Worcestershire Record No. 5 Nov 1998 p. 10


By David M. Green

A. crabroniformis L. is a large spectacular predatory fly with much of the end half of the abdomen bright yellow, of wide ranging habitat including rough grassland and heath. In Worcestershire the adult fly was found by Brett Westwood during 1996 inhabiting pony pasture on the edge of Kidderminster and has not been seen since, but the site has been seldom surveyed and apparently remains in suitable condition.

Current status. In UK A. crabroniformis is widespread throughout Wales and in the southern half of England, with records from about 37 vice-counties, but has sharply declined since 1970 from being in 111 10km squares to only 48 10km squares by the early 1990s. Areas of loss are much of east England, and the previous strongholds of Devon, Dorset, and Hampshire; elsewhere there is a sharp contraction of distribution. (Clements and Skidmore 1998.) This fly is categorised as Nationally Notable (scarce)(Falk 1991, JNCC 1997), and is on the "short list" of Biodiversity Action Plan (BAP list 1) species, recognising a decline in distribution and the need for more detailed information on ecological requirements and conservation management.

Research. Considerable on-going field study (and observation of captured flies) has yet failed to reveal the food of the larva, that grows large, lives 2-3 years, and which has not yet been found wild, at least recently, in UK. The larvae is associated with surface-dry dung (of stock typically cow dung, or mounds of rabbit dung) for nearby oviposition in soil during August-September. The larvae is probably free-living in soil, and appears possibly associated with dung beetle larvae for food. The adults take a wide range of insect prey - grasshoppers, beetles, moths, butterflies, bees and wasps (social and solitary), and flies - most frequently dung beetles Aphodius, secondly flesh flies Sarcophaga. Crusted dung is much used for perching in the sun and for feeding platforms. (Clements and Skidmore 1998, Pinchen et al 1998.)

Deterioration of current sites could be caused by
Shading out of bare ground, or sparsely vegetated ground by natural growth such as filling in of scrub areas or increasing bracken cover.
Loss of areas with short vegetation, to infill of scrub or long vegetation owing to reduction of grazing by stock or rabbits.
Use of parasiticidal compounds such as ivermectins causing reduction of dung fauna, or the sub-lethal effects of ivermectin residues.
Loss of dung of herbivores on site owing to changes in grazing stock or changes in rabbit population.
Loss of suitable roosting habitat such as nearby woodland.
Climate change.
Species-rich pasture is not a required vegetation type.

Survey - your help wanted

The known Worcestershire site is on private land and CCW request we delay in approaching owners until EN and CCW arrange some agreement with national representatives of affected parties and produce a leaflet. There is immediate need to look for more sites in Worcestershire on heathland but especially rough grazed grassland such as pony pasture, as a habitat the fly could be making increasing use of. Sheltered sites with some areas of short grassed areas are preferred, such as open areas, exposed to sun, sheltered within scrub or bracken, maybe on a south facing incline. Pony pastures around Birmingham are possible new sites that definitely should be surveyed, and places rough-grazed by travellers' ponies may be fruitful. Look on dung with dried surface for the remains of prey items. Scan dried dung or other pale or reflective surfaces, from nearby perhaps with binoculars, for flies sunning themselves. Generally robber flies, including other other species in the robber fly family, Asilidae, are attracted to such surfaces such as a white nets or polythene. They could be found resting on bracken at edge of a clearing. When the sun goes in the flies could take some cover possibly in plants around dried dung. Active at temperatures above 16C. Flight period late July-early October. Asilus crabrobiformis is large 20-25mm with obvious clear yellow on about 4 segments, more or less the end half, of the abdomen. Any sightings contact the WBRC. Observations of behaviour are useful. Collect any prey remains.

References and Bibliography:

CLEMENTS David K and SKIDMORE Peter 1998 The autecology of the hornet robber fly Asilus crabroniformis L. in Wales, 1997. CCW Contract science report No. 263. CCW.
COOKE AS 1997 Avermectin use in livestock [leaflet]. EN
FALK Steven 1991 A review of the scarce and threatened flies of Great Britain (part 1). Research and survey in nature conservation No. 39.JNCC
JNCC 1997 Recorder 3.2 [database biological recording software]. JNCC
OLDROYD Harold 1969 Tabanoidea and Asiloidea. Diptera, Brachycera section (a). Handbks Iden. Brit. Insects vol.9 pt.4. RES
PINCHEN BJ, DENTON JS, and BIRD DR 1997 The hornet robber fly Asilus crabroniformis L. - adult behaviour at selected sites in Dorset, Hampshire and Surrey in 1997. English Nature Research Reports No. 274. EN
CCW, Countryside Commission for Wales
EN, English Nature
JNCC, Joint Nature Conservancy Council
RES, Royal Entomological Society

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