White-letter Hairstreak

British Nature Guide

Wych Elm was once a cornerstone species of the Wrekin Forest, inhabiting the same damp fertile soils as Ash in places like Limekiln Wood and Chermes Dingle. The half century long ravages of Dutch elm disease mean it is now much less common but, thankfully, still relatively abundant — which is all the better for a tree top dwelling insect with some very distinctive markings. The Hairstreak family of butterflies are united by the presence of a characteristic thin white line on the undersides of their wings but the White-letter variety (Satyrium w-album) takes its detailing a stage further, doing exactly what it suggests on the tin! The ‘W’ in question appears on the undersides of its hind wings, which are typically held shut when at rest. Ordinarily, you’d do very well to see such activity with the naked eye, for this species spends much of its time high up in the woodland canopy feeding on honeydew. However, when that substance is in short supply, the White Letter Hairstreak descends, in typically spiralling fashion, to ground level to feast on thistles and brambles.  What has this to do with Wych Elm you might ask? Well, in spring, one of the easiest ways to identify this tree (which is equally comfortable growing as a hedgerow shrub) is by the orange hairs that cover its twigs and buds. It’s in the forks of these twigs that White Letter Hairstreaks lay their button-shaped eggs, and where their caterpillars eventually emerge to feed on the bright green, jagged and lop-sided leaves of their host.

Quick ID: dark, plain hairstreak with notable white ‘W’ marking on the brown undersides of the hind wings, which also have orange edges. A treetop dweller with an erratic flight pattern, it rests with wings closed.

Where and When: adults are on the wing from July to mid-August around in scrub and woodland rides where Wych Elm is present.

Links: Ercall Moths and Butterflies