White-letter Hairstreak

White-letter Hairstreak Satyrium w-album (Linnaeus 1761)

Family: Lycaenidae, subgroup Theclinae (The Hairstreaks)


Photo top above left by Peter Waterton

Photo top above right and header by Martin Partridge

Photo above left by Samantha Batty

Photo above right by Robert Hulmes

This is one of many species which keeps its wings shut when at rest, only showing the undersides. These are brown, with the white ‘hairstreak’ line forming a very distinct ‘W’ shape, from which the butterfly gets both its common and scientific names. On the rear edge of the hindwing there is a row of orange lunules edged in black, joined together to form a wavy band. Also on each hindwing there are two tails, one longer than the other, both black with white tips. These are most easily seen on freshly emerged individuals, as they soon become worn as the insect ages.

This butterfly is most commonly seen at the tops of the trees upon which it breeds. Individuals can be observed basking on leaves, where they are visible as dark, triangular silhouette. Sometimes the butterfly itself may not be visible, only the shadow it casts on the leaf it’s resting on. If a basking male spies a rival male, or a potential mate, it will fly up to intercept it. If it’s another male this results in a “combat” flight, with both butterflies engaging in a rapid spiralling flight away from the tree canopy before breaking off and each returning to its favoured spot.


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.


White-letter Hairstreak appears to be quite widespread throughout the county although it is rarely seen in large numbers, with some sites known from only a single individual. However, there are a few sites where numbers are often higher, such as Hugset Wood near Barnsley (VC63), Burton Constable (VC61), Hilton Woods (VC62), Potteric Carr (VC63), and Bramhope (VC64).

Due to its elusiveness it is likely that this species is greatly under-recorded throughout the region. It should be looked for wherever elms grow, and a useful technique is to search for eggs (see below). These are present for around nine months of the year and can be looked for throughout autumn and winter. Only a single egg needs to be found to confirm the presence of a breeding colony! There are known sites where eggs have been found but no adults seen, despite a concerted search.

Life Cycle

The adult butterflies start to emerge in late June, and are typically on the wing until late August, with a small number of sightings in early September.

Eggs are laid singly in sheltered positions and at any height, often on the girdle scars between the current and previous year’s growth, nodes and bud bases of elms. The central dome of the egg turns chocolate brown, surrounded by a pale rim and is often described as resembling a flying saucer. These eggs are fairly conspicuous to the practised eye (with a magnifying lens!), enabling searching and recording to be carried out throughout the winter months; indeed, a number of sites are known only from egg records. Eggs are often found on lower branches, and a walking stick can be useful for hooking branches to bring them closer for examination. Larvae develop inside the egg, but don’t hatch out until the following spring. Larvae usually feed on flower buds as they open out, and later move on to leaf buds and developing seed clusters. In the final moult, they eat and rest on the under surface of elm leaves, producing a characteristic dark silhouette to an observer looking up from beneath. The woodlouse shaped larvae are green and yellow with oblique pale stripes and lilac markings. They finally pupate on the underside of an elm leaf, or on a twig, or fork on a bough, hanging by a pad and girdle of silk. Pupae are speckled brown and hairy, resembling an elm bud.


Butterfly Conservation priority: High

UK BAP status: Priority Species


Dave O'Brien 15/03/21