The Giant Asian Hornet

A Predator and Pest of Honey Bee Colonies

A native hornet to Asia, the Giant Asian Hornet is the world’s largest hornet species, with workers reaching 45–55 mm long and a wingspans of ~75 mm. This also makes it the largest insect predator of honey bee colonies, killing adult bees and feeding on their larvae. For those Asian beekeepers unlucky enough to sight them, there are several hive management techniques to control them, including reducing the hive entrance size, deploying wasp & hornet traps, or locating and removing the hornet nest.

This article examines the impact of the Asian Giant hornet species on honey bees and how both beekeepers and bee populations can manage this pest, and looks at the life history of this introduced wasp species.

How can Beekeepers Control Asian Giant Hornets?

Begin with regular monitoring, not just of the hive, but in the general area around it during the spring, summer and fall. What other insects are present? Think spiders, ant mounds, wasp or hornet nests. If an Asian giant hornet is spotted hovering at the hive entrance, it is likely a scout, marking the honey bee colony with its scent by rubbing its abdomen on or near the hive walls. The abdomen contains the van der Vecht glands, which release a marking pheromone that other hornets can use as a guide for food sources; (Ono et al., 1995).

For beekeepers with Asian honey bees (particularity the Japanese subspecies Japonica), this isn’t a major issue as Apis cerana reacts to the scent by increasing the number of defenders at the nest entrance. The European honey bees Apis mellifera however do not have such defenses, and will ignore or disregard the marking scent of the Asian giant hornet, which spells fatality for that colony (McClenaghan et al., 2018).

Beehive Modifications for Hornet Control

As mentioned above, hive entrance reducers make it more difficult for the Asian giant hornet (or any other large predator of honey bees) to enter and attack bees, on the basis that this hornet species is 2 inches/ 5 centimeters long to the honey bees’ 1 inch / 2.5 centimeter length. Further protective netting can be wrapped around hives with a hole diameter that is wide enough for bees to fit through.

In the giant hornet’s native range of Japan, beekeepers construct hives with narrow, metal entrances that prevent hornets from entering, and utilize sticky sheet traps to capture scouts or an invading hornet party.

Baiting and Trapping of Predatory Pests

When engineering methods are impractical, there exist simple(r) solutions such as the mousetraps mentioned above (and featured on the YouTube channel, Japanese natural beekeeping), or commercial wasp traps. The sticky mouse traps work by gluing the hornet’s legs onto the board where they land. When a single hornet is stuck, it releases alarm pheromones that cause more hornets to gather around the trap and help their unfortunate peer, only to be trapped in the process. Over time, dozens of hornets will be adhere and die from starvation, and the container can be safely disposed of. Traps can be baited with sugar or meat, depending on the season (summer/fall & spring respectively) to increase their effectiveness

Bait-based traps can be made at home using a bottle trap (as outlined in the article on wax moths), whereby a plastic bottle is inverted to create a funnel, filled with a sweet mixture that attracts the hornets. Once they enter, they cannot get out and thus starve to death. More simple solutions involve a bait-loaded jar with a funnel on top which the Asian giant hornets can enter, but not exit and drowning in the mixture underneath. These traps are placed in the evening or early morning when hornets are less active either near the beehive or the hornets’ nest.

A word on meat-baited traps: using fresh meat can attract non-target scavenging specie like birds, large cats or (if you have them) bears, which can lead to unintended and much more catastrophic consequences. Ever seen what damage a bear can do to a hive? Well you’re in luck, here’s an image below:

How Asian Giant Hornets Impact Honey Bees

Hornets can decapitate and kill many adult bees rapidly during sustained attacks. This devastates European bees as a colony can be destroyed in a few hours. For Asian Honey bees, (particularly the Japanese subspecies Apis cerana Japonica), the impacts are not as bad – a strong colony can fight back and repel invasions (explained below). Alas, their European counterparts lack such defences and are more vulnerable to the hornet raids that occur in the autumn when food sources are scarce (they target the brood and food stores to bring back to their nests for their developing hornet larvae).

Asian Honey Bees, A. cerana Fight Back Against V. mandarinia

So how does Apis cerana withstand attacks from the Asian giant hornet? The 2021 review by Cappa & colleagues investigated the co-evolutionary history of this predator-prey interaction, and found that our hornet hero, Vespa mandarinia, developed its predatory behaviours in response to adaptions by colonies of Japanese honey bees. Since then, an evolutionary arms race has taken place between bee and hornet species:

1. Predator effect: Scout hornets rub pheromones and markers against or near the colony walls to signal its location to other hornets.
   1. Worker bees can detect the marking pheromone and recruit nestmates to guard the entrance, giving the appearance of a strong, well guarded hive to deter the scouting hornet.
   1. Worker bees gather “filth” (such as animal feces) and plaster them around the entrance to muddle scents, and perhaps give the impression that the colony is abandoned.
      1. This effect was only recently documented by Mattila and colleagues in Vietnam against the Southern hornet Vespa sorer. Only A. cerana colonies that were attacked by these hornets (who also attacked in groups)performed this activity as a way to repel future attacks from the same hornets. Colonies that were not attacked did not exhibit such behaviour.
      1. For A. cerana japonica, they perform the analogous “plant smearing” rather than faecal matter in response to raids from V. mandarinia (Fujiwara et al., 2016). Interestingly, their plants of choice happen to have hornet-repelling properties
2. A large & heavily-armoured solitary hornet enters the nest and begins consuming bees.
   1. 400-500 worker bees cluster around the hornet in groups of 30, furiously beating their wings and stinging it. This raises the temperature inside the ball to ~48°C and increasing carbon dioxide concentrations, suffocating the hornet to death.
      1. The Asian giant hornet hunt in groups, however other Vespine wasps species like Vespa velutina (Asian hornet [regular size]) and Vespa crabro (European hornet) are solitary hunters, and the colony will resume normal activities once the hornet dies from heat balling.
3. A hornet hovers in front of the nest entrance to hawk (catch) returning bees and departing foragers from the air.
   1. Visual sighting of hornets increases guard bees at the nest entrance. It also causes bees to conduct a “shimmering” maneuver by coordinating a rapid shaking of abdomen and buzzing in synchrony to gesture “(Peekaboo) I see you”.
      1. Well, maybe not the “peekaboo” part.

Similarities with Other Honey Bee Species

Other honey bee species that nest in the open, such as the giant honey bees, Apis dorsata and dwarf honey bees Apis florea also exhibit this shimmering behaviour against hornet predation, providing a visual cue to the approaching hornet in order to confuse or repel it (Kastberger et al., 2014). The intensity and frequency of the shimmering increases as the predator gets closer to the colony, however it is (unsurprisingly) unsuccessful in deterring larger predators like birds (Kastberger & Sharma, 2000).

European Honey Bees Lack Adaptations for Dealing with V. mandarinia; Colonies Suffer

In general, hornets have a higher impact on A. mellifera than A. cerana. A. mellifera has not evolved alongside group-hunting hornets like Vespa mandarinia, they will not be able to defend themselves from attacks, nor comprehend the significance of the scouting hornets. The reader may be thinking “but European honey bees also ball intruders, just look at how they kill a false queen bee!”. Correct, for they have evolved to recognized a pretender as something that should not be. The Asian giant hornet however, are not deemed a threat and so the colony is destroyed with no survivor stock remaining to learn from this experience.

Some subspecies of Apis mellifera, such as A. mellifera Cypria out of Cyprus shows a degree of adaption to their hornet predator Vespa orientalis (the Oriental hornet; Papachristoforou et al., 2011). These adaptions are similar to A. cerana, in that upon sighting the hornet, they increase the guards at the nest entrance and ball it should it come close or retreat inwardly and form a defensive line at the hive entrance, preventing the predator from entering¹. Researchers found that the choice of defensive tactics was not dependent on colony strength; colonies of equal strength and numbers randomly chose between offensive and defensive measure.

Death by Bee-balling Hornets is not Temperature Dependent

The balling method is suitable in defending from V. orientalis attacks, as they have a lower lethal temperature limit than V. mandarinia (44°C compared to ~46°C respectively; Ken et al., 2005), whereas bees do not start dying until 50°C. Indeed, the temperature inside bee balls varied between 45-46°C in A. cerana (Sugahara & Sakamoto, 2009), but only 44°C in A. mellifera. (Papachristoforou et al., 2011). Within A. cerana bee balls, Asian giant hornets die in less than ten minutes, as the carbon dioxide concentrations increase to 3.6% (compared to the ~ 0.04% CO2 in air).

Sugahara & Sakamoto, 2009 reported two phases of temperature ranges in A. cerana bee balls: a rapid rise followed by a plateau at 0-5 minutes, and then a maintenance of that plateau with slight decreases in the second half, at 5-10 minutes. There were no survivors after ten minutes. The authors suggest that these phases are deliberately timed by the bees, who sense the physiological condition of the trapped hornet, maintain the latter phase as an endurance test which the hornet does not win.

From Asian Giant Hornet to Northern Giant Hornet

“Murder hornets”, “Asian giant hornets” or “Asian hornet” (the latter being a different species, Vespa velutina). These common names were retired in July 2023 in favour of “Northern Giant Hornet”, which was selected by the Entomological Society of America- probably because it’s politically correct.

What do Asian Giant Hornets Look Like?

They look magnificent. Asian giant hornets are the largest hornet species worldwide, measuring up to 2 inches long (5 cm), with a large orange head and very striking, predatory eyes. Their abdomen is black with orange-yellow stripes (or, a yellow-orange abdomen with black stripes).

They live within temperate or mountainous regions and are known to build nests underground or in hollow tree trunks, sometimes occupying or enlarging burrows built by previous animal tenants. The nest structure features several horizontal combs made of paper, with comb components being chewed bark and saliva. The nest begins with a single queen and several workers, expanding to a thousand developing brood at its peak.

Asian Giant Hornet Lifecycle

The colony experiences a yearly life cycle. In the spring, mated queens leave their overwintering soil burrows in search of a suitable habitat. Initially, they feed on tree sap (a source of complete nutrition with sugars, amino acids, vitamins & minerals) and gather woody materials in preparation for comb building. A small number of brood cells are made by the queen (the workers will expand the nest), who lays the initial cluster of worker eggs and tends to them until they hatch.

Depending on the initial brood size, the queen may retire from nest building activities and focus solely on egg laying since the workers forage and maintain the colony throughout the summer and fall. Male hornets emerge mid-autumn and wait outside the nest to mate with the new queens (who unlike queen bees, usually mate with only one male). The now-mated queen finds a suitable spot in the soil to burrow and overwinter, beginning the cycle anew next spring.

It takes around 40 days from an adult worker hornet to emerge from the egg. Regarding the larvae, they undergo five development stages (instars), and are fed a diet of either tree sap or animal tissue. The pupation phase lasts around 18 days, and the larval stage 18-20. In Asian giant hornet nests, the comb for worker brood can be distinguished from queen based on dimensions – 11 mm in diameter and 33mm deep, whereas queen comb is 14mm in diameter and 42mm deep (Matsuura and Sakagami 1973).

Vespa mandarinia in North America

In 2019, officials in the Pacific northwest were alerted to the presence of these large wasps in both Washington and the Vancouver Island, believed to have arrived from two separate introductions. More than just being an invasive species, there are also human health concerns over their ability to sting repeatedly. Liu et al., 2016 reported cases of single or multiple organ failure as a result of Vespa mandarinia stings in China (“murder hornet” is unfortunately true here).

The introduction of this species raised concerns about the ability of V. mandarinia to establish itself in this novel ecosystem. Wilson & colleagues, 2023 sequenced the droppings of V. mandarinia larvae in North America and found that the adult hornets were feeding other wasps and insects to their young. They concluded that this hornet species is highly adaptable to the novel (yet similar) environment of the Pacific northwest, and will consume “entire families of insects that have not been previously recorded as prey”.

In October 2020, the first nest was located and eradicated in Whatcom county, Washington. Public engagement by the Washington State Department of Agriculture have provided information on how to identify the Asian Giant Hornet and report sightings, as well as guidance on what not to do (e.g. not to kill native hornet and wasp species). To the untrained (and panicking) eye, Vespa mandarinia nests may be hard to distinguish from say, the introduced European hornet, or various social wasps species like paper or yellowjacket wasps².

The good news is that there have been no new nest detections throughout 2023 in Washington. Likewise, Canadian officials reported no new detections in 2022 and 2023. Trapping efforts are still underway, as one possible criteria to confirm the eradication of the Asian giant hornet is “no new detections over multiple (e.g. 3), seasons”.

The Importance of Bees, Wasps and Hornets

Ecologically, these insect species are beneficial to the environment. Bees have been buzzed about extensively, whilst wasps and hornets (which are really, just “large wasps”) fulfill their ecological niche as pollinator and predator species that keep prey and pests in check. While terms like “slaughter phase” and “murder hornets” are alarming (and very, very, fascinating), it is all just Mother Nature doing her part in maintaining a balance.

For beekeepers dealing with wasp (or giant hornet) attacks, consider reducing the entrance size and using wasp traps to help honey bee colonies survive. For those outside of eastern Asia, if you think you see a a giant hornet, particularly the Asian giant hornet, then report it to your local department of agriculture.

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Footnotes

1. The success of this tactic lays in the hive entrance structure – colonies performing the defensive retreat had small entrance holes filled with propolis, which hornets are not able to penetrate and thus enter the hive. The strategy may sound better as it limits the number of bees lost compared to the offensive, however these bees would be prone to starvation as there is an extended hornet siege (which can be likely in the case of V. mandarinia.) ↩︎
2. Visual comparisons of the Asian giant hornet and lookalikes are provided on the USDA website. ↩︎