White-letter Hairstreak may be found in woodlands, parks, gardens, embankments, and hedgerows, wherever elms grow. In Yorkshire it strongly favours Wych Elm (Ulmus glabra), but it has also been seen on English Elm (U. procera) and Smooth-leaved Elm (U. carpinifolia). In woodlands, these are typically at the woodland edge or in a glade so that the canopy receives sufficient sunlight. A significant colony may be supported by a single hedgerow tree. It is stated that this species is rather sedentary and doesn’t wander far from its “home” tree. However, individuals have been observed at sites where no elms were seemingly present, so clearly it possesses some powers of dispersal.
It is thought that this butterfly mainly feeds on honeydew secreted by aphids, which collects on the surface of leaves, and this means that it rarely needs to come down to ground level. However, it can often be found feeding on flowers, sometime at quite a distance from the nearest elm tree. Creeping Thistle (Cirsium arvense) seems to be favoured, but it has also been seen nectaring on bramble (Rubus fruticosus), ragwort (Senecio sp.), Angelica (Angelica sylvestris), and Wild Marjoram (Origanum vulgare).
With its dependence on elms as the sole larval food plant, it is clear that the main threat to the White-letter Hairstreak is Dutch elm disease (DED). Most of the large, mature elms in the UK were lost in the 1970s & 80s, although the disease doesn’t entirely kill off the rootstock so many elms regenerated through suckers. Additionally, Wych Elm produces large quantities of seed, so in some areas there was significant regeneration from rapidly-growing seedlings. Once these trees reached flowering age many were colonised by butterflies, presumably from refugia that had escaped the worst effects of DED. However, as these trees continue to grow, they reach a size where they can be detected by the large elm bark beetle (Scolytus scolytus), the main vector for DED in the UK, and so the cycle begins again.
White-letter Hairstreak is known to breed on Ulmus ‘Sapporo Autumn Gold’ a disease-resistant elm (DRE) hybrid from Japan, and trials are ongoing to assess the suitability of other DRE species and varieties as larval foodplants. Planting of DREs could help mitigate the effects of DED on this butterfly.