0. Abstract

The Asian hornet is gradually establishing itself throughout Western Europe. To combat this generalist predator, which attacks a wide range of different prey, a reminder of its biology and its remarkable behavior throughout the season allows us to adapt our defensive strategies.

In spring, the vulnerability of founding queens prompts targeted trapping aimed at destroying primary nests.

Before the swarming of sexual adults in early autumn, locating the secondary nests hidden high in tree canopies makes it possible to destroy colonies from which hundreds of queens, and just as many males whose sole function is to fertilize the future founders of the following year, would escape…

Only by knowing this particularly well-adapted hymenopteran—currently without a regulating predator—can one hope to eliminate it (at best) or to slow its rampant spread (at least) and its impact on insect biomass, on which it feeds its voracious brood.

1. Cycle biologique

Illustration 1: Biological cycle of Vespa velutina. The outline colors of the photos correspond to those of the different phases of the cycle (after Rome et al., 2013; photos: JH = J. Haxaire, MD = M. Duret, QR = Q. Rome, CV = C. Villemant).

Vespa velutina nigrithorax is a diurnal species that, unlike the European hornet, V. crabro, ceases all activity at nightfall. It is a confirmed “generalist” predator of other social hymenopterans, notably honey bees and common wasps, but, like V. crabro, it also consumes a wide variety of other insects—flies, butterflies—and spiders.

Observers agree that the Asian hornet is not particularly aggressive and that it is possible to observe its nest from 4 or 5 m away without risk, provided the insect is not stressed.

As in all other hymenopterans, female offspring originate from fertilized eggs and males from unfertilized eggs. From March–April onward, the Asian hornet colony consists only of workers (sterile females) until the new generation of sexual males and females develops at the end of the season.

From August onward, many diploid larvae are reared to become future reproductive queens, and haploid larvae will become males whose only function is the fertilization of these reproductives. After emerging from their cells, future queens are abundantly fed (stuffed) by workers to build very large fat reserves in preparation for overwintering. Males are also fed by heavily solicited workers. This intensive feeding activity explains the peak presence of workers outside the nest and the impact on honey bee colonies, which the beekeeper feeds with concentrated syrup after the honey harvest so that colonies can store food/fuel for winter. The old founding queen dies shortly before the swarming of sexual individuals.

It is only in autumn (October to November) that reproductive females of the new generation leave the nest accompanied by males to mate; it is not known exactly whether males fertilize queens from the same nest or whether fertilization is by males originating from different nests. What seems clearer is that neither fertilized queens nor males return to the nest after mating: workers remaining on site prevent them from returning.

 

Illustration 2: Morphological characteristics: as in honey bees, male Asian hornets lack a stinger. They also show two small whitish spots at the ventral tip of the abdomen (blue arrows). The worker’s sting measures more than 5 mm.

Fertilized queens will be the only ones to overwinter, while males, the last larvae, and workers die. A few rare nests may remain active in December. Empty nests are never reused, but in early spring one sometimes finds a few late sexual females that were trapped by the onset of cold. They are unable to found a colony because they were not fertilized and often have atrophied wings.

Fertilized females overwinter singly or in groups of two or three in leaf litter, rotting logs, wood piles… in the immediate vicinity of their former secondary nest, and sometimes even in the abandoned nest.

At the end of winter, in March, after several days of fine and warm weather (temperature < 13°C), fertilized queens that survived the cold are hungry, leave their winter shelter, and feed on large quantities of nectar by foraging the available flora depending on phenology, and sometimes by stealing debris on varroa trays or honey drops under the screened bottoms of hives, to restart their carbohydrate metabolism.

It should be borne in mind that adult hymenopterans feed mainly on sugars; in contrast, feeding larvae requires large protein inputs (of pollen origin for Apidae [bees], of animal origin for Vespidae [wasps/hornets]).

In spring (from late February to May), contrary to an older notion, it appears that not all fertilized queens or “gynes” become true founders and build nests. Only about 10% of fertilized queens become true founders that build a new embryonic nest (also called a foundation nest). This embryonic nest corresponds to the nest built, developed, and maintained by the queen alone: future workers have not yet emerged and, after emergence, do not leave the nest for several weeks. This embryonic nest is spherical, has two layers of papier-mâché; it is the size of a tennis ball, with the opening facing downward, and it is built under the edge of a roof, in various shelters protected from bad weather, or in empty nucleus hives.

It should be remembered that a hornet spotted between February and early May will very likely be a “gyne,” because workers have not yet emerged. It is only later, after the emergence of the first workers, which take over from the queen in expanding the brood combs, that this nest will develop into a primary nest, reaching the size of a football, still with an opening on the lower side.

 

Illustration 3: Primary nest with the foundress.

These founding queens lay a few eggs in around ten small cells and care for their first larvae, which will become, one month to one and a half months later—depending on temperature conditions during May—small adult workers, only 14 to 16 mm long, yet fully able to take charge of construction, nest enlargement, and the food supply needed to rear the colony’s brood. If the queen initially builds small cells in the center of the nest, the workers, which gradually take over, build larger cells on the periphery, in the dozen stacked “combs” within the nest structure, and subsequent generations of adult workers will be larger than the first.

In its native range under subtropical climate, the Asian hornet lives mainly in forest environments of mountainous regions. In Western Europe, the Asian hornet thrives in urban or agricultural areas as well as in deciduous woodland, excluding conifer forests and wet habitats. It appears to take advantage of river valleys and major road corridors to disperse

 

Illustration 4: https://sizun.eu/2017.htm The first workers emerging from the small cells built by the foundress are smaller than those that will emerge from the cells built by the future workers.

By analogy, bees that emerge from cells of old combs that have contained several generations of larvae and whose successive cocoons have reduced cell volume will be smaller than those that emerge from recently built combs. When old, dark brood frames are melted down, the matrices of these cocoons remain at the bottom of the wax melter, closely matching the shape of the cells that had contained brood. These matrices are not found when melting combs from honey supers that contained only honey. It is therefore useful in three ways to renew the wax of brood frames: to keep cells of “normal” size, to avoid deposits of possible residues from treatments against Varroa, and to reduce the presence of spores of contagious diseases.

During the initial phase, the founding queen manages her nest and brood entirely on her own. She is very vulnerable because workers are not yet present to assist and protect her. Field observations suggest that many fertilized queens or “gynes,” but non-foundresses, will attempt to evict an established founding queen to take over the embryonic nest and reign there as a “secondary” queen. “Usurper” queens could evict up to 10 queens in succession from an embryonic nest… Ultimately, these “usurper” queens will be driven back once the first workers have emerged. The last queen in place will then devote the rest of her life to laying eggs until mid-September, early October, at which point she dies.

This usurpation behavior would explain the fierce, even ferocious, competition in spring among fertilized females, which considerably reduces the number of expected nests relative to the large number of sexual females (between 500 and 1'000!) that leave the nest in autumn. The beginning of the season is therefore very dangerous for the queen because she is alone in maintaining the nest. If she dies, the colony’s disappearance is almost certain unless a usurper takes over.

2. Le développement du couvain

Brood development begins in April/May. Three or four days after the first egg laying, the eggs hatch and the larvae must be fed. The queen collects nectar on plants (flower nectar) or honeydew (exudates on trees or others) and proteins from captured prey, from carcasses of various animals, or from food scraps in open dumps…

 

Illustration 5: Queen tending brood in an embryonic primary nest.

For about 3 weeks, larvae go through 4 molts and grow at each one. They can increase in size only through a molt (because of their exoskeleton). The queen brings them the proteins necessary for their development by hunting innumerable insects, clearly impacting insect biomass.

At the end of the last larval stage, the larva produces a silk cocoon with its salivary secretions and seals its cell with a cap, in which it will remain for 3 weeks. It then undergoes its 5th molt to become a pupa.

At the end of the pupal stage, the individual becomes an adult, pierces the cap, cuts it, and extracts itself from the cell. It takes about 45 days for an egg to develop into a worker

 

Illustration 6: From egg to imago in 45 days.

With the appearance of the first generations of workers, colony activity intensifies considerably and the nest size increases to reach the volume of a ball. Workers assume different roles within the colony : foraging (sugary on plants + protein-rich from captured prey or other carcasses) ; brood care ; ventilation and thermoregulation of the nest (by wing beating), key factors for brood development ; cleaning the nest ; collecting plant fibers to enlarge the nest, develop new cells for the queen’s egg laying, and repair damage caused by bad weather or predators such as the European honey buzzard, which feeds on the larvae of many hymenopterans ; defense of the colony (guarding the nest entrance). At present, there are not yet scientific data to know whether all these roles change over the course of life as in honey bees (age polyethism).

Unlike the combs built by honey bees, the cells of hornet nests serve only to house brood. The Asian hornet, like all vespids, does not store food in the cells.

3. Le nid secondaire (12'000 d'alvéoles)

Due to lack of space, in July–August this primary nest will be abandoned by ¾ of the colony in favor of a secondary nest, with a lateral opening, often built in the foliage of a tree at more than 10 meters height. This pear-shaped nest will reach its maximum volume in early autumn, sometimes measuring up to 100 cm high, containing 2’000 workers, 500-1’000 future queens (gynes) and as many males (Rome et al., 2013b ; Rome et al., 2015, Villemant & Rome, 2017).

 

Illustration 7: Secondary nest with a lateral opening.

By the end of the year, the nest may contain up to 12'000 cells (versus 3'000 for the European hornet). The queen lays up to 15’000 eggs over her year of life. It is probably with the decline of chemical control (pheromones) related to the queen’s age that the development of reproductive organs in larvae, both diploid and haploid, occurs through epigenetic derepression. These generations of larvae will produce very many sexual adults. The mechanism(s) that enable the queen to lay unfertilized eggs (or “ova”) from which males arise are still unknown.  

By analogy, it is known that a queenless honey bee colony can turn into a drone-laying colony when one or several workers develop their reproductive organs and begin to lay unfertilized eggs. These behavioral changes are possible because epigenetic inhibition by royal pheromones is no longer present after the queen disappears from the colony. 

 

Illustration 8: Female Asian hornet (large 2nd-generation worker) taking nectar from a Camellia flower.

Adult Asian hornets feed only on sugary liquids (honeydew, nectar, honey…) and on the protein-rich liquid regurgitated by larvae when solicited (trophallaxis). Workers carry these liquids in their crop to feed, by trophallaxis, the queen, other workers, males, and future foundresses remaining in the nest.

In autumn, they also eat the flesh of ripe fruits—apples, plums, grapes… To feed the expanding brood, workers attack a very wide variety of protein-rich prey, including wasps, flies, butterflies, spiders, not forgetting honey bees. It should be remembered that, unlike bees, Asian hornet workers do not produce royal jelly to feed brood and future queens.

4. Sa tactique de chasse

Careful observation of the hornet’s predation behavior on honey bees makes it possible to understand its hunting strategy. The hornet positions itself in hovering flight in front of the hive, where it waits for the return of a forager heavily loaded with pollen and nectar. The hornet seizes its prey with its forelegs and first cuts off the bee’s abdomen with its mandibles. It sucks the nectar contained in the crop of its prey for its own needs, before cutting up the thorax, rich in muscular proteins, intended to feed the brood, and carries it back to the nest by following a more or less winding flight path based on the landmarks it has recorded. 

 

Illustration 9: Asian hornet that has captured a bee and is cutting off its abdomen in order to suck the nectar contained in the crop of its prey. It will use part of this sugary liquid for its own energy needs. It will then cut up the thorax, rich in muscular proteins, which it will carry to the nest to feed the voracious larvae.

5. Résumé : le cycle tout au long de l'année

 

		Overwintering December to end of January Nests are clearly visible, often high in the trees, but they are inactive. Future founding queens have hidden to get through the winter.
		Emergence of foundresses February to April Founding queens emerge; they look for a sheltered place to build the primary nest (or embryonic nest). At this time the foundress is alone ; she will choose a site that protects her from bad weather: garden shed, roof overhang, barn, window frame, etc.
		Birth of the first workers Late April, during May, and even early June The first workers emerge ; they begin caring for larvae and enlarging the primary nest.
		Start of predation June to September The population increases in the primary nest. Workers look for a new place to build a new nest (secondary nest) in July. Often located high in a tree, it is nevertheless sometimes found at human height, near ground level, in a hedge or against a building.
		Increase in predation September to October The population has increased sharply ; the colony has abandoned the primary nest in favor of the secondary nest, in which the founding queen spends all her time laying eggs. Workers need to bring back a lot of protein to feed larvae. Considerable pressure is observed on apiaries.
		Reproduction October Males and sexual females leave nests by the hundreds ; they will mate. Fertilized females will become “gynes” or future foundresses.   Dispersal of future foundresses November After leaving the nest, fertilized females look for hiding places where they will spend the winter. They will re-emerge the following spring to begin a new cycle.  Males die. The coefficient multiplying Asian hornet populations is close to 10 from one year to the next.   Death of workers December The last workers continue to occupy the nest ; they still attack apiaries. They will die with the cold of winter. The nest will be damaged and naturally degrade.

The activity of the Asian hornet begins about forty days before that of V. crabro. Over the season, a colony of V. velutina is estimated to be ten times more populous than a colony of V. crabro. The

competition between the two species can cause a change in the hunting behavior of V. crabro, which becomes much more present in apiaries.

 

► see also: Vespa velutina, Asian hornet

 

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