CYNGOR CEFN GWLAD CYMRU COUNTRYSIDE COUNCIL FOR WALES The autecology of the Hornet Robberfly Asilus crabroniformis L. in Wales, 1997-1999 D.K.Clements & P.Skidmore 2002 CCW CONTRACT SCIENCE REPORT No. 525 CCW CONTRACT SCIENCE REPORT TITLE: The autecology of the hornet robberfly Asilus crabroniformis in Wales, 1997-1999 REPORT No.: 525 AUTHOR: D.K.Clements & P.Skidmore PUBLICATION DATE: July 2002 NOMINATED OFFICER: Dr. Mike Howe RESTRICTION: Nil DISTRIBUTION: Core: CCW HEADQUARTERS, LIBRARY ENGLISH NATURE, LIBRARY SCOTTISH NATURAL HERITAGE, LIBRARY BRITISH LIBRARY, DOCUMENT SUPPLY CENTRE NATIONAL LIBRARY OF WALES, ABERYSTWYTH CCW SOUTH AREA (CARDIFF), LIBRARY CCW WEST AREA (ABERYSTWYTH), LIBRARY CCW EAST AREA (LLANDRINDOD WELLS), LIBRARY CCW NORTH-WEST AREA (BANGOR), LIBRARY CCW NORTH-EAST AREA (MOLD), LIBRARY INVERTEBRATE ECOLOGIST, CCW HQ Others: ENGLISH NATURE, INVERTEBRATE SPECIALIST SCOTTISH NATURAL HERITAGE, INVERTEBRATE SPECIALIST CCW SENIOR CONSERVATION OFFICER, SWANSEA CCW SENIOR CONSERVATION OFFICER, GWENT CCW WARDEN, CRYMLYN BOG GWENT WILDLIFE TRUST A.E.STUBBS The Autecology of the Hornet Robberfly Asilus crabroniformis L. in Wales, 1997-1999 CONTENTS Summary Section 1.0 : Introduction Section 2.0 : Approach & Methods Section 3.0 : Results Section 4.0 : Discussion & Conclusions Section 5.0 : Summary of the Three-Year Study Results Section 6.0 : References Acknowledgements Figure 1 : Map showing the eastern area of Kilvey Hill and western pastures of Crymlyn Bog, indicating main areas of occupation by Asilus crabroniformis in 1999 Figure 2 : Habitats of pasture areas 1 & 2 of Crymlyn Bog Figure 3 : Habitats of pasture area 5 of Crymlyn Bog Figure 4 : Habitats of pasture areas 3, 4 and The Mound at Crymlyn Bog Figure 5 : Recorded movements of five adult Asilus crabroniformis in Crymlyn Bog and the eastern pastures of Kilvey Hill in 1999 Figures 6-10 : Recorded movements of adult of adult Asilus crabroniformis at Brockwell’s Meadows in 1999 Figure 11 : Crymlyn Bog & Kilvey hill area, showing recorded distribution of Asilus crabroniformis in 1999 Figure 12 : Survey of the Usk Valley, Monmouthshire, for Asilus crabroniformis in 1999 Figure 13 : Frame used for captive rearing of Asilus larvae (empty) Figure 14 : Geotrupes larva in-situ in rearing frame Figure 15 : Mature larva of Asilus crabroniformis, dorsal view Figure 16 : Mature larva in side view, showing polygons on sides Figure 17 : Head capsule of mature larva Figure 18 : Typical field excavation for larvae of Asilus crabroniformis Figure 19 : Adult Asilus showing simultaneous capture of a Sarcophaga sp. and Enallagma cyathigerum Appendix 1 : Survey Results - Day Notes Appendix 2 : Tabulated Results Appendix 3 : Survey of Kilvey Hill and the northern parts of Crymlyn Valley, Glamorgan Appendix 4 : Survey of the Usk Valley, Monmouthshire The Autecology of the Hornet Robberfly Asilus crabroniformis L. in Wales, 1997-1999 Summary This report sets out the results of the final year of a three-year study into the autecology of the Hornet Robberfly Asilus crabroniformis L. in Wales. The main objectives of this part of the study were to use mark-recapture techniques on adults at Brockwell’s Meadows in Gwent, and Crymlyn Bog/Kilvey Hill in Glamorgan, to investigate adult population sizes and other factors such as longevity and dispersal. Efforts were also continued to find larvae in the field and to study larval behaviour. A subsidiary objective was to investigate the distribution of the species in the valley of the River Usk in Monmouthshire (Gwent). The study estimated that the total population of adults at Crymlyn Bog varied between 67 to 179 in the period between 27 July - 11 September, peaking between 18-19 August. On the adjacent Kilvey Hill a more limited survey estimated numbers between 155 - 231 in the period 30 July to 11 September. The study at Brockwell’s Meadows estimated adult numbers varying from 51 to 104 in the period 29 July to 1 September, peaking between 31 July - 1 August. The study concluded that the numbers of adults observed by traditional means at any one time typically represent between about 10-20% of the estimated total population, and that the populations at the two sites therefore probably reside in the low hundreds throughout much of the flight period. Slightly greater numbers of males than females were present, possibly suggesting that females disperse away from emergence sites although it was impossible to detect any clear trends. Adults were found to be capable of travelling over distances of at least 430m. Typically this appears to be by means of short movements from one patch of dung to another (‘dung-hopping’) although in some instances there was evidence of sustained flight. Long movements of this order could either occur over short time periods (ie several hours) or over longer periods of several days. No clear sexual differences could be found. Adults longevities of at least 10 days were found to be fairly common, with an average of 15.9 days and a maximum of at least 34 days longevity being observed in the field. These results are broadly in line with those of two independent studies carried out by researchers in Gwynedd and Oxfordshire. Rearing experiments and field searches showed that the larvae of Asilus are free-living in the soil, and are almost certainly predatory on the larvae of geotrupine beetles, although this latter point could not be proven definitively. A final instar Asilus larva was encountered in the field, photographed and reared in captivity but showed no inclination to feed over a two month period, before successfully pupating and hatching. The rearing experiments suggest that a one-year life- cycle is possible but may not always be the norm, and that larvae may go without feeding for long periods. It is suggested that the life-cycle period varies according to food availability. The distributional studies in the valley of the River Usk in Monmouthshire found no evidence of this species in any of the areas examined, even at sites where there were historic records. However, a secondary study of the distribution around Crymlyn Bog and Kilvey Hill in Glamorgan found evidence to suggest that a large area of suitable habitat surrounding Crymlyn Bog and Kilvey Hill is occupied by Asilus. The Autecology of the Hornet Robberfly Asilus crabroniformis L. in Wales, 1997-1999 Crynodeb The Autecology of the Hornet Robberfly Asilus crabroniformis L. in Wales, 1997-1999 1.0 INTRODUCTION 1.1 This report has been prepared by David Clements (DKC) and Peter Skidmore (PS) on behalf of the Countryside Council for Wales (CCW). 1.2 This is the final report of a three-year study into the autecology of the hornet robberfly (Asilus crabroniformis) in Wales, carried out during the period 1997-1999. The report sets out the results of survey activities carried out during the summer of 1999, together with a résumé of the findings from the previous two years. Preparation of the final report was delayed by the personal circumstances of the main author. 1.3 Project objectives in the 1999 survey season included the use of mark-recapture techniques to investigate factors such as population dynamics, adult longevity and dispersal. Continuing efforts were also made to find larvae in the field, and to establish their life history and mode of feeding. A subsidiary objective was to carry out surveys to locate new breeding localities for this species in the floodplain of the River Usk in Monmouthshire. 1.4 As in 1998, some additional input was made by Dr Jonty Denton, who has been independently studying this species at Thursley Common in Surrey. The late completion of the present report has also meant that data from two other, independent studies could be incorporated in the discussion. These studies were carried out in Oxfordshire and Gwynedd during 1999-2000 (J. Smith 2000; Lloyd 2001) and also looked at Asilus population dynamics using mark-recapture techniques. 1.5 Full accounts of the previous two years’ work are provided by Clements & Skidmore (1998) and Clements et al (1999), to which reference should also be made. CCW Contract Science Report No. 525: July 2002 1 The Autecology of the Hornet Robberfly Asilus crabroniformis L. in Wales, 1997-1999 2.0 APPROACH AND METHODS 2.1 As in previous years, research was concentrated at two sites in South Wales: Crymlyn Bog SSSI near Swansea in Glamorgan, and Brockwell’s Meadows SSSI near Newport in Gwent. 2.2 Crymlyn Bog was primarily studied by PS. The Asilus population at this site is distributed throughout a series of traditionally grazed pastures at the edge of an extensive area of lowland fen, bog and swamp near the mouth of the River Neath. The pastures comprise generally acid dry and marshy grasslands on peaty soils, grazed by ponies, cattle and sheep. The study at this site also extended onto the adjacent Kilvey Hill, which comprises a mixture of grassland, scrub, woodland and revegetated spoil habitats on rising ground adjacent to the SSSI. 2.3 Brockwell’s Meadows was primarily studied by DKC. This site comprises a group of three dry, species-rich neutral (and some calcareous) pasture enclosures on base-rich soils, grazed by cattle, with the Asilus population highly localised on one 2ha slope in the largest field. 2.4 Both of the main sites are described in greater detail in Clements & Skidmore (1998). 2.5 Mark-Recapture Studies 2.5.1 At Crymlyn Bog, individual Asilus adults were captured by hand-net and marked on the wings with an alpha-numeric code, using coloured Staedtler Lumocolor Permanent pens (blue, red, green and black). Adults were captured at random throughout the SSSI (ie no fixed transect or route was followed) over periods of about 4-5 hours. As far as possible, each day spent marking individuals was followed by a repeat visit the next day, when the same procedure was followed.