Appearance and behaviour
A dark little butterfly that spends the majority of its life in the tree tops, feeding on honeydew, making it best observed through binoculars. The uppersides are a dark brown with a small orange spot in the bottom corner of the hindwing. The male has a small pale spot on the forewings made up of scent scales. The undersides are a lighter brown with a thin white line, the "hairstreak", which gives this group of butterflies their name. On the hindwing this streak zigzags to form a letter W (or M) from which this species gets its name. The outer edge of the hindwing has an orange border, but there is no orange on the forewings as on the similar black hairstreak and there are two short tails on the hindwings. Part of a group known as "lateral baskers", they always rest with their wings closed.
The insect has a widely disjunct distribution across the Northern Hemisphere; it is found throughout much of Europe, from Wales to the Urals, including Scandinavia, and again in the Far East, including Japan. It is absent from North America.
It is widely but patchily distributed across most of England; the butterfly is absent from Ireland, western Wales and most of Cornwall. In 2017, the butterfly was spotted in Scotland again after a gap of 133 years. The spread of second, far more lethal, strain of Dutch elm disease in the 1970s inevitably had a major impact on the British population, but it has since recovered well in places and is still increasing its range.
To safeguard the future of this butterfly, some conservation organisations within the UK, such as the Hampshire and Isle of Wight branch of Butterfly Conservation and the Forestry Commission, have planted disease-resistant cultivars, notably Lutece. The beetle-resistant European white elm has also been planted, notably by the Cheshire Wildlife Trust.
Life cycle and food plants
Eggs are laid singly, usually on the girdle scars near the terminal buds of elm trees. Unlike other hairstreak eggs, white-letter hairstreak eggs are harder to find during the winter months as they are flatter, turn brown with age, and are thus far less distinct on the twigs. Wych elm is reputedly preferred, but field elm and European white elm are also used, as are Asiatic species, notably Japanese and Siberian elm, as well as hybrid cultivars such as Lutèce. The caterpillar hatches in March. Under normal circumstances, where sexually-mature trees occur, it immediately feeds on the flowers, later the seeds, before progressing to the emergent leaves in April. However, research in the Low Countries has shown that the larvae are remarkably adept at using non-flowering suckers; provided they are able to hydrate on emergence, they can survive dormant for up to six weeks, until the leaves flush. The larval stage lasts on average for 57 days, but this can vary from 37 to 78 days, depending on food availability.
Pupation takes place underneath a leaf or twig, or in a bark crevice. Adults are on the wing from late June until the middle of August in the UK, where there is one brood a year. The butterflies occasionally fly down from the canopies to nectar from flowers when honeydew is unavailable, notably after heavy rains have washed it from the leaves. The much-preferred flower observed in the UK is creeping thistle, but bramble and others are also used. In France, the butterfly has been observed nectaring on Buddleja davidii, but the flowers of the lime tree appear to be its favourite.
Elm cultivar Lutece, Isle of Wight, hosting white-letter hairstreak 2015
- "Rare butterfly spotted in Scotland after 133 years". BBC. 16 August 2017. Retrieved 16 August 2017.
- Bink, F., Cuvelier, S. & Russell, P. (2015) Butterflies in the Benelux. www.phegea.org, Wergroep Dagvlinders. 
- Cribb, P. (1983). Breeding the British butterflies. The Amateur Entomologist, Vol. 18. The Amateur Entomologists' Society, Hanworth, Middlesex, UK.
- Var-Wild (2014) The White-letter Hairstreak Satyrium w-album in the south of France.
- Brookes, A.H. (2016). Disease-resistant elms, Butterfly Conservation trials report, 2016 Butterfly Conservation, Hants & IoW Branch, England. 
- Davies, Martyn and Ceney, Bryan (1992). The White-letter Hairstreak Butterfly. Butterfly Conservation, Colchester, UK, pp. 27. ISBN 0-9512452-7-9
- The Vale Royal White-letter Hairstreak Project  Cheshire Wildlife Trust, UK.