Monitoring the abundance of a canopy-dwelling butterfly species
(Warren et al. 1997). However, it is thought that the conservation priority of S. w-album may be an
error due to under-recording because of its secretive habits (Corke 1997). Consequently, the BMS
transect method may lead to canopy dwelling species appealing 'rarer' than they actually are.
Satyrium w-album is an elusive butterfly; it spends most of its time in the tree canopy feeding on
aphid honeydew (Davies 1986; Thomas & Lewington 1991; Pollard & Yates 1993; Corke 1997).
The butterfly lays its eggs on Ulmus spp., and the larvae feed on the elm buds and leaves (Oates
1984; Davies 1992; Thomas & Lewington 1991). Satyrium w-album is a highly sedentary species
with individuals rarely flying further than one or two tree tops away from the breeding tree (Oates
1984; Thomas & Lewington 1991).
The British population of S. w-album has been severely reduced as a consequence of habitat
degradation caused by Dutch Elm Disease (Davies 1986; Oates 1984; Easterbrook 1988; Thomas &
Lewington 1991; Corke 1997). Indeed, there were concerns about the survival of this species in the
1970s due to the epidemic of the disease, which resulted in the loss of many known S. w-album
colonics (Oates 1984; Davies 1986; Corke 1997).
Ulmus spp. in Britain have recently been shown to be re-infested with Dutch Elm Disease, with
certain infestations similar in magnitude to those of the 1970s (Brasier 1997; Pain 1997; Pinchin
1999). Since 1990, almost ninety percent of young elm frees around London have been killed by this
disease (Pain 1997). The fungus (Ophiostoma novo-ulmi), which is responsible for Dutch Elm Disease,
is predicted to retain a continuing cycle.
In the period after the 1970s epidemic, S. w-album has been found to survive in greatly reduced
numbers on 'immature' elm trees and regenerating sucker growth, which have a reduced susceptibility
to the disease (Oates 1984; Thomas & Lewington 1991). Efforts to locate S. w-album colonies have
shown that the species is not as rare as was once thought (Davies 1992; Corke 1997).
The primary objectives of the study reported here were to develop a monitoring technique that
would accurately record the abundance of S. w-album by observation of the tree canopy, and provide
information on the habitat requirements of this species. The secondary objective was to compare the
new technique with the BMS transect method for recording the abundance of S. w-album.
Study Site
The site chosen for the investigation was Langdon Nature Reserve, Basildon, Essex, UK (Ordnance
Survey grid reference TQ 660 875). The site was selected as it has recently been linked with high
populations of S. w-album colonies (N. Wood pers, comm.), some of which were in association with
Ulmus spp. which were infected with Dutch Elm Disease. The reserve has been owned and managed
by the Essex Wildlife Trust since 1989 and is unique due to its 'plotlands' history (Gunton 1994).
The reserve covers an area of approximately 186 hectares and supports a diverse array of habitats
including flower rich meadows, ancient and secondary woodland, plantations, scrub and ponds.
The reserve offers one of the most diverse populations of butterflies in Essex, with twenty-nine
species being recorded there in recent years (N. Wood pers. comm.). Four butterfly species of national
conservation priority have been observed at Langdon Nature Reserve: Pyrgus malvae (drizzled
Skipper) and S. w-album, classified as medium priority species, and Callophrys rubi (Green
Hairstreak) and Melanargia galathea (Marbled White), which are of low conservation priority
(Corke 1997).
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Essex Naturalist (New Series) 18 (2001)