Asilus crabroniformis (Diptera - Asilidae) In Worcestershire, Kidderminster Area, 1999

By David M. Green

Asilus is a large nationally notable (scarce) true fly, on the priority list of the national Biodiversity Action Plan (UK Steering Group 1995), and considered to be in decline, though survey is perhaps needed for clearer information as there could be unknown sites. Asilus was first recorded in Worcestershire on the outskirts of Kidderminster on pony dung (Brett Westwood 1997) during 1995. I wrote in Worcestershire Record on the need for survey of Asilus in Worcestershire and its known biology (Green 1998, 1999) largely derivative of the reports of contract field research by CCW and English Nature. This year, August 1999, Brett revealed Asilus again in the same Kidderminster area, resulting in further survey and observation, including by Mike Averill who has provided a list of sites and revealingly photographed Asilus feeding on a beetle. In the Kidderminster the soil is very sandy with steep slopes and hillocks in places; on the steep slopes the grass is sometimes less dense, less improved, so that the vegetation in a single field can be variable. I characterise the condition of the areas of fields used by Asilus that I observed personally this year, also recording insect carcasses left by Asilus after feeding on the horse dung with some interpretation and reasoning noting Clements and Skidmore 1998. My observations made during the last week of August 1999 and the first week of September, mainly on warm sunny days, were written on around 9th September 1999 with some additions and editing later.

The sites

A(about SO850780) Hurcot Pasture (SSSI)acid grazed grassland that suffers lack of grazing presently. A small amount of old horse dung remained. At the end of August1999, 4 Asilus seen, including pair in copulation; beginning of September, 1 Asilus seen. All on the second hillock, on south side not far from the south boundary, where there is some old horse dung, the grass longer but sparser, not lush as in many other parts of field where the grass someplaces shorter. Some dung was turning into ants nests; amount of dung limited and most shaded out; the grass long, not grazed for months, or if was then at low levels. Some of dung turning to soil with roots within, merging with the ground; this reflecting the state of the dung generally. Fauna in the dung was less evident compared with site D in terms of Aphodius dung beetle larvae for example (despite that some such larvae can live in debris like grass litter). These various reasons apparently accounted for the site becoming unsuitable for Asilus. Over visits in last 10 days the numbers of flies decreased from 4 to 1, then 1 again; then none seen, even in hot weather. Grazing is urgently required by horse (or cattle) in good numbers to provide more dung and rough grazed grass (not reduced to lawn-like length judging by preferences of Asilus in the areas) on the slopes and tops of hillocks and other places. There is no need for continuous grazing apparently, but when grazed there need be impact, and periods between grazing not so long the dung is obscured by grass. There was scattered ragwort in this field but not much and was going over and brown. Over the north fence of this field are previously arable fields, to be houses site, now grassed over and covered with dense ragwort. Other fields to the west and north to the road were little grazed, if at all, with little dung (varying amounts of ragwort), except the small pony paddocks that are over grazed down to bare soil. Thus these fields north and west of the Hurcot Meadow SSSI were reduced value for Asilus. Rabbits had made little impact little in the way of piles of rabbit dung and grazing that Asilus is reported to use.
B(about SO848783)Asilus seen by Mike Averill in August 1999 at least one seen; none were seen myself. Lack of appreciable grazing for months, lack of dung, and shading out of dung resulted in deterioration of site from borderline to no good over last weeks apparently. The lack of grazing or limited horse grazing finally results in the site deteriorating to failure in the last week or so. Otherwise the site would be viable judging by appearance.
C(about SO849783)A single Asilus was seen by me in first week of September, beyond the fence forming the south boundary of this field and someway into the field on gentle south facing slopes, where the grass was grazed but not lawn-like, not lush and dense. The grass was ideal apparently in agreeable comparison with main site in field to the south (D): grazed but not too short and not dense, with plenty of suitable old horse dung few weeks old and dried on surface and fresh dung. This site is more exposed to wind than the upper part of site D sheltered with scrub. (When I visited this field latter in September after some wet weather the character of the grass had changed to longer and denser so that the distinctive patch of non-dense grass was no longer to be found the quality of a particular field can quickly change.)
D(about SO849784)Main site with up to 20 flies estimated on my visits, maximum of 15 flies definitely counted at one visit, all perched on horse dung including fragments. Many Asilus were seen in copulation. Asilus was seen, up to 5, mainly on the very upper slopes and top of slope of the south side of this field. The colony continued, another 10, over the east fence (not on OS maps) in the scrub at the top and down the slopes quite steeply northwards, then flattening, reaching the middle of the field up to the area where the grass became short grazed to lawn length. I did not see Asilus here where the grass was very short, despite Asilus being found in low numbers further north in the middle of next field where the grass is longer again. Perhaps Asilus prefers shelter from wind of longer grass, but not so long and dense that the dung is shaded, preventing Asilus from perching on the dung in the sun. Asilus appeared to be distinctly biased to the scrubby upper parts on average. A north-facing slope would not be thought ideal for warmth but apparently entirely suitable on this site. In these fields there were number of small horses, plenty of dung. The dung somewhat kicked and scattered, to no bad effect apparently, and plenty of dung with well-dried surface is available, but Asilus was seen perched on fresh dung as well (horse dung being fairly dry when fresh unlike fresh cow dung that is not used). The scrub provides sunny sheltered areas of grazed grass complete with horse dung. The grass where Asilus is, appears not well nutrified, lush or dense; well grazed but mostly long enough to sweep net. Dung is exposed to the sun so Asilus can rest on the dung in the sun. Well-drained sandy soil, as usual on the sites and the Kidderminster area, are typical of Asilus, but not essential. The living dung fauna was evident. Aphodius larvae and adults were present in a number of dung piles, and a cantharid (soldier) beetle larva was found (identified to family by Paul F. Whitehead). The burrows of Geotrupid beetles were under the dung. Geoptrupes spiniger (a door beetle) was found alive and there with dead remains in fox dung around the horse dung. (The food of Asilus the adult fly is known to be notably Aphodius dung beetles (adults) and sarcophagid flesh flies). I saw Asilus feeding on Sarcophaga sp. (flesh fly) and found dead Sarcophaga subvicina with a sizeable hole in the body between the front coxa. A few dead Aphodius foetens a nationally local species (Ball 1994) were found here (D) and on site A (Hurcot Pasture SSSI) on the dung surface with wings out; all except one of these not visibly holed. Apparently the saliva of Asilus is very lethal enabling Asilus to overcome quickly beetles that are much more powerfully built, and the intersegmental membranes may be penetrated by the proboscis of Asilus leaving the beetle with no obvious hole. But one dead A. foetans was obviously holed taking out much of two abdominal tergites (site D), presumably the prey of Asilus, confirmed later by a photograph by Mike Averill of Asilus feeding on apparently A. foetans. Some specimens retrieved of A. foetans had unusually dark abdominal sternites, differing to the usual reddish-yellow to light brown as described by Jessop 1986 (identification confirmed by Paul F. Whitehead).

All considered these fields of site D appear in good condition for Asilus as presently, as described. Removing the scrub could be detrimental owing to loss of sheltered areas on breezy days. On the cooler days, or cooler parts of the day when visited, Asilus tended to within the scrub or not far bellow during the September visits. Opposite westwards across the road from main site D and C, a single horse grazed fields with little dung, and the grass is long, or short and lush. If this area was more grazed, parts would be similar in grass to the main site. Despite being well populated a few days previously, my last visit to site D, 8.9.99, revealed no Asilus presumably because of the cooler weather: total cloud cover, wind force 3.

Threats of ivermectins and biology

Asilus seems to require rich dung fauna on site (Clements and Skidmore 1998). The Asilus larvae itself in the field could be affected by ivermectins used as antiparasitic digestion treatments on cattle and horses that kills the dung fauna, but the larva has proved illusive causing lack of information how it might be affected. There could be sublethal breeding effects on Asilus. Ivermectins could be damaging to Asilus populations, but there has been lack of firm knowledge of when and how. Horses can not be given bolas treatments the same as cattle because of the different digestive system, so the dung receives less lengthy treatment with horses after the dose. After drench by mouth, the dung from horses is contaminated with ivermectins for about 2 weeks; at the New Forest, ponies take about 2 weeks to make their way back to sensitive sites after treatment (pers. comm. Martin Drake, EN, 1999). In at least one Asilus pasture around Kidderminster the horses are taken off the fields when being treated; the dung during this treatment time piled in one place not scattered on fields (pers. comm. Mike Averill 1999). The ivermectins in deposited dung naturally break down apparently after some weeks (pers comm. Malcolm Smart 1999). Definite advice on when and how treatment by ivermectins would be least environmentally damaging is lacking, owing to lack of research that is perhaps referable to MAFF (pers. comm. Martin Drake). Ivermectins clearly could be a great risk, or be relatively harmless, perhaps depending on the circumstances and timing. Asilus has been known to survive on sites where ivermectins have been used (pers. comm. Malcolm Smart).

The Asilus larva has been illusive, causing more uncertainty on how its biology might be disrupted by ivermectins. But there is apparently some progress very recently by Peter Skidmore in recent work not yet published. And marked Asilus individuals revealed large turnover of flies leaving a site. (Pers. comm. Mike Howe, CCW, December 1999)

In conclusion, at the limited Kidderminster sites I have observed, for this limited period, Asilus is discouraged to use pasture where there is either of the following: lack of grazing, lack of horse dung, presence of dense well-improved grass, tall grass shading out the dung that is limited, or lawn-like grass (that provides no shelter from wind). Asilus tends to the slopes or peaks in fields observed, including a north facing incline on the main site, possibly because this is often where the grass is usually less lush. The SSSI pasture particularly requires grazing and would appear to be an illustration of how SSSIs may not well cared for owing to management difficulty: a national issue. Horse dung appears entirely suitable, even could be advantageous to Asilus compared with cow dung owing to the lesser period of effect of a dose of an ivermectin on a horse, as the bolas treatment is not applicable to the horse. At the main Asilus site (D), the dung had obvious beetle fauna including Geotrupes spiniger.


My thanks to Paul F. Whitehead for useful discussion, and all mentioned above as pers. comm.


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  • CLEMENTS David K & SKIDMORE Peter 1998 The autecology of the hornet robber fly Asilus crabroniformis L. in Wales, 1997. CCW Contract Science Report No. 263
  • GREEN David M 1998 Asilus crabroniformis L. hornet robber fly. Survey in Worcestershire help wanted. Worcestershire Record 5: 10
  • GREEN David M 1999 The mysterious biology, and survey, of Asilus crabroniformis L. (Diptera Asilidae) hornet robber fly Worcestershire Record 6: 12
  • JESSOP L 1986 Dung beetles and chaffers. Coleoptera: Scarabaeoidea. Handbk Ident. Br.Insects. RES
  • 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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