White-letter Hairstreak (Satyrium w-album)
I have recently been checking nearby localities for the presence of native elm (Ulmus spp.). Unless you live in Brighton of course, where the national collection is housed, elm has nowadays become a somewhat rare component of the British countryside, mainly due to the devastating effects of Dutch elm disease (DED) - one of the most serious arboricultural diseases in the world. Our main native species, English elm (Ulmus procera), smooth-leaved or field elm (U. minor) and Wych elm (U. glabra) are all susceptible to Ophiostoma novo-ulmi, a highly aggressive pathogen of the disease first noted in Britain in the late 1960s. It is disseminated by various species of elm bark beetles. Within a decade of its arrival about 20 million elms out of an estimated UK population of 30 million were dead. By the 1990s this number was probably well over 25 million. The disease still persists in the UK. The less virulent Ophiostoma ulmi first reached the UK in 1927.
Elm is unfortunately the sole foodplant of S. w-album and this species has undoubtedly suffered as a result of DED, especially in its southern distribution. As all species of elm are affected there was concern that this species might become extinct in the British Isles as a result. Surviving colonies were subsequently looked for in order to obtain a better understanding of the distribution of this species. Several new colonies were discovered which gave hope for the future of this vulnerable butterfly. In addition, there has been a concerted effort to find disease-resistant elms that exhibit the appropriate qualities to support this butterfly (such as flowering at the right time of year - since young larvae generally rely on flower buds as a food source).
S. w-album is never found far from its larval foodplant, U. glabra being the preferred choice. Flowering elms are usually essential for successful larval development and this therefore suggests a certain maturity of tree, although there is some evidence that this species has successfully used non-flowering elms on occasion. Favoured sites are elms on the edge of deciduous woodland, but this species can also be found in more open habitat including roadside verges, such as the individual female figured above, if suitable elms are present.
The butterfly forms discrete colonies, which are sometimes very small containing only a few dozen individuals. These are typically focused on a small clump of trees or even an individual tree. They are not great wanderers and will reuse the same locality year after year. This elusive species is found throughout England, south of a line stretching between South Lancashire in the west and South Northumberland in the east. It is found more locally in Wales, and is not found in Scotland, Ireland or the Isle of Man.
Check out the elms near you …