Hornet robberfly (Asilus crabroniformis)

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Hornet robberfly, lateral view
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Hornet robberfly fact file

Hornet robberfly description

KingdomAnimalia
PhylumArthropoda
ClassInsecta
OrderDiptera
FamilyAsilidae
GenusAsilus

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.

Size
Body length: 25 mm
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Hornet robberfly biology

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.

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Hornet robberfly range

The hornet robberfly is found across Western Europe, and in some 50 - 100 scattered populations in southern England and Wales. It appears to be declining across the whole of its range.

You can view distribution information for this species at the National Biodiversity Network Gateway.
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Hornet robberfly habitat

This species requires unimproved grassland and heath, preferably grazed, as animal dung is known to play an important part in the robberfly's lifecycle.

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Hornet robberfly status

Classified as Nationally Scarce in the UK.

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Hornet robberfly threats

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.

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Hornet robberfly conservation

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.

There may be further information about this species available via the National Biodiversity Network Gateway.
The UK Biodiversity Action Plan for this species is available at UK BAP.
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Authentication

Information supplied by English Nature.

http://www.english-nature.org.uk

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Glossary

Larvae
Stage in an animal's lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
Pupating
The process of becoming a pupa, the stage of an insect's development, when huge changes occur that reorganise the larval form into the adult form. In butterflies the pupa is also called a chrysalis.
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References

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Image credit

Hornet robberfly, lateral view  
Hornet robberfly, lateral view

© Geoff Dore / naturepl.com

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