Byctiscus populi, a leaf rolling weevil dependent on Aspen

The Biodiversity and Management of Aspen woodlands: Proceedings of a one-day conference held in Kingussie, Scotland, on 25th May 2001

Jon Mellings
Centre for Biodiversity and Conservation, University of Leeds, Leeds LS2 9JT. Email: bgyjhm@leeds.ac.uk

Steve Compton
Centre for Biodiversity and Conservation, University of Leeds, Leeds LS2 9JT. Email: pab6sgc@leeds.ac.uk

Introduction

Byctiscus populi (Coleoptera: Attelabidae) is an attractive, metallic green or coppery coloured leaf-rolling weevil associated with Aspen Populus tremula and occasionally Poplars. Fowler (1891) stated that B. populi was found 'on young Aspens', and Morris (1999) refers to 'anecdotal evidence that these weevils prefer young growth of suckering and regenerating trees to large, mature individuals'. The UK range of B. populi appears to have declined in the past few decades. It is classified as Red Data Book 3 'rare' and UK Biodiversity Action Plan (BAP) - Priority listed species (Morris, 1999). In accordance with recommendations in the UK Species Action Plan, research was initiated in 2001 at the University of Leeds, the designated Lead Partner for this species. Former and extant sites are being resurveyed to update records, and thereby establish a picture of the species' current status. Since ecological knowledge of B. populi is largely anecdotal, priority is being given to field and laboratory-based research to examine aspects of its biology and habitat requirements. These findings will be used to advise on practical management actions to aid recovery of B. populi. Here we discuss the current UK distribution of B. populi, describe some initial findings regarding its biology and habitat requirements and suggest possible causes for its decline.

Ecology

Field studies were carried out during summer 2001 at Monkwood (just north of Worcester, Worcestershire), which supports a strong population of B. populi. Comparative studies were also carried out by Lianne Evans (University of Leeds) and Dmitry Telnovs in Latvia, where B. populi is still common.

Besides Aspen, B. populi is said to be associated with White poplar (Populus alba) and Black poplar (Populus nigra) (Hyman and Parsons, 1992). However, in the UK and Latvia it appears to occur almost exclusively on Aspen. Field observations and captive rearing of Latvian and UK specimens have provided a reasonably complete picture of its lifecycle.

Emergence of adults, mating and oviposition

The adult insects appear from May onwards, when they can be seen feeding on the leaves of Aspen. Mated females lay batches of between one to four eggs in cigar-like rolls formed from one, or occasionally two, Aspen leaves, with more eggs deposited in longer leaves (Evans, 2001). The larger, longer leaves found at the growing tips of Aspen suckers are the favoured oviposition sites, with plants as short as 30cm utilized. Trees above two or three metres are used more rarely than smaller individuals. As many as four individuals, both females and more rarely males, may cooperate in the rolling of a single leaf roll. This suggests that some rolls may contain the eggs of more than one female. The rolls are sometimes detached almost immediately, or may remain on the trees for several weeks.

From egg to larvae

Eggs of captive-reared Latvian beetles took an average of just under four days to hatch. The first instar larvae then proceeded to feed on the wilted leaf roll from within. Usually at this point the leaf was still attached to the food-plant. The number of larval instars remains undetermined, but mature larva of 4-6mm in length, vacated the roll after an average of around 16 days. It is possible that larvae occasionally complete their development while the roll is still attached to the tree.

Prepupa, pupation and emergence

In common with B. populi's slightly larger sister species B. betulae, larvae were found to pupate in pupal chambers in the soil at a depth of between 5-60mm. A prepupal stage lasts up to four weeks, followed by a comparatively short pupation, lasting less than a week in some cases. Whereas B. betulae overwinter as adults in the pupal chamber (Bily, 1990), captive-reared B. populi specimens emerged above the soil surface and began feeding shortly after.

Number of broods and hibernation of the adults

In Monkwood, B. populi adults were observed rolling leaves throughout the summer between May and the beginning of August, when suckers were still seen to be producing fresh leaves. The peak activity was around the end of June, when both beetles and leaf rolls were very numerous. It is likely that the beetle is bivoltine or even continuously brooded through the summer. Captive-reared beetles provided with a choice of rough pieces of bark and soil in outdoor conditions in late October 2001, settled beneath the soil and within cracks in the bark and appeared to enter diapause. Whether any final generation adults overwinter as adults in their pupal cells, as described for B. betulae, is unknown.

Distribution

Internationally, B. populi occurs over the whole of the Palaearctic region, being fairly common in central Europe (Harde, 1998). In the UK, Morris (1999) stated 'There are post-1970 records from east Sussex and east Kent, but historically it was more widely distributed, being recorded from much of southern England northwards to east Norfolk, east Gloucestershire and Worcestershire'. Now considered to be 'rare' and declining, it is clear from Fowler's (1891) summary: 'very local, but not uncommon where it occurs', that B. populi was patchily distributed even at the end of the 19th century.

A meeting on the conservation of UK BAP phytophagous beetles, funded by English Nature, was held in London in February 2001. It became evident that B. populi has continued to decline, having probably become extinct at two of the seven sites for which post 1980 records were available. Ian Menzies, who had regularly recorded the species at Bookham Common Special Site of Scientific Interest (SSSI), Surrey, noted that he had recorded no further specimens since 1991. Similarly, four miles away at Wisely Common, Peter Hodge reported the beetle's recent demise as a result of extensive clearance of pioneer Aspen scrub, removed for conservation purposes!

Four possible post-1990 B. populi site records are available, two of which are based on our surveys this year. The first is Oversley Wood, a Forestry Commission site in Warwickshire. Here the most recent of several records since 1987 was made by Lane & Forsythe in May 1999 (Lane & Forsythe, 1999). They beat a single specimen from Aspen and described the Oversley population as 'small and vulnerable'. Aspen is one of the dominant species in sections of this site and our 2001 site visits found several leaf rolls on ride-edge aspen suckers, though no adults were seen. This suggests the population is still extant at Oversley, but remains very small.

The second recent B. populi site record was added by Darren Mann (pers. comm.), who found the beetle at Wappenbury Wood (a Warwickshire Wildlife Trust Reserve approximately 30km from Oversley Wood) during the 1990s. Aspen suckers proliferate along rides at Wappenbury, though conservation efforts have apparently reduced their abundance in recent years. Leaf scars consistent with B. populi were found during our 2001 site visit, but we failed to confirm the continued presence of the beetle.

Orlestone Forest, a Kent Wildlife Trust-managed ancient woodland, near Ham Street was visited in mid-August 2001. B. populi had last been recorded at the site in 1972, with records dating back to the mid-1960s. No adult beetles were found, but two leaf rolls, almost certainly created by B. populi, were discovered in a sunny, ride edge location. Interestingly, the habitat at this site was very similar, in terms of vegetative composition, structure and management, to Monkwood, another site where the beetle survives.

Morris (1990) has described Worcestershire as a 'blank spot' because of the apparent under-recording of Coleoptera there. We investigated two woods in the county where there were old (1950s) records for B. populi, one of which was Monkwood, where we found what may be currently the strongest UK population of this species. This wood is a Worcester Wildlife Trust Reserve and SSSI. An ancient coppice woodland, and once a 'Harris brush wood', it is now managed jointly by the Worcester Wildlife Trust and Butterfly Conservation. Leaf rolls were noticed even before the site was entered, on low aspen suckers at the woodland boundary adjacent to the road. Subsequent searches within the wood revealed a number of discrete patches of Aspen sucker growth along ride edges and in areas of recently-coppiced Hazel (Corylus avellana). B. populi adults and leaf rolls were found in reasonable numbers on virtually all the patches of pioneer, ride-edge Aspen throughout the site. In contrast, the beetle and its rolls seemed largely absent from mature trees (although rolls sometimes occurred on the lower branches of standards in sunny conditions). More tellingly, where aspen suckers occurred as an understorey in areas heavily shaded by mature trees, B. populi was almost always absent.

The second site in Worcestershire, which has recently been acquired by Worcester Wildlife Trust, was Randen Wood, between Bromsgrove and Kidderminster, where the beetle appears to have been lost. Aspen was present in this wood in small isolated patches, but the woodland canopy was closed, allowing little light to penetrate. There was no sign of recent management and no evidence that B. populi was present.

The contrasting fortune of the beetle at the two Worcester sites and its distribution within Monkwood and elsewhere provides strong indications of the habitat needs of B. populi and the management required for it to persist. Small Aspens, growing in sunny, sheltered conditions are what this species requires. The Dark-bordered beauty (Epione vespertaria) appears to have very similar habitat requirements to B. populi. This moth is known mainly from Aspen sites in Scotland, and it would be well worth looking for B. populi at these sites, even though it has not been recorded previously north of the border. Suitable small Aspens are typically found along woodland rides at the English sites, but in Latvia they are also a common feature of roadside verges, derelict land, footpaths and other habitats, hence the beetles much greater abundance there. Perversely, maintenance of open rides is important for the persistence of this species, but thorough ride clearance that results in the elimination of Aspen suckers and bushes deprives it of suitable host plants.

Acknowledgements

Our work on Byctiscus populi has been funded by the English Nature.

References

Bily, S.,1990. A colour guide to beetles, ed. London: Hamlyn.

Evans, L. 2001. A study on Byctiscus populi (L.1758) (Attelabidae) in Latvia and implications for conser-vation management in the UK. Unpublished MSc. project, University of Leeds.

Fowler, W.W., 1891. The Coleoptera of the British Isles, vol 5. London: Reeve and Co.

Harde, K.W. and Severa, F., 1998. A Field Guide in Colour to Beetles. Leicester: Blitz Editions.

Hymen, P.S. and Parsons, M.S., 1992. A review of the scarce and threatened Coleoptera of Great Britain, Part 1. Peterborough: JNCC.

Lane, S.A. and Forsythe, T.G., 1999. Noteworthy beetles found in Warwickshire (VC 38) in 1999. The Coleopterist, 9: 102 - 104.

Morris, M.G., 1999. Byctiscus populi (a leaf-rolling weevil) Action plan. In UK Biodiversity Group Tranche 2 Action Plans, Vol 5: Invertebrates (March 1999). Peterborough: JNCC.

2001, Minutes of meeting on phytophagous beetle conservation, Royal Entomological Society, London. (unpublished).

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