Yellow-legged Hornet photographed again near the Soignes in September

A Yellow-legged Hornet (Vespa velutina nigrithorax) was photographed on Henri Schoofslaan in Auderghem on September 8, a few streets from the edge of the Forêt de Soignes.¹ It's the invasive Asian hornet known for preying on honeybees, and iNaturalist records show it turning up repeatedly across the wider Brussels region this year: well over a hundred sightings logged in the past twelve months alone, spanning communes from Auderghem, Ixelles and Watermael-Boitsfort to Tervuren, Wemmel and Leuven.¹ That pattern of records doesn't establish population size or trend — sightings track observer effort as much as anything — but it does show the species is being found widely and repeatedly around the forest, not just this once.

Telling it from the native European Hornet takes one look at the color. The European Hornet is a big yellow wasp with black bars and a reddish-brown head and thorax. The Yellow-legged Hornet is the reverse: velvet black or dark brown, each abdominal segment edged with a thin yellow line, a single broad orange band toward the tail, an orange face, and legs that end in bright yellow as if dipped in paint. It usually runs a little smaller. The yellow feet are the mark most people catch first.

September is when its colonies are as large as they will get. A nest at this point holds a few thousand individuals, and the workers are provisioning larvae at full tilt. This is the month for the behavior beekeepers dread: workers hang in the air in front of a hive entrance, pick off single bees returning heavy with pollen, then cut away head, wings and legs and carry the thorax home. Honeybees are not the whole diet, though. Nests studied in southwest France took honeybees for about 38 percent of their prey, flies for about 30 percent and other social wasps for about 20 percent, with more than 150 prey species in all. Colonies near forest leaned harder on native wasps; colonies in town leaned on honeybees.² A single nest can take roughly eleven kilograms of insects in a season, which is why the concern extends to native pollinators and not just to hives.

The nest itself is usually invisible right now. A queen starts small in spring, low in a shrub or under a roof edge, and by summer the colony shifts to a large paper nest high in a tree canopy. In beech and oak stands like the Soignes those only show once the leaves drop in October and November, by which point the colony is already producing next year's queens. Workers and the old queen die with the cold. Only the mated young queens overwinter, each one capable of founding a new nest in spring.

Brussels handles this through Bruxelles Environnement, which trains a referent in each commune to take sighting and nest reports. A hornet hovering at a hive, or a large grey nest appearing in a bare canopy this fall, is what to report.