The importance of Aspen for Lepidoptera

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

Mark Young
Centre for Ecology, University of Aberdeen, Culterty, Newburgh, Ellon, Aberdeenshire, AB41 6AA. Email: m.young@abdn.ac.uk

Introduction

The species of Lepidoptera that use Aspen as a larval foodplant are summarised by Emmet (1991), who lists all British Lepidoptera and their life histories. Depending on interpretation of the use made of Aspen by generalist feeders, there are around 40 moth species regularly found on the tree in UK overall, about 25 in Scotland and, of these, 26 are specialised Aspen feeders in UK, whereas only about 14 are mainly confined to Aspen in Scotland (Table 1). Southwood (1961) originally counted the number of herbivorous insects on British tree species, (although this analysis has been upgraded recently for Lepidoptera by Young (1997)). By this reckoning, Aspen harbours a rather modest total, compared with Oak, Birch and Sallow, for example. However, the number found on different trees is positively correlated with the abundance and distribution of each tree and, although Aspen is widespread, it does not compare with other forest trees in its abundance, nor in the tendency to form extensive woodland. Its 'apparency' to moths is relatively low. In Scotland, every large stand of Aspen tends to have a regular attendance of 10-12 moth species, which form a distinct assemblage.

Characteristic species on Aspen

Since Aspen is closely related to other Poplars, and reasonably closely related to Salix spp., it is not unexpected to find that it shares a number of species with them. The Pale prominent (Pterostoma palpina) and the Poplar hawk (Laothoe populi) are examples of species which will readily use most Sallow and Poplars species, whereas the Swallow prominent (Pheosia tremula) and the Poplar grey (Acronicta megacephala) are restricted to Poplars. However, the latter illustrates an interesting feature in that it is apparently restricted to Aspen and does not use other Poplars in the north of its UK range. This extra specialisation applies to several species. Finally there are some species, such as the Seraphim (Lobophora halterata) and the Lead-coloured drab (Orthosia populeti), that are always restricted to Aspen.

Most tree species have some moth larvae that feed on the bark, flowers or that bore into the twigs. However, all Aspen's specialised feeders use only the leaves, except that the Lead-coloured drab also feeds on the flowers at first. It is traditionally believed that the flat petiole, leading to the trembling of the leaves, makes it difficult for insect herbivores to remain attached to Aspen leaves, and it may not be coincidence that most species live between spun or folded leaves. For example, the Poplar lutestring (Tethea or) uses a series of strong silk pads to attach one leaf on top of another and Ancylis laetana makes a neat chamber of a folded leaf.

Even small stands of Aspen will harbour the commoner species of moth, especially those that will also use Sallow or other Poplars. However, it is only Aspen woodlands of a significant size that include the rarer, specialist species. The Chocolate-tip (Clostera curtula) is found locally only in the larger Aspen woods. No-one knows how large such a woodland needs to be for this species to survive, but it is clear that it is associated with mature trees, as is the Lead-coloured drab. As will become clear below, other species require regenerating growth and so there is a clear conservation need for large stands of Aspen that include both mature and regenerating stems. The majority of the species that live on Aspen can apparently survive in rather generalised woodlands, but the rare species described below clearly need very much more specialised conditions.

The Dark-bordered beauty and its conservation

The Dark-bordered beauty (Epione vespertaria) has always been a rare species but is now very localised indeed and, by nature of this localisation, must be considered threatened in UK. Its English and southern Scottish localities are already published in the literature and so can be safely repeated here, whereas its northern Scottish localities are referred to by a generalised name (Table 2).

At Strenshall Common, Yorkshire and Newnham Bog, Northumberland Dark-bordered beauty larvae are known to feed exclusively on Creeping willow (Salix repens) and they have been confirmed recently from these sites, but not from the small handful of other historic English sites. At Adderstonlee Moss, Roxburgh it is also assumed that S. repens is the foodplant, in the absence of Aspen, but no larvae have been found to confirm this. Nor has the adult moth been seen there recently, but it must be admitted that the few recent searches have been in less than ideal conditions (K.P. Bland, D.A. Barbour, pers. comm.).

Dark-bordered beauty moth, Epione vespertaria © Roy Leverton This rare and very localised UK BAP Priority species requires very low, regenerating Aspen growth. It is now known from only six small sites, three of which are in the Cairngorms and is considered threatened. Both the female and male Dark-bordered beauty moth are illustrated in this photograph.

In contrast, at the 'Balmoral' and 'Grantown' sites, the larvae feed exclusively on Aspen (Leverton et al., 1997) and are confined to regenerating shoots. There is no clear height above which such growth becomes unsuitable, but recently larvae were found on shoots of less than 50cm height and at the 'Balmoral' site the moths are seen only where there is abundant suckering of less than 1m height. At 'Balmoral', this regrowth has been favoured by irregular cutting of a roadside verge and by the recent clearance of mature woodland to create a pylon wayleave. At 'Grantown', intermittent grazing seems to have allowed regrowth, although the number of available shoots is very low there and only a small number of moths have been found. The colony seems to be in serious danger.

Visits have been made to several possible sites, near those on Deeside and Speyside, but so far with only limited success. In summer 2001, a new site on Deeside was discovered, taking the total to three sites in the Cairngorms. Suitable regeneration of Aspen does occur in places near the known sites and there is no convincing reason why these should not be used. However, a successful site will have to have had a continuity of regrowth always available and the adult females are rather sluggish, so that colonisation may only be possible over short distances.

The clear conservation priority is to secure management of the existing sites, so as to maintain and extend the availability of suitably low growing shoots; and then, secondly, to consider the suitability and security of other nearby sites, with a view to possible introductions.

Species in need of relocation

Finally, there are two species that used to be found on Aspen in Strathspey but are now apparently extinct. Urgent survey work is needed to relocate these, or to confirm their absence. Gypsonoma nitidulana was found until 1911 on old Aspens near Aviemore but has not been seen in Britain since then. Its relocation may seem to be a hopeless cause but there are ample examples of other species that have been rediscovered after such an interlude such as Ethmia pyrausta, recently re-recorded after over 100 years (Smith and Young, 1997). G. nitidulana is rather nondescript and its relocation will require collection of larvae by a specialist.

Paraleucoptera sinuella, by contrast, should be reasonably easily recorded, for its larvae make rather characteristic leaf blotches. It was recorded by Bankes, a well-known and reliable lepidopterist, in an Aspen spinney near Aviemore Railway Station in 1910 and it survived there until the 1950s, since when it has inexplicably vanished. It was also found in 1945 near Grantown, indicating that it was not wholly restricted to one site. Abroad, it ranges from Europe to Japan and is often rather common on other Poplars and Salix spp., as well as Aspen. The adult is generally found in June and again in August, and the larvae in July and September but there may be only one generation per year in Scotland. Adults are tiny white moths, with minute gold streaks on the wings, but the larvae form oval blotch mines on the leaves. These are 1-1.5 cm long and 0.5-0.8 cm wide when fully formed and are at first brown but then later black. The larval droppings (the 'frass') are placed at the centre of the mine, often in a spiral array of fine grains, leaving clear margins to the mine. However, old mines go black and indistinct as the leaves decay. Furthermore, there are other insects that also mine Aspen leaves and so confuse matters. Once the larvae are fully fed, they emerge from the mine and spin a small and sparse rectangular white silk web on a leaf within which they form a dense yellowy-white cocoon, in which to pupate. No other insects form such a spinning and its presence is diagnostic for this species. There is a real hope that P. sinuella will be refound in Strathspey in future, by searching for its mines and cocoons.

Conclusions

Conservation of Aspen feeding Lepidoptera requires a general effort to provide a continuity of all ages of tree, in sufficiently large stands to provide for the larger, specialist species. The Dark-bordered beauty has more specialised and exacting requirements, however, and its three remaining sites in northern Scotland will need careful management, to sustain the continuity of regenerating growth. It has survived by accident so far, but we must now act deliberately to secure its future.

References

Emmet, A.M. 1991. Chart showing the Life History and Habits of British Lepidoptera. In: Emmet, A. M. and Heath, J. The Moths and Butterflies of Great Britain and Ireland. 7:2. Harley Books, Colchester.

Leverton, R., Young, M.R. and Barbour, D. 1997. Epione paralellaria D. & S. (Lep.: Geometridae) and its association with Aspen (Populus tremula) in the Scottish Highlands. Entomologists' Record & Journal of Variation. 109: 49-55.

Smith, R. and Young, M.R. 1997. The rediscovery of Ethmia pyrausta (Pallas, 1771) (Lepidoptera: Ethmiidae) in Britain. Entomologists' Gazette 48 (2): 85-87.

Southwood, T.R.E. 1961. The number of species of insect associated with various trees. Journal of Animal Ecology 30: 1-8.

Young, M.R. 1997. The Natural History of Moths. T. & A.D. Poyser, London.

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