The White-letter Hairstreak (Satyrium w-album) butterfly is so called because of the markings on its wings that suggest a scrawled letter ‘w’ (indeed it has previously been called the White w-Hairstreak). This helps to distinguish this butterfly from other hairstreaks such as the Black. The White-letter Hairstreak is a species that thrives in the high canopy of mature trees, drawing on nectar from flowers and laying eggs on small branches to overwinter. Elm tree flowers are the main recognised food source of the White-letter Hairstreak. As well as English elm, the butterflies are associated with Wych elm (which are Dutch elm disease resistant) and hybrid elms. The Huntingdon elm is an example of a hybrid elm, thought to be partially resistant to Dutch elm disease because of being a cross between Wych elm (resistant) and English elm (not resistant). With so few mature elm trees remaining in the landscape because of Dutch elm disease, a mature elm tree in any location is a rare sight.
There has been a marked decline in the abundance of White-letter Hairstreak, estimated as a loss of 99% of the population over 25 years (1984-2003 Defra/JNCC UK Priority Species Report). Being a high canopy butterfly, it can make it difficult to spot this elusive butterfly and assess population changes but given the loss of mature English elms from England in the 1970s onwards, it is not surprising to see such a decline in this species. The butterfly is considered to be endangered and is listed on the Butterfly Red Data List for the UK.