How many Asian hornets are in a nest in Belgium?

Contents

• Life cycle and number of hornets in a nest over the seasons

• Situation in Belgium: number of nests in Flanders, Wallonia and Brussels

• Proliferation factors: why are colony numbers soaring?

• Conclusion

A nest of Asian hornets hanging in a tree may seem harmless from a distance. A grayish ball, rather like papier-mâché. Except that inside, in autumn, there are potentially over 6,000 individuals. Six thousand of them. And each nest can produce several hundred future queens capable of founding their own colony the following year. It's easy to see why Vespa velutina is gaining ground at a rate that surprises even entomologists.

In Belgium, the situation has changed. We're no longer dealing with anecdotes or isolated observations: data from Vespawatch, the national monitoring platform, show a progression year after year, with striking disparities between Flanders, Wallonia and Brussels. This article puts the figures on the table. How many hornets are there in a nest? How does the colony evolve from spring to autumn? And where do we stand in Belgium in 2026? We break it all down, backed up by scientific data.

Things to remember

• This article combines a precise demographic analysis of a colony (seasonal evolution of the number of individuals) with an unprecedented survey of the infestation in Belgium.

• Based on scientific data and regional monitoring reports (Vespawatch), we explain the density of nests in Flanders, Wallonia and Brussels to provide a global view of the invasion.

• Life cycle and number of hornets in a nest over the seasons

• Number of nests in Flanders, Wallonia and Brussels

Life cycle and number of hornets in a nest over the seasons

It all starts with a single female. The founding queen, fertilized the previous autumn, emerges from her winter diapause around February-March, when temperatures rise above 13°C. She is alone. Completely alone. Her first job is to build a primary nest, often the size of a ping-pong ball, in a sheltered spot - a garden shed, an electricity meter, under an awning. This primary nest houses the first cells in which she will lay her eggs.

At this stage, the nest population evolves slowly. The queen has to do everything herself: build, lay eggs, hunt and feed the larvae. The first workers emerge only after around 45 to 50 days. We're talking about 5 to 10 individuals at the end of May, beginning of June. Fragile. In fact, according to a study published in the Journal of Pest Science (Monceau et al., 2014), around 70 % of primary nests fail before reaching this stage. The queen dies, the nest is destroyed by a predator, or the weather conditions don't cooperate.

Those that survive move up a gear. Once the first workers are active, the queen concentrates exclusively on laying eggs. The nest grows. Often, the colony moves to build a secondary, larger nest, usually high up in the tree canopy (10-15 meters, sometimes more). This secondary nest is the one spotted in autumn when the leaves fall. And that's where the numbers get impressive.

In summer, from June to August, the colony grows from a few dozen to several hundred workers. Growth is exponential. The queen lays up to 100 eggs a day, according to observations by Rome et al. (2021). Workers live between 30 and 55 days, but are continuously replaced. By mid-August, an active nest easily contains 1,000 to 2,000 individuals.

The peak occurs in September-October. Fall numbers peak at between 4,000 and 6,000 hornets in the most prosperous colonies. Some studies, notably those carried out by the Muséum National d'Histoire Naturelle in Paris, have documented nests containing up to 13,000 cells. Not all are occupied simultaneously, but this gives an idea of production capacity. At this point, the nest can measure 80 cm in diameter, sometimes more than a meter.

It's also in autumn that the colony produces its sexual offspring: males and future queens (called gynes). A single nest can produce between 200 and 500 gynes. After mating, these future queens leave the nest and seek shelter for the winter. The males, workers and former queen die with the first cold spells. The nest is abandoned and never used again.

Let's recap the evolution over the months:

• March-April : 1 individual (the founding queen alone)

• May-June : 5 to 30 workers

• July-August : 200 to 2,000 individuals

• September-October : 4,000 to 6,000 individuals (peak)

• November-December : rapid decline, death of the colony

Why is it not advisable to crush an Asian Hornet? Because a stressed or crushed hornet releases an alarm pheromone that attracts other hornets. Near an active nest at the height of the season, this can trigger a mass attack. It's best to report the nest and call in a professional.

Situation in Belgium: number of nests in Flanders, Wallonia and Brussels

7447 confirmed nests in Flanders alone in 2024, according to figures compiled by Vespawatch. More than 6,000 estimated by CRAW for 2024 in Wallonia. Probably more than a thousand for Brussels. This is the kind of statistic that makes you think. The Asian hornet in Belgium is no longer an emerging phenomenon: it's an established reality, with a dynamic that accelerates every year.

To understand the regional density, we need to go back a little further. The first nest of Vespa velutina in Belgium was confirmed in Tournai in 2016. For some years, reports remained concentrated in Hainaut, on the French border, which was logical since the species had been colonizing southwest France since 2004. Progress northwards seemed slow. But it wasn't.

Wallonia remains the region most affected. Hainaut, Namur and the province of Liège account for the majority of Walloon nests. This density is due to the region's proximity to France, its hilly terrain, which offers a variety of nesting sites, and its semi-rural setting, which is ideal for hunting pollinating insects. More than 60 % of Belgian nests are regularly counted here. Walloon beekeepers are on the front line: some are reporting hive losses directly linked to Asian hornet predation.

Flanders seemed spared for a long time. But no more. Since 2021, the number of Asian hornet nests in Flanders has exploded. The province of Antwerp, Flemish Brabant and East Flanders show increases of 40 to 80 % per year, according to Vespawatch data. The flat, urbanized terrain does not hinder the species, contrary to what one might have thought. Hornets adapt perfectly to suburban areas, parks and gardens. They find plenty to eat (bees, flies, caterpillars) and abundant nesting sites (tall trees, roofs, sheds).

Brussels-Capital has not been spared either. Regional density is lower in absolute terms, which is to be expected given the size of the area, but reports are increasing. Nests have been found in public parks, in trees along avenues, and even on buildings in the city center. The Brussels monitoring network relies on Vespawatch and citizen reports, which have become an essential tool for early detection.

Vespawatch plays a central role in all this. This platform, developed by the Institut de recherche de la nature et de la forêt (INBO) and Honeybee Valley (UGent), enables anyone to report an Asian hornet sighting, with photo support. Experts then validate the reports. This citizen science system has made it possible to map the progress of the species with remarkable precision. In just a few years, the database has grown from a few hundred to several thousand annual reports.

What we're seeing is a settlement front moving north and east. The border provinces were affected first, then the interior of the country. Today, virtually no Belgian province is unscathed. The question is no longer «will the Asian hornet come to me?», but rather «when will it be dense enough to pose a real local ecological problem?».»

For comparison: in France, it is estimated that the number of nests exceeds 100,000 per year throughout the country. Belgium isn't there yet, but the trajectory is the same, with a time lag of a few years.

Proliferation factors: why are colony numbers soaring?

Three words: biology, climate, absence of predators. The life cycle of Vespa velutina is a colonizing machine. Let's go over the figures again: a single nest produces 200 to 500 future queens in autumn. Even if 90 % of them don't survive the winter (predation, cold, diapause failure), there are still 20 to 50 capable of founding a new colony the following spring. A nest that generates 20 to 50 the following year represents colossal demographic pressure.

The survival rate of founding queens is the key factor. And it seems to be increasing. Why is this? Belgian winters are getting milder. Data from the IRM (Royal Meteorological Institute) show that winters with prolonged periods of frost are becoming rarer. Yet it is frost that kills some of the queens in diapause. Milder winters mean more surviving queens in the spring, so more primary nests, so more colonies reaching maturity.

Global warming also affects the length of the active season. Colonies start up earlier in the spring and remain active later into the autumn. This gives them more time to grow and produce offspring. A study by Barbet-Massin et al (2013), published in Biological Invasions, had modeled the potential expansion of Vespa velutina in Europe according to climate scenarios. Belgium was clearly in the planned colonization zone.

The absence of effective natural predators is another determining factor. In Asia, its continent of origin, the Asian hornet is regulated by other hornet species, by specific parasites and by Asian bees that have developed defense strategies (the famous «heat ball» where the bees surround the hornet and kill it by hyperthermia). Our European bees, Apis mellifera, They are easy prey. They are easy prey.

The impact on biodiversity is direct and measurable. The Asian hornet is a generalist predator, but it particularly targets domestic and wild bees. A hornet colony can capture between 25 and 50 kg of insects per season, including a significant proportion of bees. In heavily infested areas of southern France, beekeepers report losses of 30 to 50 % of their colonies. But the impact is not limited to bees: syrphid flies, butterflies, social wasps and other pollinators are also affected. The entire pollination network has been weakened.

Another underestimated factor is the species' ability to adapt. Vespa velutina nests in a wide variety of environments. Tree tops, low hedges, roofs, garden sheds, electrical boxes. This flexibility makes detection difficult. Many nests are only discovered in autumn, when the leaves have fallen and the colony has already produced its future queens. Too late to prevent dispersal.

What scent does the Asian Hornet hate? You read a lot about it. Cloves, burnt coffee, citronella. Let's be honest: none of these methods have been scientifically validated as effective repellents on a garden or apiary scale. Beer and syrup traps may catch a few individuals, but they also catch non-target insects and have no significant effect on colony dynamics. The only method that really works is nest destruction by trained professionals.

The proliferation of the Asian hornet in Belgium is not inevitable, but it does require a coordinated response. Early detection of primary nests in spring, systematic reporting via Vespawatch, and rapid destruction of active nests are the three pillars of a strategy that can at least slow expansion. Researchers are also working on biological control options, in particular a parasitic micro-fungus (Beauveria bassiana) and more effective selective trapping techniques. Nothing miraculous yet, but research is making headway.

Conclusion

The figures are clear. An Asian hornet nest can house up to 6,000 individuals at its autumn peak, and produce hundreds of future queens. In Belgium, colonization by Vespa velutina now affects all regions, from Wallonia to Flanders and Brussels, with a clear acceleration in recent years.

If you spot an Asian hornet or a suspicious nest, do the right thing: photograph it and report it on Vespawatch. Every report counts. Every nest destroyed in spring means potentially 500 fewer queens the following autumn. The fight against this invasive species is a collective effort, and it starts with observation.

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