Invaders bring new challenges

Research to better understand the Asian Hornet

The Asian Hornet arrived in Europe in 2004 and has quickly and surely expanded its distribution area since then. This is bad news for honey bees – the hornet’s favorite prey. Working with academic partners in France, Bayer has supported a study on the Asian Hornet, advancing the knowledge on this little-investigated predator species. The goal was to understand more about the lifecycle, reproduction cycle and habitat preferences, so as to identify measures to control hornet numbers in areas where it currently occurs and, if possible, slow its spread across mainland Europe and beyond. The European Honey Bee already faces numerous challenges to its health. Addressing and looking for early ways, to minimize additional threats, such as this predator, can only help the situation.

Invaders bring new challenges
The Asian Hornet, a destructive bee-eater


// When it comes to the Asian Hornet, Vespa velutina, there are considerable knowledge gaps relating to the lifecycle, reproduction cycle and food distribution practices within a colony.

// A study in France has investigated these topics, in order to control hornet numbers and slow its spread

// Potential methods using entomopathogenic fungi for the control of the invasive hornet species in Europe were investigated as part of the study.

// Further work will be needed to see if any of the control methods identified can be turned into practical solutions.

“With this PhD project, Bayer recognized an opportunity to support beekeepers by looking for ways to limit proliferation and control the impact of the Asian Hornet, a new and additional threat to French honey bees.”

Benedicte Labourie, Biodiversity & Bees Expert at Bayer France

It hovers, hanging in the air, waiting and watching … ready to attack when a suitable prey passes unwarily into its path. The predator in question is the Asian Hornet, Vespa velutina nigrithorax (also known as the Yellow-legged Hornet), and one of its favored prey is honey bees. A hornet attack can leave a honey bee colony weak and low in numbers. The situation is aggravated as worker bees avoid venturing out to forage in case they are attacked when they do so, with the result that there is insufficient food to sustain the whole colony. A weakened colony is also more vulnerable to attack by other predators, such as wasps, and to parasites and diseases.

This is the scenario now being played out across France and other Western European countries (figure 1) since the arrival of this alien, invasive species which originates in Asia. The hornet is thought to be an accidental arrival in a shipment from East China. This highlights one of the difficulties being faced by border controls around the world on a daily basis, as they handle the import of global trade goods while trying to prevent the entry of other unwanted, uninvited guests.

With this foreign invasion threatening the local apiculture of its new home area, further action was needed. Bayer colleagues in France sponsored a PhD student, Juliette Poidatz, who has since received her doctorate for her contribution to advancing the knowledge about the Asian Hornet. Her work looked at the hornet’s lifecycle, reproduction cycle and habitat preferences in order to identify measures to control its widespread establishment in France and beyond. The aim was to find ways to control numbers and reduce the hornet’s impact on honey bees while also slowing its spread in Europe. Many others were involved in providing specific support and working with Juliette on various aspects of this study, including her host laboratory at the INRA (French National Institute of Research in Agriculture) in Bordeaux, some local French beekeepers, the French association “Association Anti-Frelon Asiatique” (Asian Anti- Hornet Association), the CNRS’s Biological study center of Chizé (CEBC) and the IRBI (Research Institute on Insect Biology) in Tours.

Honey bees are the favored prey of the Asian Hornet.

In fact, the Asian Hornet topic was not a new one for Juliette, who had had an interest in honey bees from an early age and, as a result of this, had already worked with INRA researchers two years before starting her PhD, supporting a post-doctoral study on this hornet species. “I found the work we were doing on Asian Hornet really interesting and so, when the chance to work further on this species came up, I was intrigued enough to want to learn more,” she explains.

Global trade and spread of invasive species

Dr. Juliette Poidatz, received a PhD for her work on this project which was sponsored by Bayer.

Preventing non-native plant or animal species from arriving in shipments, resulting from global trade around the world, is almost impossible these days. While the invaders may settle into niches alongside “native” species without apparent disturbance to the ecological balance, some of these new species can pose a serious threat to native species or agriculture.

Many do not warrant particular attention in their native region where they have evolved in local habitats alongside other species. However, once they are introduced and start to spread across a new region, they can impact native species. The distribution potential and impact of an invasive species depends on many factors. These include the range of environmental factors it can adapt to in its new home range, the influence of natural enemies in the new environment as well as the ability to adapt to new conditions, all of which will influence the most important factor which is its competitiveness, compared to native species. These factors govern the success of any invasive species in their new distributional ranges.

Invasive species introductions which have had severe consequences for native species include the highly poisonous cane toads in Australia, introduced to eliminate the native grey-backed cane beetle. Alas, the toads have poisoned and decimated local wildlife trying to eat them. The introduction of rats from cargo ships into New Zealand caused the near-extinction of several ground-dwelling birds including the iconic, flightless Kiwi and the American grey squirrel, in the UK, has driven the native, smaller red squirrel from large areas of its original habitat to small, secluded areas where it continues to thrive, away from its bigger relative.

Initial studies on reproduction

The initial studies, which began in 2014, focused on the reproductive behavior and sexual maturation of the Asian Hornet. “We wanted to know more about how the queens choose males to mate with. This hornet species, unlike other hornet species where queens choose only one partner, is more promiscuous and mates with up to eight partners,” Juliette comments. Due to this strategy, the Asian Hornet queen has the opportunity to store twice the amount of sperm in her body compared to the native European Hornet (Vespa crabro). This may explain the fact that Vespa velutina nests tend to be larger to accomodate a higher number of offspring than those seen for V. crabro. Unfortunately, there was no successful breeding seen in the experimental approach as the hornet queens would not reproduce normally in the laboratory. More studies in the wild will, therefore, be needed to investigate how queen hornets choose their mates. The studies did show, however, that Asian Hornet queens usually mate in fall. The queen hornet carries the males’ sperm in a specialized organ, the spermatheca: The sperm will be used all through the following year to fertilize the eggs she will produce.

In a similar way to bumble bees, the Asian Hornet queens disperse, overwinter in a suitable hibernation place and emerge the following spring to start a new colony from scratch, using the sperm they each have carried all winter. The ovaries of these queens mature before spring so egg-laying and colony initiation can take place as soon as the weather is optimal, beginning much earlier in the year compared to their native European Hornet relatives.

Native European Hornet (Vespa crabro).

Asian Hornet (Vespa velutina).

Close-up of an Asian Hornet nest.

Rare view of an Asian Hornet nest, which is usually hidden by dense foliage.

A hornet colony will produce, on average, some 15,000 new offspring throughout the season.

“The Asian Hornets’ nests, once the colony is stronger and bigger, are well-concealed and hidden at the top of high trees, so not easy to find. This may be a particular challenge in controlling and reducing the spread of these invasive pests.”

Dr. Juliette Poidatz

“The Asian Hornets’ nests, once the colony is stronger and bigger, are well-concealed and hidden at the top of high trees, so not easy to find through dense summer foliage. This may be a particular challenge in controlling and reducing the spread of this invasive pest,” explains Juliette. “The old queen will die at the end of the year, having reared her new colony, but she will also have produced some 300 new young hornet queens who will leave the nest, mate with males and overwinter, ready to start their own colonies the following spring. Even if not all of the new queens make it through the winter, enough of them will survive to start new nests and colonies. The large number of young queens setting up individual nests will make it impossible to eradicate all the Asian Hornets from France and beekeepers wanting to protect their honey bees should put their efforts into controlling the nests close to beehives,” she concludes.

A-hunting they will go

Figure 2

Understanding the homing ability and range of foraging worker hornets

Asian Hornets were fitted with RFID trackers to record when they returned to the nest.

The homing ability was tested using 320 hornets from one colony which were released at six distances between 500 m and 5 km away.

Apart from finding a suitable spot for nesting or reproduction, invasive species, such as the Asian Hornet, also need a source of nutrition in their new distribution range. For European Honey Bee colonies and beekeepers alike, this presents a problem as the honey bee is among the Asian Hornets’ favored food sources. Studies1 have shown that honey bees can contribute two thirds of V. velutina’s diet in an urban environment.

To better understand the homing ability and range of foraging worker hornets, the research team fitted the hornets with small tracers (RFIDs) which had proved successful in tracking honey bee movements in other research studies. The homing ability was tested using 320 hornets from one colony whose nest was placed in the INRA garden. Each hornet was fitted with a tracer and detectors, placed at the nest entrance, recorded their return to the colony. Hornets were released at six different distances, ranging from 500 m to 5 km, and from four different directions, with eight hornets being released from each direction, making 32 hornets from each distance (see figure 2). A second set of eight hornets were also released from each of the four directions on a different day, under similar weather conditions, for the three furthest distances only.

The results, gathered over a two-month test period and factoring in changes due to environmental conditions such as temperature and humidity, proved very educational. Of the 320 released, 112 made it back to the colony. Over 90 percent of the hornets released at 500 m made it home in less than five hours. From a distance of two kilometers away, roughly 50 percent returned to the nest, taking an average 16 hours to do so. Further tests, carried out to see how far they tended to travel before returning to the nest, suggest that their homing range is less than two kilometers from the nest. The study also showed that only 14 percent made it back from three kilometers away, taking on average 80 hours (over three days) to do so, and only five percent of those released returned from five kilometers away, taking around 176 hours (just over a week) on average. This again indicates that the Asian Hornets could not orientate themselves very well to return directly to the nest beyond two kilometers distance from it but that it was possible for hornets to still return from at least five kilometers away. In additional experiments, hornets were again fitted with tracking equipment and their foraging time from leaving to returning to the nest was recorded.

Using this and a calculated flying speed, the researchers could derive the normal transmission processes. a foraging distance for the hornets of up to 800 m from the nest, where they were most likely to be successful in orienting themselves back to their colony again with food. This fits with subsequent findings from UK studies2 but can differ, depending on other variables such as the surrounding landscape which the hornets may use for navigation, food sources available in the vicinity of a nest and other external factors.

On the menu tonight

Nests at various stages of development were given radio-labelled food to see how this was distributed to individuals in the nest.

To learn more about food distribution among those in the hornet colony, laboratory studies were undertaken over a two-year period with each test lasting three weeks. Colonies were taken from the wild. “This is not an easy task and we relied heavily on the beekeepers’ support in finding and capturing the colonies’ nests which had to be small, so no bigger than 30 cm in size, otherwise they could not be kept in our cage,” explains Juliette. Once back in the laboratory, they were put into a fridge for 24 - 48 hours to cool them and make them torpid before being transferred to the cage, a glass containment area.

The researchers thought that the sugars from the hornets’ diet would go mainly to adult hornets whereas the proteins would be distributed to larvae, as is the case in the colonies of some other social insects. From literature, it was also known that the hornet brood gives food to other hornets in the colony with an ‘offering’ of regurgitated sugar which is mixed with protein. “I heard,” says Juliette, smiling, “that this mixture is so nutritious that energy drinks are made from this in India, but I am not certain.”

What we do know is that the hornets cherish this brood food, the queen is fed this in high amounts and worker hornets, returning fatigued from foraging trips, are also awarded this ‘food’ when they need re-energizing before returning to their food-provision activities.

Using special heavy metal markers in sugars and protein, placed into food which was made available for the hornets to find and bring back to the colony, the researchers were able to study the colony food dynamics. “We looked at seven nests of various sizes up to 30 cm, reflecting differing stages of hornet colony maturity, and controlled the conditions such as temperature and humidity of their surroundings,” Juliette says, continuing “We gave two workers labelled food (sugar and proteins), then put them back in their colony, to see which individuals received the labeled food they had eaten.” The bioassay markers in the food, such as rubidium and cesium, when ingested by the insects, stayed in their bodies and accumulated over time. This meant the researchers could then look at the levels of each marker in the different castes and developmental stages of hornet in the colony over time to determine how much of each food type it had consumed or been given.

The results revealed that all was not as had been assumed previously. The worker hornets in the nest received mostly sugar, as had initially been thought, with some protein as well, but the larvae actually received both sugar and protein. Even the eggs were marked with proteins from the collected food sources though this may have been transferred from adult hornets as they moved or nursed the eggs. Unfortunately, it was not possible at this stage to differentiate between food that was provided directly to others from the first hornet bringing the food into the nest and that which was then subsequently distributed and eaten by different groups within the colony. More will have to be done, on location in the wild, to answer more detailed questions, the researchers acknowledge. “The results look very interesting and similar experiments will need to be continued in order to better understand the food distribution in the nest between the different inhabitants. For example, we were able to see that the biggest and heaviest larvae had been given more sugars and less proteins and that the bigger the workers were, the more protein markers they had. Of course, this shows just a small window of feeding behavior over the three weeks of each experiment and doesn’t reveal how much food they receive over their whole lifetime, but it is a start,” says Juliette. This experiment clearly underlines that when food is brought into the nest by foragers, it is highly distributed to all the castes and developmental stages present in the nest. Further work is needed to understand the true dynamics of Asian Hornet food distribution, the reasons for this and factors which may disrupt or perturb the normal transmission processes.

Interview with Dr. David Aston: Master Beekeeper and former President of the British Beekeepers Association (BBKA) in the UK.

David trained as a biologist in the 1970s, studying at the Universities of Aberystwyth and Aberdeen. He spent his career in the international wood preservation industry. In 2002, he set up in consultancy, serving this industry.

David has a keen interest in beekeeping, which he took up as a hobby in the early 1980s. Then, he was attracted by the ecological aspects of the honey bee and how it has become enmeshed in human society over thousands of years, providing humanity with honey as food and light from beeswax candles. And, of course, not forgetting the honey bee’s critical importance as a pollinator.

“Once the hornet is established in the UK, the potential impact on beekeepers could be catastrophic – when a honey bee colony is attacked, it is usually doomed!”

Dr. David Aston

David, what could be the impact of the arrival of the Asian Hornet for British beekeeping?

Before we can answer that question, we need to understand how beekeeping in the UK differs from continental Europe. Here in the UK, as in many European countries including France and Germany, most beekeepers have amateur status. That is not to say they are not highly trained, just that the number of hives they look after is, on average, around 20 or less. Generally, what we see is that, as you move from the northwest of Europe towards the southeast, beekeeping becomes more commercial. In the UK amateur sector, hives are more widely spread across the whole country compared to many other European countries, so the arrival of the Asian Hornet would be gradual, as it spreads across the country.

How is the commercial beekeeping industry distributed in the UK?

The commercial operations are found throughout the UK, reflecting the spread of forage habitat and availability, which I see as key to honey bee health. Each honey bee colony needs some 30 kg of pollen and 120 kg of nectar each year to stay healthy, so beekeepers need to consider this when siting their hives.

How likely is it, that Vespa velutina will be able to invade the UK?

The likelihood is quite high, unfortunately, as some sightings have already been reported. Invasion of the UK doesn’t necessarily mean flying directly across the Channel. More likely is that they come in via the back door, arriving on containers or goods, imported into the UK from abroad. Once the hornet is established in the UK, the potential impact on beekeepers could be catastrophic – when a honey bee colony is attacked, it is usually doomed!

What precautions can beekeepers in the UK take?

At the moment, there is no requirement in the UK for amateur beekeepers to register that they keep honey bees. Therefore, it is very difficult to alert local beekeepers in a region of the arrival of the Asian Hornet, so that they are aware and know to look out for nests. There is, currently, a big push for British beekeepers to register via a national database ( so that they receive warnings which will alert them of not only Asian Hornet sightings but outbreaks of disease or other pests in their area as well. If farmers and beekeepers work together, be it through initiatives that encourage exchange like the BeeConnected platform ( or by helping each other locate and announce when they find hornet nests on agricultural land in the future, then both parties will benefit.

By supporting each other and learning from our beekeeping friends in continental Europe, we can look for solutions or control methods. It may not be possible to stop the hornets, but hopefully at least, the spread of this new and highly destructive, invasive pest species can be slowed down.

Control options

Entomopathogenic fungi can parasitize the Asian Hornet (see photo above) but this option has not been studied enough to know if it has the potential to significantly control the invaders.

So how can we use all of the information and knowledge, gained from this research, to try to control this new threat to honey bee health which is hovering on the border of many European beekeepers worst thoughts?

Previous research3 shows that trying to find realistic control methods to trap worker hornets or destroy new nests is not an efficient solution in Europe at the moment. Around a quarter of Vespa velutina nesting sites which were located during the field studies were “cryptic”, hidden from view and well-concealed, which makes them very difficult to find and destroy. In addition, the hornet queens’ main reproductive period is anytime from end of August into November or December so there is no short timespan when eradication measures could prove highly successful. As such, one method for a targeted approach which may be worth further investigation could be to develop a ‘Trojan Horse’ concept whereby food, spiked with a pesticide or fungal spores, is placed in the local environment of the hornet nest. In order to be attractive to Asian Hornets, it could carry specific pheromones attractive to this species, so that only they take this back and distribute it throughout their nest. Care will have to be taken, though, that this would not affect native hornet colonies and non-target insects in the vicinity.

So would a control that keeps the numbers of this devastating bee-eater below a manageable threshold within a given location be possible? The researchers looked at alternative measures which may reduce the impact or spread of the invasive hornet in some regions. This included organisms which parasitize the hornet, and certain viruses and fungi which could infect Vespa velutina.

Facts & figures

// The Asian Hornet is found from Afghanistan to China, Japan and Korea and feeds on different sources including bees, flies, wasps, as well as dead fish meat, food and meat remnants from garbage bins.

// Following its arrival from China to the Mount Bongrae region of the Yeong-do islet of South Korea in 2003, the Asian Hornet is probably the most abundant hornet species there now.

// The Asian Hornet is identified by its yellow-tipped legs and the front section of its black head between its antennae, which is orange in color. // The adult queen is up to 3 cm long and the workers are up to 2.5 cm long.

// Reports suggest that Asian Hornets are capable of killing up to 30 percent of a honey bee colony in just a couple of hours.

// Some honey bees in Asia have learned to defend themselves against the Asian Hornet by forming a compact ball around it. It is believed that this drives the internal temperature of the mass to 45 degrees Celsius, killing the hornet and potentially many bees as well, but allowing the colony to survive.

Asian Hornet an Asian pest, too

The Asian Hornet, although a relatively new problem for European beekeepers, is wellknown as an unwelcome invasive species in other areas of the world as well. Its arrival in Korea in 2003 and subsequent spread to Japan (first detected on Tsushima Island, near Korea, in 2012) has had a big impact for many of these countries’ native hornet species, as it occupies a similar ecological niche. Vespa velutina eats the native honey bees and other insects, including flies, wasps and wild bee species, which may be important crop pollinators in their native countries.


The Asian Hornet, Vespa velutina, may be here to stay in Europe, as studies indicate that current control methods are unlikely to be successful in eradicating it. Its impact on European honey bees may be reduced, in the future, if information gathered from these investigative studies can be used to develop further control options.

Some studies4 were carried out to look at biological control options, such as with pathogenic fungi, which may prove more successful in slowing the spread of this species across Europe and limiting its impact in areas where it has already invaded. More research will be needed here.

Beekeepers can protect their beehives, to some extent, by controlling and destroying hornet nests located close to beehives if they are able to find them. This is not so easy and, therefore, a true limitation to the hornets’ control. Also, by staying connected to information sources which indicate when hornets are in a region, beekeepers can work with farmers and landowners to look out for nests and monitor the situation.

1 Villemant C. et al. (2011). Bilan des travaux (MNHN et IRBI) sur l’invasion en France de Vespa velutina, le frelon asiatique prédateur d’abeilles. Barbançon J.-M., L’hostis M. (eds). Journée Scientifique Apicola, Oniris-Fnosad, Arles, Nantes, France, pp 3-12

2 Budge G.E., Hodgetts J., Jones E.P., Ostojá-Starzewski J.C., Hall J., Tomkies V., et al. (2017). The invasion, provenance and diversity of Vespa velutina Lepeletier (Hymenoptera: Vespidae) in Great Britain. PLoS ONE 12(9): e0185172.

3 Franklin, D.N., Brown, M.A., Datta, S. et al. (2017). Invasion dynamics of Asian Hornet, Vespa velutina (Hymenoptera: Vespidae): a case study of a commune in south-west France. Appl Entomol Zool (2017) 52: 221.

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