sanctuaryofwings
White-letter hairstreak (Satyrium w-album), a species in the family Lycaenidae.
Photo: Ian Kirk from Broadstone, Dorset, UK · Wikimedia Commons · CC BY

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Contents
  1. Identification
  2. Range and habitat
  3. Life cycle and host plants
  4. Behaviour and flight period
  5. Similar species

The white-letter hairstreak, Satyrium w-album, is a small brown butterfly of the family Lycaenidae, named for the thin white line that zigzags into a “W” on the underside of its hindwing. It ranges across Central, Northern and Eastern Europe and western Asia, from Wales east to the Urals and from southern Scandinavia to the Mediterranean, with further populations in Siberia, the Russian Far East, Korea and Japan; it is absent from North America and the Himalayan region. Across that range it lives in elm edges of deciduous woodland, hedgerows and roadside verges with suitable elms, rarely found far from elm trees, with wych elm the preferred host.

Identification

The white-letter hairstreak has a wingspan of 25–35 mm. The upperside is dark brown, with a small orange spot near the lower corner of the hindwing, and males carry a pale patch of scent scales on the forewing. The underside is a lighter brown, crossed by the thin white line that gives the species its name; on the hindwing this line zigzags into a “W” shape, with orange borders along the hindwing’s outer edge but not on the forewing. Each hindwing carries two short tails, and the sexes differ mainly in tail length, with females showing longer tails than males. Adults never settle with their wings open, so the diagnostic white “W” has to be looked for on the closed underside.

Range and habitat

The species is spread widely through the temperate Old World, from Wales east across Central, Northern and Eastern Europe to the Urals, from southern Scandinavia south to the Mediterranean, and on through Siberia and the Russian Far East to Korea and Japan. It is tied closely to its host trees, keeping to elm edges of deciduous woodland, hedgerows and roadside verges wherever suitable elms grow, and rarely straying far from them.

Life cycle and host plants

Caterpillars feed on elms, chiefly wych elm (Ulmus glabra), but also English elm (Ulmus procera), field and small-leaved elm (Ulmus minor) and, in Asia, Japanese and Siberian elms. Females crawl along elm twigs searching for a place to lay, usually choosing a branch fork, and lay their eggs singly during late July and early August. The egg is a flattened sphere, green at first and darkening to brown, and the species overwinters as a fully formed caterpillar still inside the egg. In spring the caterpillar eats its way out through a small hole and feeds first on flower buds, then on young leaves, changing colour from pink or lilac to green and then dull purple before pupating, over four instars and roughly 60 to 70 days. The chrysalis is fastened by a silk girdle to a leaf or stem, closely resembling an elm bud, and pupation lasts three to four weeks. The white-letter hairstreak produces a single generation each year.

Behaviour and flight period

Adults are on the wing from late June to mid-August in the UK, with a single brood a year and numbers peaking in mid-July. The butterfly is strongly arboreal, spending most of its time high in the canopy feeding on honeydew, where it flits about as little more than a dark speck against the sky, best picked out with binoculars. It comes down to nectar at flowers such as thistle, bramble and privet mainly in the early morning or late afternoon, and like other lateral baskers it rests with its wings closed, angled side-on to the sun.

A more lethal strain of Dutch elm disease spread in the 1970s and had a major impact on the British population, though it has since recovered well in places and is still increasing its range. Even so, the decline has continued by other measures: occurrence fell by 5% between 1976 and 2019, with abundance down 78% over the same period, including a further 27% drop in the 2010s alone.

Similar species

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