). European honey bees can form balls but these are much less efficient at killing the hornet than A. cerana balls (Tan et al., 2005 and 2007). They might be able, however, to adapt with time.

Public Awareness

In the UK, beekeepers are encouraged to be alert for the presence of V. velutina, and anyone seeing it is asked to report the sighting to www.nonnativespecies.org/alerts/asianhornet, or alertnonnative@ceh.ac.uk (NNSS, undated), or use the Asian Hornet Watch app.

Eradication

Eradication of V. velutina from France is now impossible as it has become too well established over a large area (Villemant et al., 2011; Rome et al., 2009, 2012, 2013; Chauzat and Martin, 2009).

Physical/mechanical control

In India, control strategies such as killing hornet queens in early spring, destroying hornet nests and swatting hornets at hive entrances have been advocated. Unfortunately, none of these have been reported to be effective, although mass trapping in jars half-filled with fermented honey-water drove numbers of hornets hawking at the apiary from 10-25 per hive to 0-3 (Shah and Shah, 1991). In France similar methods have been tried with similar results; for example in September 2007, a beekeeper in the Bordeaux area killed about 80 hornets a day over several weeks without any decrease in the hornet pressure on his honey bee colonies (Chauzat and Martin, 2009).

Chemical control

Hornets have two basic dietary requirements: sugars for energy for the adults, and proteins for the nourishment of the brood: baiting (and poisoning) could therefore be an option. However, this could have adverse effects on non-target species (Beggs et al., 2011).

Biological control

Studies on indigenous species of entomopathogenic fungi in the genera Beauveria and Metarhizium from France show some potential for control of V. velutina nigrithorax (Poidatz et al., 2018).

France: EPPO, European and Mediterranean Plant Protection Organization, 21 boulevard Richard Lenoir, 75011 Paris, http://www.eppo.int/

UK: National Bee Unit, The Animal and Plant Health Agency (APHA), National Agri-Food Innovation Campus, Sand Hutton, York,

21/05/19: Updated by:

Sarah Bunker, consultant, UK

10/08/10 Original text by:

CRCNPB Australia, CRC for National Plant Biosecurity, Canberra, Australia