BYCTISCUS POPULI: THE STATUS AND BIOLOGY OF A RARE
SPECIES ACTION PLAN (SAP) WEEVIL IN WORCESTERSHIRE.
LEFT: B. populi adults constructing leaf roll, Monkwood, Worcester, June 2001
RIGHT: Aspen sucker with leaf rolls and new roll in the process of construction, Monkwood, Worcester, June 2001
Byctiscus populi (Coleoptera: Attelibidae) an attractive, metallic green or coppery coloured, leaf-rolling weevil associated with Aspen Populus tremula and White Poplar Populus alba, has suffered serious decline in the UK in the past few decades. It is now classified as RDB3 'rare' and is the subject of its own SAP - 'Species Action Plan' (Morris,2001). In response to specifications in the SAP, research is currently being carried out at Leeds University, English Nature's lead partner for this species. Many former, extant and potential sites have been surveyed to update records, and a truer picture of the species' current status has emerged. Results from this survey together with research into aspects of the biology and habitat preferences of this weevil, will be used to advise practical management actions necessary to aid recovery of B. populi's UK populations.
During the summers of 2001 and 2002 more than twenty sites were surveyed nationally. Of only six sites where B. populi survives, two have been located in Worcestershire. Whereas the remaining U.K. populations of this weevil, one in Warwickshire and the remaining three in B. populi's former stronghold of south east England, are very small and in danger of extinction, both Worcester populations are thriving.
A common feature of historic and extant B. populi populations is that they have, with very few exceptions, occupied ancient woodlands or common land with a history of coppicing. Interestingly, there are several parallels between the two Worcestershire sites. Monkwood and Trench Wood have very similar management histories; they are both ancient coppice woodlands that were managed by Harris the brush-makers in the post war period, before being taken over by Worcestershire Wildlife Trust and Butterfly Conservation.
B. populi is one of only two British representatives of the genus. Both are compact, metallic weevils with squarish elytra and straight rather than elbowed antennae. Byctiscus males are characterized by the presence of a conspicuous, forward-pointing spine on the lower margin of the pronotum in the anterior third on each side. This feature is a particularly useful identification aid, being unique to Byctiscus in this group. B. populi's sister species B. betulae is very similar to B. populi which can be distinguished by the inky blue-black underside contrasting markedly with the typically greenish or coppery coloured upper surface. B. betulae is normally a uniform metallic blue in the male and metallic green or golden green in the female, the underside being concolorous with the upper surface in both sexes. Whereas the elytral apices are entirely glabrous in B. populi, in B. betulae these are sparsely clothed with fine, pale pubescence. B. betulae tends also to be a somewhat larger species of 4.8-7mm compared with 4-5.5mm in B. populi.
Adult beetles appear around the beginning of May and can be found from this time onwards until the end of September. The weevils then feed on the developing aspen leaves before commencing to construct neat, cigar-like leaf rolls into which 1-4 pearly white eggs are deposited on the innermost edge. Prior to rolling, the female weevil punctures the petiole causing the leaf to wither. The eggs hatch after about a week and the creamy white larvae begin to feed in earnest on the decaying leaf. At some point during the larval development the roll falls to the ground, the larva continues to feed before burrowing beneath the soil to pupate.
Leaf rolls are continually produced throughout the summer but peak activity is between June and July, activity trailing off considerably and ceasing around mid-September. In captivity, B. populi specimens were found to take as little as one month to develop from egg to adult at room temperature, so it seems probable that peak activity at the end of June and beginning of July corresponds to the emergence of individuals developed from eggs laid by the over-wintering brood in May. Captive reared adults emerging in August were successfully hibernated in outdoor conditions in late October. Given a choice of rough bark and sandy soil, weevils hibernated both just beneath the soil and within cracks in the bark. Despite this, the lack of late season records of B. populi adults, may indicate that the majority of late brood weevils remain beneath the ground until the following year. Bily (1990) states that this is the case with B. betulae.
B. populi's need to produce leaf rolls is evidently important in determining the weevil's choice of habitat as it is linked to the early successional stages of aspen. The weevil almost exclusively uses newly regenerating aspen 'suckers' for leaf rolling. In contrast to more mature aspens which produce few new leaves after the initial Spring bud-burst, in their first two years of growth suckers produce a continual supply of new leaves throughout B. populi's flight season. Importantly, pliable new leaves are much easier to roll than older ones, furthermore, the leaves of young suckers tend to be much larger than those of mature aspens and older suckers. Interestingly, a study by Evans (2001) of B. populi in Latvia, showed that more eggs tended to be laid in longer leaves. Meanwhile, in Monkwood two or more females were occasionally observed to cooperate in the rolling of a single large sucker leaf.
The low sucker growth is also beneficial in providing a warm, sheltered microclimate. Like many thermophilous invertebrates, B. populi becomes more active as temperature increases. Although sexual activity and roll production continues throughout periods of cooler weather and even in moderately wet conditions, in periods of direct sunlight activity speeds up markedly, weevils fly readily in such conditions and have been observed to fly for 100 metres or more on occasions. In June and July they are active for long hours, continuing to produce rolls throughout the day from early morning until dusk.
B. populi is another example of a species which suffered greatly from the decline of traditional woodland practices in the latter half of the last century. Surveying has revealed that the beetle has disappeared from former strongholds which have become shaded. A good example in Worcestershire is Worcestershire Wildlife Trust owned Randan Wood near Kidderminster where B. populi was recorded in 1951 by Fred Fincher. Whilst both mature aspen and suckers still occur, albeit thinly on this site, the lack of light penetration evidently led to the demise of the weevil. (Fred Fincher allowed parts of the wood to develop naturally long before it became a WWT reserve - Ed)
Being a species associated with early successional re-growth, B. populi seems well adapted to survival in a dynamic habitat. The rotationally managed ride and coppice system of Monkwood and Trench wood seems very suitable for the insect and in mid-summer ride edges and young coppice stands in parts of the reserve are festooned with B. populi leaf-rolls. The weevil may have benefited from the open habitat created during the Harris brush era at a time when many other former coppice woods were neglected. At Monkwood the beetle may also have inadvertently benefited from management for the Wood white butterfly Leptidea sinapis which favours a patchy ride edge habitat of tall grasses and light scrub. An overall impression is that the weevil is perhaps more at home on ride edges than in all but the earliest stages of the coppice cycle. This may partially explain why this species has died out in sites like Bradfield Wood in Suffolk. Although this site has been continuously coppiced since mediaeval times, the ride edges are regularly manicured and kept almost entirely scrub-free.
At present a report detailing work completed over the past two field seasons at Monkwood and other sites is being prepared for English Nature. Plans for the next year include a large scale G.I.S. habitat mapping project to establish the whereabouts and condition of habitat suitable for B. populi, in woodlands in the areas surrounding core sites such as Monkwood and Trench wood. Any information regarding Byctiscus populi records in the Worcester area would be gratefully received.
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 conservation management in the UK. Unpublished MSc. project, University of Leeds.
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.
Jon Mellings. Email: email@example.com
Dr Steve Compton. Email: firstname.lastname@example.org
Or locally Helen Woodman or Harry Green at Worcestershire Wildlife Trust.
The map below shows national distribution of B. populi (all records)
ASPEN LEAF ROLLS
On a Management Committee visit to Monkwood on 6th June 2002 we noticed the leaf rolls on young aspen as shown in the pictures. At that time we were not sure what Bycticus populi rolls looked like so I sent a picture to Jon Mellings for comment - see the footnote to his article. Following his response I sent the pictures to Tony Simpson who said that the most likely causer was a micro-moth Anacampsis populella (Fam Glechidae) which produces 95% of aspen leaf rolls. Less likely was one of two species of Tortricinae, which also make aspen leaf rolls. If any reader sees rolls in Worcestershire resembling B. populi please let us know as a matter of urgency - thank you. Harry Green
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