Allien Species: Vespa Velutina Nigrithorax (Hymenoptera: Vespidae) – Proliferation and Methods for Its Control

Abstract

Vespa velutina nigrithorax has become an invasive species since its introduction in Europe, as it is a predator of native fruits and insects, mostly honeybees. In this chapter, the morphology of the Vespa velutina is described together with its life cycle and the morphology of the nest that this hornet builds. The proliferation of this invasive species in Europe and the economic, environmental and human health impact that it produces makes necessary the use of methods for its control. The methods for the inactivation of nest, trapping hornets and minimising the impact on apiaries are the established methods to fight against this species. Behavioural study of these hornets in captivity has been carried out to increase the knowledge on the ethology of Vespa velutina. For this purpose, two secondary nests and one embryonic nest were captured and kept under controlled environmental conditions for up to 13 weeks for the secondary nest and 6 weeks for the embryonic nest. Adaptation to captivity, defence against disturbance, colony evolution and hibernation were the different behaviours observed. The results of this research will allow us to obtain additional information on this species, which are crucial to develop effective control methods for this invasive species.

1. Introduction

Vespa velutina nigrithorax (Lepeletier, 1836) is a eusocial hymenopteran belonging to the genus Vespa, one of the four genera in the subfamily Vespinae [1, 2, 3]. This species, better known as the Asian or yellow-legged hornet, like most Vespa species, is native to the Asian continent, extending from Afghanistan to Eastern China, Indochina and the Indonesian Archipelago [1, 4, 5].

This type of insect is characterised by the most highly developed social behaviour, including some species of bees, hornets, wasps, ants and termites. As a eusocial insect, Vespa velutina spends most of its life cycle in large colonies of hundreds to thousands of individuals, organised in castes, with different assigned roles [6, 7].

The caste system of Asian hornet is divided into queen, workers and males. The queen is responsible for the laying of eggs, from which new individuals will hatch. The workers, who are females, take care of the other functions. They feed the colony by collecting proteins, sugars and water, which will be supplied by trophallaxis, from adults to larvae and vice versa. This way of feeding means eusocial insects are not able to survive alone. They are also responsible for building, cleaning and repairing the nest, as well as protecting it, acting as guardians at the nest’s entrance. The males have as their only function to fertilise the new queens [7, 8, 9].

2. Vespa velutina’s morphology

Vespa velutina is physically similar to Vespa crabro (Linnaeus, 1758), which is the native European hornet, although they are easy to differentiate. Vespa velutina is slightly smaller, measuring between 2.5 and 3 cm, while Vespa crabro can grow up to 3.4 cm. Their colours are completely different, which is the best feature to differentiate between the two species. Vespa velutina has an orange head and a black face, and the thorax is entirely velvety black. The abdomen is also blackish except for the fourth segment, which is orange, as well as a thin orange band between the first and second segment. The dark colour of its body continues up to the middle of its legs, which are yellow at the ends. Its wings are dark with brownish tones.

Vespa crabro has a reddish head and yellow face. Its thorax is a combination of reddish-brown and its abdomen has the first two black segments separated by a yellow band, while the rest are yellow. Its legs are completely brown, the same colour as the wings (Figure 1).

Figure 1.

Differences between Vespa velutina (left) and Vespa crabro (right). Source: Image modified from http://anti-frelon-d-asie-jp33.over-blog.com (accessed on 9 January 2024) and from [10].

Other characteristic of both hornet species is the shape of the end of the abdomen to identify sex differences. In males, it is flattened because they lack a sting, while in females it is conical, where the sting is stored (Figure 2).

Figure 2.

Physical characteristics of Vespa velutina and differentiation between males and females. Source: Images modified from J. Urkiola and M. Lopez of Apicultura Ibérica journal.

3. Life cycle

The life cycle of Vespa velutina (Figure 3) is similar to the life cycles of other species of Vespa [12]. It starts when the queen emerges from its lethargy, who was overwintering, builds the embryo nest in a protected site and lays the first eggs. This time is the most vulnerable for the queen because she has to forage for itself and hunt to feed the first larvae alone until the first workers emerge, as well as collecting material for building the nest [7, 8, 13].

Figure 3.

Life cycle of Vespa velutina. Figure slightly modified from [11].

The larvae are fed on various insects and arachnids, which provide protein, while adult individuals, on the other hand, are unable to digest protein, and their diet is based on sugary liquids (nectar from flowers, blossoms or fruit) [14, 15]. In addition, after eating, the larvae secrete a substance rich in amino acids, which serve as food for the workers and the queen in case of need [16].

Between late spring and early summer, the colony increases, causing a lack of space in the primary nest, which leads to the construction of a secondary nest. They usually abandon the embryonic nest and build a new one in the upper part of leafy trees. In midsummer, due to the large number of larvae in the nest, this is the phase that gives rise to a great deal of insect predation, especially in apiaries of bees. From the construction of the secondary nest until autumn, hundreds to thousands of hornets may emerge [17].

With the arrival of autumn starts the reproduction season, the gynes (future queens) and males, from another colony, mate outside the nest. This is to avoid the inbreeding of the species. With the onset of winter, due to lack of food, comes the extinction of the nest with the death of the queen, workers and male, while gynes mated and unmated search a place to overwinter until next spring, when the new cycle starts [8, 9, 10, 13].

4. Nests’ morphology

The nests of Vespa velutina (Figure 4), like other species of wasps and hornets, are mainly made of bark from different trees. The bark is shredded by the hornets with their mandibles and mixed with saliva to form a paste with which they shape the nest [18, 19].

Figure 4.

Primary nest (left) and secondary nest (right) of the invasive species Vespa velutina. Source: Image modified from J.M. Perez de Ana.

As indicated in the section on the life cycle, Vespa velutina generally builds two nests during its lifetime. Although both have similarities in the way they are built, there are some differences between them [10, 20].

Both nests have an outer shell made up of several protective layers, with secondary nests having between 5 and 10 layers, and primary nests having between 1 and 3 layers. The nest structures have a single entrance hole, which is at the bottom of the primary nests and at the side of the secondary nests, being much smaller in the last one (approx. 1.5 cm).

Primary nests are usually about 5–10 cm in diameter, and their shape has a usually circular form. Secondary nests are usually circular or oval in shape and can be as large as 60 x 80 cm [8, 10, 12].

Inside the nests, we can find the combs with the cells, where the queen lays her eggs. These combs are built by the workers as the colony grows, and the first combs are found in the upper part of the nest. In the embryonic nests, there are usually no more than three combs, and they can contain about 30 cells, while the secondary nests at the end of the cycle can have up to 10 combs with an average of 150 cells each and can house up to 1500 individuals [12, 17, 19].

As for their location, primary nests are constructed in places sheltered from the weather and possible predators and are mostly found in the eaves of windows, inside garages, shacks, under roofs or in very leafy bushes. Secondary nests are usually built outdoors at high altitudes in trees near a water source. However, they can also be observed in urban areas, in roof eaves, garages, empty hives and even on the ground [21, 22, 23].

The differences with the Vespa crabro nest must be focused on the secondary nest. The primary nests are very similar and can only be differentiated by identifying the insect inside. This species usually builds the secondary nest in holes in the bark of trees, holes dug in the ground or in cavities in buildings such as roofs. These are usually well camouflaged. Therefore, the shape of these nests depends on the location where they are built. There are occasions where they build them in plain sight, but they are usually in enclosed, sheltered places, such as warehouses, animal huts or garages. Another major difference with the Vespa velutina nest is the opening of the secondary nest, which is in the lower part of the nest and is large enough to allow the cells inside to be seen [18, 24].

5. Proliferation in Europe

The increase of global connections, especially in the commercial sphere, has resulted in a constant flow of people, goods and technologies. This movement has sometimes resulted in the accidental introduction of new species into countries where they did not previously exist. The adaptation of most invasive species to the new environment gives rise to a negative impact on ecosystems.

Regarding hornets, not all invasions are successful in establishing themselves in the invaded country, but there are exceptions. This is the case of the recent Vespa velutina, which was accidentally introduced into Europe in 2004, via the port of Bordeaux in cargo from China, which contained a dormant, fertilised queen [12, 25, 26]. Although the pathway of introduction remains somewhat uncertain, evidence that the spread into Europe began with a single queen was confirmed in 2022 by the research group of Dillane et al. [27]. It was not until a year later, in 2005, that the first record of the species was made in south-west France (Lot-et-Garonne). Since then, over the course of two decades, the insect has expanded its presence exponentially, expanding rapidly in Europe, with its presence confirmed in countries, such as Spain, Portugal, Belgium, the Netherlands, Germany, Italy, the United Kingdom and Ireland (Figure 5). Therefore, it represents the first successful invasion of the Vespidae family on this continent. It is estimated that Vespa velutina can extend its territory by 80 kilometres per year [13, 28]. The successful invasion and spread of the Asian hornet in Europe have been due to a combination of factors. The favourable climatic conditions have played a crucial role, being similar to their place of origin, with moderate temperatures and sufficient humidity. In addition, the inexhaustible source of available food and the absence of natural predators that could control their population have provided them with a suitable habitat to adapt and reproduce rapidly and successfully throughout Europe [29].

Figure 5.

Expansion of the Asian hornet in Europe in 2023. Created by https://www.mapchart.net/ (accessed on 9 January 2024). Figure from [11].

The massive attack on apiaries, fruit orchards and crops due to the need for protein and carbohydrates to feed larvae and adult hornets, respectively, is causing significant damage. These include crop losses, reduction in the number of pollinating insects and honey production, resulting in large economic losses in the agricultural and beekeeping sector, as well as in biodiversity [14, 15, 30]. In addition, Vespa velutina is also a danger to public health. Its sting can cause serious human health problems, especially for allergy sufferers. Although the attack on humans is scarce, they can occur if the colony feels threatened or if people come too close to their nests [12, 26, 31].

6. Control methods

The damage caused by Vespa velutina has led to the development of various control methods, with the aim of reducing its spread and minimising its impact.

The control methods developed up to now can be divided into those that inactivate the nest, those that trap hornets and those that help to minimise the impact on apiaries.

6.1 Inactivation of nests

This group of methods is based on eliminating or inactivating nests in order to minimise the damage that a nest can cause, as well as to prevent the hatching of new hornets, thus preventing the survival of the colony. It is worthwhile to consider that most of the methods in this category require specialised personnel, such as firefighters or forestry experts. In addition, the use of a specific Vespa velutina suit is mandatory for nest removal work. The costume is similar to the beekeeping costume but it is made with a stronger and thicker material, because of the larger size of sting of the hornets. Furthermore, it is necessary to wear special gloves and inside the hood, it is recommended to wear a helmet with a protective shield to avoid skin or eye contact with the biocide or venom. The latter is because the hornets spit venom when they feel threatened.

When the nest to be removed is primary, it can be easily removed without calling the professionals. This can be done by applying a common insecticide to the nest, by placing it in a container which must then be frozen to kill the individuals inside or by placing the nest in hot water (> 65°C) [10].

However, secondary nests must be removed by professionals due to the difficult access, where they usually build these nests, and/or the large size of the colony they contain. The most commonly used method to inactivate nests at high altitudes is by inoculation of a biocide with the help of a telescopic rod that can reach 3–25 m in length [10, 32]. In Figure 6, two workers can be seen using a telescopic stick to inactivate a secondary nest with biocide.

Figure 6.

Two workers inactivating a secondary nest of Vespa velutina with a biocide applied by means of a telescopic stick. Source: Daniel Martínez.

If the nest is in an area that cannot be reached with the telescopic pole and is not a danger to humans, it is usually not treated. However, if it does pose a danger, there are other methods of inactivation:

• Shotgun shots to destroy the nest (Figure 7a), the least reliable method, due to the high probability of colony survival, as the nest is broken up leaving surviving hornets that can rebuild the nest and continue to cause damage, even if the queen dies [10].

• Shooting with an adapted paintball gun using the system created by the company Asiatic Wasp Balls (Cantabria, Spain). It is based on projectiles to which the biocide chosen by the user is added and frozen until use. These are fired like any other projectile into the nest, but once they enter the nest they release the biocide as they thaw, killing both the individuals inside the nest and those that return later. However, sometimes, either due to lack of aim or because of the large size of the nest, it is necessary to fire several bullets, which can cause the nest to break up, and with it a high survival rate of the hornets. In addition, all bullets that do not hit the nest remain in the environment and can be ingested by other animals.

• Drones with a telescopic stick (Figure 7b) is very effective, but it is very expensive, and it is also necessary to have a clean flight path, because any brush with a tree branch can cause the drone to crash and break [33].

Figure 7.

On the left, a man shooting at a nest (source: Monica Torres) and on the right a drone with a stick attached inactivating a Vespa velutina nest (source: Miguel Souto).

Within this group of methods, it is important to highlight the work carried out by Ruiz-Cristi et al., in which they have developed a new green control method for the inactivation of nests. Unlike traditional methods that inject different types of biocides, this method is based on the injection of hot steam inside the nest, reaching lethal temperatures for Vespa velutina [34].

Before carrying out any of these methods, it is recommended to make sure that the queen is inside the nest. The best time to do it is at night, when the entire colony is in the nest, and using only a red-light source as illumination, because Vespa velutina is not able to perceive it.

6.2 Trapping of hornets

The aim of individual trapping is to catch as many hornets as possible using attractive traps (Figure 8), usually made with sugar-rich ingredients. If the aim is to capture as many foundress queens and future queens as possible, to prevent growth and the construction of new nests, these traps should be set in early spring and late autumn. This is the period of greatest effectiveness against the invasion, since one queen caught means one nest less.

Figure 8.

Attractive traps to control Vespa velutina. Source: Lioy S [35].

It is important to highlight that this method is not selective for Vespa velutina and therefore attracts a multitude of insects, mainly Diptera and Vespidae.

Since the introduction of Vespa velutina, there has been an increase in the use of traps by private individuals, both commercial traps and traps made at home with natural ingredients, such as beer, white wine and sweet fruit juices. This uncontrolled trapping leads to a reduction of the local insect community and thus to possible consequences in future local biodiversity. Therefore, the placement of these traps in the Basque Country and in other regions of Spain must be requested and authorised by the competent local environmental institution [36].

For this selectivity problem, managing to create other types of traps or methods with more selective attractants for Vespa velutina is of special relevance for the control of this invasive species.

It is important to note that among the attractant traps are those made with Vespa velutina sex pheromones. Several research groups have worked on both the isolation of pheromones and the development of traps. The main objective of these traps is to capture sexually mature males, thus avoiding the mating of future queens [37, 38, 39].

6.3 Minimise the impact on apiaries

As is well known, the Asian hornet is a great predator of honeybees (Apis mellifera), for which Vespa velutina waits by gliding in front of the apiary, and when the bees are about to enter or leave it, they are easily hunted by their predator (Figure 9).

Figure 9.

Vespa velutina attacking an apiary (left) and tearing apart a hunted bee before transporting it to the nest (right). Source: Jean Haxaire and Quentin Rome [40].

Because they only need the parts of the bees that are richest in protein to feed the larvae, adult hornets form a small protein pellet from the bee thorax, discarding all unnecessary body parts, such as the legs, wings, abdomen and head, and in some cases the cuticle of the thorax itself. If the apiary is weakened, the hornets are able to get inside and plunder it completely, thus obtaining the carbohydrates that the adults need to feed on the honey inside.

Due to the predatory behaviour towards bees and the absence of defensive techniques of bees against hornets, the application of specific control methods to treat apiaries is of great necessity.

Within this group of methods, the bee-hive muzzle and the electrified harp (Figure 10) stand out. Both are physical methods that are placed in the hive to block the entrance of hornets. The first is based on a net placed in the hive entrance, which has holes with a diameter that allows the bees to pass through without difficulty while blocking the entrance of hornets. Although it is a cheap method that can be constructed at home, it does not kill hornets [32].

Figure 10.

Bee-hive muzzle (left) and electrified harp (right). Source: Lavignotte a. [32] and Rojas-Nossa, S. V. [41], Respectively.

The electrified harp is a structure that is placed between the hives, which can be square or rectangular, and which has wires in the middle connected to an electric current. These wires are placed at a distance from each other to allow only insects smaller than the Vespa velutina to pass through. When the hornets pass through this structure, they receive an electric shock. This is an effective and selective method of combating the species, however, on the downside, for treating apiaries with large numbers of hornets, it can be a great economic investment [32, 41].

A technique that combines the reduction of pressure in the apiaries with the inactivation of the nests is the placement of biocidal protein baits inside the nests [42, 43].

The use of biocidal baits is a practice already used to combat other types of insect pests, such as Diptera, Hymenoptera, Hemiptera or Dictyoptera [44]. However, a few studies have been conducted on the use of protein baits to mitigate the destruction of bee apiaries by Vespa velutina.

The Basque Institute for Agricultural Research and Development, Neiker(Derio, Bizkaia) and the company supplying the baits, DTS OABE S.A. (Orozko, Bizkaia) carried out experiments to test the efficacy of these protein baits. These are placed near the apiaries to act as a protective barrier. The hornets are attracted to the pieces of bait that they take and introduce them into the nest to feed the larvae, and the effect of the biocide kills them by temporarily inactivating the nest, thus reducing the pressure on the apiary. In their research, they observed how the hornets effectively took the protein bait and flew away with it in their mouths (Figure 11). The presence of the bait caused the hornets to be attracted to them, allowing the bees to continue foraging.

Figure 11.

Asian hornets taking bait pieces from the study conducted by Neiker and DTS OABE S.A. in 2022.

Within 48 hours of the trial, a decrease in the number of hornets in the apiary was observed, maintaining a constant number of hornets for at least 2 weeks after treatment. However, after a few weeks, the number of hornets in the apiary began to increase. This is because the larvae that were operculated did not receive bait, hatching and continuing to grow the colony. Therefore, it is important to re-treat the apiary when an increase in the number of hornets is observed [36].

The protein baits used consist of different animal proteins and the insecticide phenylpyrazole, fipronil, as a biocide. This broad-spectrum insecticide is the used biocide in commercial protein baits [44]. It is important to consider that it would be necessary to study the possible risks that dead larvae and hornets and the remains of bait may present for birds, other insects and the environment.

Even knowing the proven effectiveness of bait in apiaries, due to the difficult access and search of nests in the wild because of the location of their constructions, it is not possible to know the effect caused by the bait on larvae and adults inside the nest. Studies with hornets under controlled environmental conditions in the laboratory represent a useful tool to obtain knowledge on the Vespa velutina in the nest.

7. Vespa velutina’s behaviour in captivity

Very few studies in the literature have focused on the behaviour of Vespa velutina in captivity. The behavioural studies that have been reported are nest defence, colony activity and olfactory attraction [45, 46, 47].

In our research group, an observational study on the behaviour of Vespa velutina in captivity has been carried out [11]. Two secondary nests and one embryonic nest were captured and kept under controlled environmental conditions for up to 13 weeks for the secondary nest and 6 weeks for the embryonic nest. The different behaviours studied included adaptation to captivity, defence against disturbance, colony evolution and overwintering. In Figure 12, the locations of the different nests are given.

Figure 12.

Location of the Basque Country in Europe (left). Map of the Basque Country, with its three counties (right). The numbers indicate the locations of the Vespa velutina supplied nests. Image modified from paintmaps.com. Image from [11].

The nests were collected in transparent plastic boxes, which had little holes to allow hornets to breathe. An adhesive tape was used to close the boxes.

In order to be able to handle and introduce the Vespa velutina nests into the captive cage, the hornets were made to sleep using diethyl ether (99.7%) as an anaesthetic. Once all the hornets were asleep, they were placed, together with the nest, in a wooden cage (1 x 1 x 1 m) (Figure 13).

Figure 13.

Scheme of the cage used for the captivity study of Vespa velutina. Image from [11].

On each day, hornets were fed with a honey-water mixture (50:50), which was placed inside the cage and on the top grid impregnated in a common tissue and different pieces of fruit (banana, grape and apple) [16, 17, 20], which were placed on the side grids and inside the cage. Furthermore, fish protein, which is the main component of the tested bait pieces, was provided for the purpose of feeding the larvae. Both water and the honey-water mixture were supplied ad libitum, as reported in other studies [11, 17, 21].

To simulate outdoor conditions, the nests were adapted to a temperature-controlled environment (25–28°C) with light cycles (12 hours). In addition, different materials were provided to serve as supplementary food or to condition the nest (tree bark, fruit plants, stones and grass, pieces of nests) [20].

Each morning and evening, the different behaviours of the hornets were observed and recorded for at least one hour.

It must be considered that animals living and growing up in captivity experience drastically different conditions to those of the species in the wild, and therefore may act differently [23].

7.1 Behavioural observations in captivity

7.1.1 Adaptation to captivity conditions and defence against disturbance

After awakening from the anaesthetic, during the first two days, the hornets in the three nests showed aggressive behaviour towards their own colony and defensive against any kind of external disturbance. It was observed how they attacked and destroyed the nest, breaking the outer layers, fighting among themselves and even killing each other and extracting the larvae from the cells (Figure 14). Although this behaviour also occurred in the embryonic nest, the aggressiveness was less pronounced, especially with respect to the larvae. According to Perrad et al., this may be due to the fact that primary nests of Vespa velutina are not as aggressive towards humans, and therefore also towards their own colony, since colonies close to residential areas have been observed to become accustomed to the presence of humans [46].

Figure 14.

Adult Vespa velutina hornet removing a larva from the nest, with another previously removed larva (left). Vespa velutina female hornets fighting with each other shortly after the nest was collected (right). Image slightly modified from [11].

Nervousness was another behaviour observed during the first days in captivity. In fact, when food and water were provided, they ignored it. This behaviour is not new to this study, as a study by Perrard et al. [46] on the activity of a colony kept in semi-captivity showed that when the nest was manipulated, it caused a disturbance during the first two days of captivity.

After approximately two days in captivity, the hornets showed a more relaxed and less aggressive behaviour towards the outside world and started to take food and drink water.

Regarding the protein food provided, the hornets did not take it on any of the occasions. This behaviour was related to the possible lack of live larvae inside the nest or due to the fact of being out of their habitat, which could have affected them when taking care of the larvae.

However, due to the absence of scratching noises inside the nest, which are characteristic of larvae when begging for food, the most reliable hypothesis for not taking the protein food is the lack of live larvae inside the nest [46].

Due to the lack of live larvae, it was not possible to obtain information on what happens in the nest when bait rich in protein is introduced.

However, in the embryo nest, the survival of the larvae and how they were fed with protein food by the workers could be verified. These results provide an opportunity to carry out in vivo studies using embryo nests to evaluate the effectiveness of protein baits in the control of this invasive species.

7.1.2 Evolution of the colony and overwintering

Over the first two weeks, the operculated cells in the nest reception area began to hatch, and new hornets, mostly males, emerged from these cells. The new individuals were identified by their characteristic whitish colouring on the thorax and wings attached to the body, which occurs during the first days after hatching, due to a lack of keratin. As expected, due to the period of the life cycle in which the embryo nest was located, most of the newborns were females.

As the weeks went by, the number of new hornets decreased. In the research carried out by Monceau et al. [47], this evolution was also observed, detailing when the queen is caging it stopped laying more eggs. It is thought that this could be due to a lack of nutrients or resources to regulate the colony or its sanitary maintenance, as well as the stress generated by captivity.

The separation of individuals in the colony by sex was another of the behaviours observed, as occurs in wildlife. With the sexual maturity of the males, they were placed in the corner of the cage furthest away from the nest. In nest 1, when males approached the nest, several female hornets were observed to act aggressively towards them, attacking them with bites and stings to the point of death. Following these attacks, dismembered male hornets were observed in the cage. This type of reaction, previously observed and reported in Vespa simillima (Cameron 1903), could be due to a mechanism of females to avoid inbreeding [47].

This behaviour was not observed in the embryonic nest, as only three males hatched and died the following day.

Mating between the hornets in nest 1 was detected up to 5 times between the fourth and sixth week of captivity (Figure 15). As in the wild, mating took place on the cage walls furthest away from the nest.

Figure 15.

A couple of Vespa velutina hornets from nest 1 mating in captivity. Image from [11].

With the passing of the weeks, in both secondary nests the number of individuals in the colony started to decrease, with new dead hornets appearing every day. This started to occur from weeks 7 and 10 for nests 1 and 2, respectively. When the observed number of live hornets in the cage was less than 15 and females were present, in order to simulate the arrival of autumn and the entry into dormancy of the future queens, the laboratory temperature was lowered (from 25–28°C to 16–17°C) and food was gradually reduced, until there was no food.

Following some weeks under non-feeding conditions and low temperatures, in nest 1, it was observed that only three female hornets had survived, so they were considered as possible future fertilised queens. Therefore, in order to try to complete the life cycle of Vespa velutina in captivity, a hibernation study was carried out. For this purpose, the environmental conditions were as follows. The temperature was lowered to 13°C and the light cycle was removed, simulating the hiding place they seek for the winter.

To ensure that hibernation was not interfered with, the condition of the hornets was checked once a week without turning on the light. Figure 16 shows how during one of the weeks of the hibernation study, one of the hornets shifted from the corner and moved slightly from its initial position, while the other one and the one on the branch did not move.

Figure 16.

Vespa velutina hornets from nest 1 in state of lethargy in captivity. Image from ref. [11].

After 5 weeks of hibernation, the hornets awoke from dormancy. This made it necessary to start a new biological cycle, for which the environmental conditions had to be modified, increasing the temperature (21°C), repeating the light cycles, and supplying food and water. The hornets were active again, eating and flying normally. However, after one week, all three hornets were found dead. No conclusion could be drawn as to why this happened. Different factors such as the environment, which is not ideal to start the cycle again, or the existence of other possible fertilised queens in the same environment, as in nature, for survival reasons, they do not hibernate in groups and build the primary nest individually far away from other gynes.

8. Conclusions

Vespa velutina nigrithorax is considered as invasive species in Europe and represents a threat to the biodiversity, agriculture, apiculture and human health. Its expansion in Europe is estimated at up to 80 km/year.

The use of protein baits with fipronil in apiaries when in the presence of Vespa velutina can reduce the pressure of the hornets on the hives for at least two weeks after baiting. Furthermore, bees can recover their normal activity.

Apart from the known control methods, more selective and environmentally friendly methodologies are being developed to fight against this species.

The observational study of Vespa velutina’s behaviour in captivity has showed the possibility to maintain secondary nests up to 13 weeks under controlled environmental conditions.

The captivity studies allow deepening the knowledge surrounding Vespa velutina’s ethology, obtaining useful information on the behaviour of this invasive species.

Acknowledgments

Omaira de la Hera thanks the University of the Basque Country (UPV/EHU) for her predoctoral grant. Authors thank the Basque Country Government (projects PUE 2018_1_0007 and PUE 2021_1_008) and the University of the Basque Country (UPV/EHU) (project US21/35) for financial support. We thank the public company BASALAN and Avispa Asiatica Association (Villarcayo, Spain) for providing nests for the laboratory tests.

Conflict of interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.