This fearsome looking fly, large and giving every appearance of aggression is, nonetheless, completely harmless to man. Most of the body is hairy and golden-brown, except for the first few segments of the abdomen, which are black. Another identifying feature common to this family is the deep groove between the eyes.
Hornet robberflies start to emerge in mid-June with the bulk of the insects appearing in July. They are predatory flies, and the adults intercept their prey in mid-flight. The favoured food species seem to be dung beetles, but other prey recorded has included grasshoppers, bees and wasps. The robberfly hunts from a perch, which can be a stick, stone or a dung pat. The larvae of this species are themselves predatory. The adult female lays her eggs on or around a pile of animal dung. The grubs enter the soil immediately after hatching and, having fed on beetle grubs, the larvae burrow into the soil before pupating.
There are two main threats to the hornet robberfly. Much of Britain's unimproved grassland and a lot of its heathland have been lost to agriculture or urban development. Those remaining have largely been fragmented, a situation which isolates populations of many species. A bigger threat is the use of persistent chemical treatments for parasite infestations in grazing stock. These include ivermectin, which can persist in an active form in an animal's dung for some time. This chemical not only kills the internal parasites but also kills or causes serious deformities in the insects that feed on the treated dung. This in turn can affect the robberflies and their larvae.
The hornet robberfly is listed as a priority species in the UK Biodiversity Action Plan (UK BAP) and is included in English Nature's Species Recovery Programme. The Countryside Council for Wales leads the work being carried out to recover the fortunes of this species.
A number of different endangered species are associated with animal dung, including a number of well-known ones such as the greater horseshoe bat and chough. The main conservation targets are to increase the available habitat for these species, incorporating agricultural management changes, and particularly to control the frequency and the timing of parasite treatment for stock. One recommendation is to produce an information leaflet for distribution to the owners of stock in the areas where the hornet robberfly is still found. Convincing some people, however, about the importance of conservation for invertebrates, especially large aggressive-looking ones, may prove tricky.
The UK Biodiversity Action Plan for this species is available at UK BAP.
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