Worcestershire Record No. 6 April 1999 p. 12


(Diptera - Asilidae) 

David M. Green

As I wrote (Green 1998) in Worcestershire Record, Asilus crabroniformis L. hornet robber fly was once found 1995 in pony grazed acid grassland on the outside of Kidderminster by Brett Westwood (Westwood 1997). Please look out for this fly that is large, easy to recognise, and of great importance being a Biodiversity Action Plan priority list species nationally and in the Worcestershire BAP (UK Steering Group 1995; Bruce 1999).

The biology of the larvae of Asilus (only one British species in genus) remains an area of overwhelming speculation; information is partly old, scarce, without consensus, and the larva is currently illusive, but I expect CCW will do more research this year. Meanwhile the nutrition in the larval stage remains unknown: faculative or obligate predation, saprophagy or parasitism, or possibly even herbivory or detritivory, therefore all remain possible modes of nutrition. Evidence points to carnivory, probably predatory, but possibly parasitic or saprophagous; and it is likely to be free living in the soil (Clements and Skidmore 1998). There is much speculation. That the larva is free-living in the soil feeding on earthworms is a popular explanation. If the larvae spent much time living on dung, feeding on invertebrates, which is one often stated description, then I would have expected confirming records of such an obvious big beast on dung - dung fauna is well studied. The mystery of the larvae remains unsolved.

The fly spends much time resting on the top of dried dung, moving from dung to dung or bare earth, when warm, sometimes feeding mainly on dung beetles (Aphodius) and sarcophagid flies (but also many other insects) in the field or heath, or land of other description, around or on the dung. Habitats known are varied: calcareous, acid and neutral grassland, semi-improved and improved, species poor pastures, dry heathland, wet heath, marshy acid grassland, clearings in broad-leaf woodland, re-vegetated colliery spoil and sea cliff grasslands. Sites in England also include sandy soils, including near coast and downlands; an old record exists of sites in clearings in coniferous forest in Sweden. The main habitat requirement appears to be light soil and rich coprophilous fauna, including geotrupid beetles, and the presence of usually ungulate dung or dung of herbivore for (at least) egg laying; grazing is often important to prevent infill with scrub etc. Aspect and topography with respect to warmth may be important. Populations tend to be confined to small or very small areas of habitat, possibly with a metapopulation structure within and between sites, with some evidence of dispersal of adults. The flight period late July to early October, probably living 4-6 weeks, becoming worn and damaged, requiring air temperature of 16C at least for activity, going to roost (upside down curiously - for camouflage or to contend with rain?) at lower temperature apparently and at night, with maximum activity 19C+.

Asilus is in decline in Britain and Europe generally with no clear explanation. The fly does not seem to require a typical conservation habitat such as species-rich grassland, but the the effects of ivermectins could be sub-lethal, or could reduce the availability of Aphodius (dung beetles) that are a important part of the diet of the adult Asilus; the other main food item the flesh fly Sarcophaga survives in the most ordinary field. But this is speculation. Geotrupid beetles (that make burrows beneath the dung - Typhaeus typhoeus (L.) minotaur beetle) have been noticed to often coexist on the same site as Asilus but no specific link is exposed. Further research towards understanding of the biology - especially of the mysterious larval life that is presently such an area of speculation - and habitats will hopefully reveal reason for decline and enable improved BAP for reversal.

Come late July to October please remember look out for Asilus - as it is likely that there are unknown sites in Britain - look particularly in warm weather, especially look at dung with dried surface. A fly with large variable size up to 28mm, the abdomen boldly marked bright yellow, a single area of yellow over about half the abdomen, should indicate the species likely from a distance, especially if the fly is perched on dried dung. If you think you have a sighting, your information is urgently required without any delay, (and for further information or colour picture) please contact immediately: tel. 01386 710377.

CLEMENTS David K and SKIDMORE Peter 1998 The autecology of the hornet robber fly Asilus crabroniformis L. in Wales, 1997 CCW. [includes review of literature]
BRUCE Bronwen (ed.) 1999 Biodiversity Action Plan for Worcestershire. Worcestershire Biodiversity Partnership
GREEN David M 1998 Asilus crabroniformis (L.) Hornet Robber Fly. Survey in Worcestershire - help wanted. Worcestershire Record 5: 10.
UK STEERING GROUP 1995 Biodiversity: The UK Steering Group Report. HMSO.
WESTWOOD Brett 1997 Pats and robbers. Worcestershire Record 3:7.

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