Yellow-legged Hornet spotted at Waldshut, a river-crossing from Zürich

A Yellow-legged Hornet (*Vespa velutina*) was photographed near Waldshut, on the German bank of the Rhine, in the last week of August.¹ This is the introduced Asian hornet that Swiss cantons have been asking the public to report, and Waldshut is a short flight across the river from the Unterland woods around Bülach, Eglisau and Kloten.

The native European Hornet (Vespa crabro) is the thing it gets confused with, and size does not separate them. Colour does. The Yellow-legged Hornet is a dark insect. Its thorax is velvety black. Its abdomen is mostly black, with a single broad orange band on the fourth segment and a thin orange line nearer the waist. Its legs are dark down to the middle and then bright yellow to the tips. The European Hornet, by contrast, is a warm-coloured insect. Its thorax is reddish-brown. Its abdomen is largely yellow, with black markings on the front segments. Its legs are brown all the way down. The leg tips are the quickest check. Yellow feet on a dark hornet mean the Asian species.

The colony is an annual structure. A mated queen wakes in March and builds a small paper nest in a sheltered spot, often on or in a building; the primary nest the City of Zürich removed in Witikon last year was in a shutter box. Once the first workers emerge in June the colony usually moves to a second, much larger nest high in a tree, often above ten metres, which can reach 60 centimetres across and hold around 2,000 workers at once. Mid-September is when these colonies are at their largest and are producing males and next year's queens. The big nests are almost impossible to see against summer foliage. Most of them are only found after the leaves drop in October and November, and by then the new queens have usually already flown. Those queens spend the winter alone in soil, in rotting logs and stumps, or under loose bark, exactly the material a broadleaf forest floor supplies. Everything else in the nest dies with the frost.

The hornet earns its reputation at beehives. Workers hover at the hive entrance and snatch foragers coming home with pollen or nectar, drop them to the ground and paralyse them. Bees under this pressure stop leaving the hive, honey yields fall, and winter losses rise. It is not only a bee predator, though. A study of 16 nests in southwest France found honeybees made up about 38 percent of prey, flies about 30 percent and social wasps about 20 percent, with 159 prey species in all; colonies in forest took more wasps and colonies in towns took more honeybees. Bumblebees and solitary bees were a very small share. A single colony is estimated to take roughly 11 kilograms of insects over a season.

The regional picture comes from the cantonal authorities rather than from this one sighting, and the figures below reflect how much reporting effort has gone into finding and logging nests, not necessarily how the population itself is changing. Canton Zürich received about 100 reports through the national platform in 2025. The City of Zürich removed a primary nest in Witikon and a secondary nest at a forest edge in Schwamendingen last year, and by mid-July this year had already logged four primary nests. A nest was also found in the Eigental reserve near Kloten, and the canton says it is now drawing up criteria for which nests to destroy first in sensitive habitats. Canton Aargau, further along the Rhine, had sightings of the hornet as early as 2022 and 2023.

Anyone who sees a dark hornet with yellow feet on the Swiss side can report it, with a photo, at asiatischehornisse.ch; on the Baden-Württemberg side, the state nature conservation administration runs a reporting portal. Spring is when the small primary nests on buildings are easiest to find and remove, but the coming weeks of leaf fall are when the big canopy nests finally show themselves.