White-letter Hairstreak

• Latin name: Satyrium w-album
• Family group: Hairstreaks
• Countries: England, Wales
• Size: Small

Resident

Range expanding in some areas after major decline. Recent population decline.

The White-letter Hairstreak is a small butterfly with an erratic, spiralling flight typical of the hairstreaks. It is distinguished by a strongly-defined white 'W' mark across the undersides. The dark uppersides are seen only in flight as the butterflies always settle with their wings closed. Adults are difficult to see because they spend so much time in the tree canopy, although they occasionally come to ground level to nectar on flowers near elm trees or scrub saplings.

The species declined during the 1970s when its foodplants were reduced by Dutch Elm Disease, but it seems now to be recovering in some areas.

Conservation status

• UK BAP status: Priority Species
• Butterfly Conservation priority: high
• European threat status: not threatened
• Protected in Great Britain for sale only

European/world range

Across Europe as far north as southern Scandinavia, and east to Japan. Its range is stable in much of Europe, but it has declined in several countries and is spreading in Finland.

Foodplants

The butterfly breeds on various elm species, including Wych Elm (Ulmus glabra), English Elm (U. procera), and Small-leaved Elm (U. minor). Research at one site has indicated a preference for (and a higher success rate on) Wych Elm. It breeds on mature trees or abundant sucker growth near dead trees. It has also been shown to survive on the Dutch Elm Disease-resistant variety of U. japonica, Sapporo Autumn Gold.

Habitat

It breeds where elms occur in sheltered hedgerows, mixed scrub, and the edges of woodland rides, and also on large, isolated elms. It was previously thought to have a strong preference for large, mature elm trees, but is now found frequently on younger elm growth.