UK Butterflies

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White-letter Hairstreak

Satyrium w-album

sa-TI-ree-um double you-AL-bum

Wingspan
25 - 35 mm

Checklist Number
61.006

White-letter Hairstreak

Satyrium w-album

sa-TI-ree-um double you-AL-bum

Wingspan
25 - 35 mm

Checklist Number
61.006


The White-letter Hairstreak is one of our more-elusive butterflies as it flits high in the treetops, often appearing as a dark speck against the sky. It gets its name from the letter "W" that is formed from a series of white lines found on the underside of the hindwings.

Elm is the sole foodplant and this species suffered as a result of Dutch elm disease in the 1970s and early 1980s, especially in southern sites. All species of elm were affected and there was concern that this species of butterfly might become extinct in the British Isles as a result. Surviving colonies were subsequently looked for, to obtain a better understanding of the distribution of this species. Several new colonies were found which gave new hope for the future of this butterfly. In addition, there has been a concerted effort to find disease-resistant elms that exhibit the appropriate qualities to support this butterfly (such as flowering at the right time of year since young larvae generally rely on flower buds as a food source).

This butterfly forms discrete colonies which are sometimes very small containing only a few dozen individuals. Colonies are typically focused on a small clump of trees or even an individual tree. These butterflies are not great wanderers and will reuse the same site year after year. This butterfly is found throughout England, south of a line stretching between South Lancashire in the west and South Northumberland in the east. This species is found more locally in Wales, and is not found in Scotland, Ireland or the Isle of Man.

This species was first defined in Knoch (1782) as shown here (type locality: Leipzig, Germany).

 
Family:LycaenidaeLeach, 1815
Subfamily:TheclinaeButler, 1869
Tribe:EumaeiniDoubleday, 1847
Genus:SatyriumScudder, 1876
Subgenus:  
Species:w-album(Knoch, 1782)

Description to be completed.

The distribution data (2000-2009) has been made available through the generosity of Butterfly Conservation and any subspecies distribution is taken from Riley (2007). Click here to see the distribution of this species together with site information overlaid.

This butterfly is never found far from its larval foodplant of elm, Wych Elm being preferred. Flowering elms are usually essential for successful larval development and this therefore suggests a certain maturity of tree, although there is some evidence that this species has successfully used non-flowering elms on occasion. Favourite sites are elms on the edge of deciduous woodland, but this species can also be found in more open habitat such as roadside verges if suitable elms are present.

This species is in serious decline and is therefore a priority species for conservation efforts.

The table below shows the occurrence (distribution) and abundance (population) trends, using information from The State of the UK's Butterflies 2015 (Fox, 2015). Any UK BAP status is taken from the UK Biodiversity Action Plan (UK BAP) (2007 review).

UK BAP Status
Occurrence Change
1976-2014 (%)
Abundance Change
1976-2014 (%)
Occurrence Change
2005-2014 (%)
Abundance Change
2005-2014 (%)
Priority Species
Click here to access the Biodiversity Action Plan (BAP).
Decrease -45Large Decrease -96Decrease -41Large Decrease -77

The adults emerge toward the end of June, building up to a peak in mid-July. There is one brood each year.

The adult butterflies are best seen early in the morning or late in the afternoon, when they will come down from their treetop resting place to nectar on various flowers, Thistle, Bramble and Privet being favourites. Most of the time, however, they remain elusive as they feed on honeydew in the tree tops. Like the Black and Green Hairstreaks, the adults never settle with their wings open.

When egg-laying, the female exhibits the same behaviour of many other hairstreaks, slowly crawling along twigs of the foodplant looking for suitable places in which to deposit her eggs.

Adults feed primarily on Honeydew (N/A). brambles (Rubus spp.), Hemp-agrimony (Eupatorium cannabinum), ragworts (Jacobaea spp.), thistles (Carduus spp. and Cirsium spp.), umbellifers (Apiaceae spp.), Wild Marjoram (Origanum vulgare) and Wild Privet (Ligustrum vulgare) are also used.

Eggs are laid singly at all heights on elm trees and are a most-curious shape - a flattened sphere that has been compared to a flying saucer, with a flattened rim surrounding a central dome. It is typically laid on the scar that separates new and old growth and is green when first laid, but soon changes to a dark brown. The fully-formed caterpillar remains in the egg until the following spring.

"The eggs are laid at the forks of the branches during the end of July and beginning of August and remain in the egg state about nine months. Eggs laid during the first week of August, 1905, commenced hatching on March 3rd, 1907. The egg is only 0.45 mm. high and 0.80 mm. wide, of a very compressed button-like shape. The micropyle is sunken, the crown flattened, and gradually slopes to a prominent projecting rim surrounding the base, which is slightly convex. The whole of the upper surface of the rim is covered with very fine reticulations and minute spinous projections, which develop into a dense series of creamy-white points encircling the egg, forming the rim; the base is likewise covered with reticulations, which are more developed than on the upper surface. All the reticulations are creamy-white and glassy. The surface is very finely granulated of an olive-leaden colour. When first laid the egg is sea-green, with a cream-white rim; it gradually deepens in colour, assuming a clear chocolate-brown in about a week; it then remains unchanged throughout the autumn and winter; in early spring, previous to hatching, it turns a purplish-drab." - Frohawk (1924)


The larva emerges in the spring by eating a hole in the top of the egg and, without eating the remaining eggshell, moves to a nearby flower bud where it feeds by burrowing its head deep within the tissues. The larva changes colour as it grows, always having excellent camouflage. It continues to feed on flower buds, leaving the bud scales intact, before gradually moving to young leaf buds and eventually mature leaves. The larva changes colour to a dull purple just prior to pupation.

The primary larval foodplants are English Elm (Ulmus procera), Small-leaved Elm (Ulmus minor ssp. minor) and Wych Elm (Ulmus glabra).

1st Instar

"Upon emerging from the egg the larva eats away the micropyle, leaving a perfectly circular hole just large enough to admit its body to pass through; as soon as it has emerged it does not eat more of the shell, but it immediately crawls away to feed on the expanding bud. On March 4th, 1906, at 10.45 a.m., a larva started emerging from the egg; the act of emergence occupied ten minutes from start to finish. Directly after emergence the larva measures 1.3 mm. long. The body is humped dorsally with a medio-dorsal longitudinal furrow, the sides sloping and somewhat dilated laterally; the first and last segments are rather flattened above and each with a shining disc of a dark olive colour; that on the first segment is triangular in form and the largest. The head is shining black variegated with deep olive. There are two longitudinal rows of hairs bordering the dorsal furrow, in clusters of three on each segment, i.e., three each side of the furrow; the first hair is very small, the second long and curved backwards, the third much shorter and also curved backwards; below the first shortest hair is a shining white disc or lenticle, outlined with a dark ring resembling a spiracle. The true spiracle is very inconspicuous, of the same colour as the rest of the surface and only to be detected with difficulty; below the spiracle are four long, slightly curving hairs with black shining bases, placed in a quadrangle; these and the dorsal hairs are slightly serrated and blackish; the two large dorsal ones have shining pedestal-shaped bases, the upper parts glossy black and lower half olive coloured. Along the lateral region, including the claspers, are several simple, whitish, straight hairs. The ground colour is olive-ochreous; the entire surface is densely sprinkled with black shining points, giving the whole a dark olive appearance. As soon as the young larvae emerged from the eggs they were placed on the opening blossoms of common elm (Ulmus campestris), upon which they very soon started to feed. Before the first moult, six days old, it measures 12.2 mm. long, of a dull lilac-pink, slightly tinged with ochreous on the dorsal surface (the general colouring greatly resembling the young larvae of Nomiades arion), thus rendering it very inconspicuous, being similar in colour to parts of the elm blossom. It feeds on all parts of the bloom and grows quickly." - Frohawk (1924)

2nd Instar

"The first moult March 21st, 1906. Before second moult, twenty-three days old, it is 5.4 mm. long. In general structure and form it much resembles the previous stage. The second to ninth segments are strongly humped dorsally, with a longitudinal furrow, the sides flattened and slightly sunken. The first segment is projecting, rounded and compressed dorsally, the anal segment is similar, and the tenth and eleventh segments are likewise compressed, and slightly sunken. The first segment overlaps the head while at rest. The entire surface is covered with very fine cellular granulations and studded with whitish serrated hairs, each surmounted on an olive-coloured pedestal base with black buttresses up the sides; the dorsal hairs are the longest. There are numerous lenticles sprinkled over the surface. The spiracles are small, projecting and conical, with very small apertures, and are shining black and olive. The head is shining black; on the first segment is a central dorsal blackish disc. The ground colour is a clear yellow-green, variegated more or less in different individuals with pink-lilac and lilac-brown; generally these colours occupy the third, fourth, fifth, ninth, tenth, eleventh and twelfth segments, giving the larvae a banded appearance exactly resembling the unexpanded elm buds, which are coloured precisely like the larvae; as the buds expand they form a banded pattern. In some specimens the lilac shades cover the whole surface with the exception of the anterior and posterior segments, which remain pale green like the ventral surface, and whitish rims to the dorsal humps. They feed on various parts of the expanding buds and young leaves, being especially fond of the former, which they perforate, burying half their bodies in them." - Frohawk (1924)

3rd Instar

"The second moult April 6th, 1906. Shortly before the third moult, thirty-five days old, it measures 7 mm. long. After moulting it is less pink, and during this stage it gradually becomes greener, until shortly before next moult it has acquired a generally green colour all over, most of the specimens only retaining a sub-spiracular lilac band on the ninth segment. The green colouring increases with the development of the young expanding leaves which the larva very closely resembles. The ground colour is a clear yellow-green, with the dorsal furrow and oblique side stripes of a pure bright green; in other respects it resembles the previous stage. Many of the unopened buds the larvae had completely hollowed out, leaving only the thin layer of the outer covering." - Frohawk (1924)

4th Instar

"The third moult April 14th, 1906. After the third and last moult, forty-five days old and fully grown, the larva while crawling measures 15.9 mm. long. The segments are prominently humped dorsally, with a deeply sunken medio-dorsal longitudinal furrow, and the segmental divisions clearly cut. The first segment is rounded, somewhat compressed and projects over the head, which is completely withdrawn while resting, the sides sloping and lateral ridge dilated. On the first segment is a dorsal quadrangular whitish glassy disc. On the tenth segment is a dorsal transverse gland, similar in structure to that exhibited by the larvae of many of the Lycaenidae, but the author failed to induce it to exude liquid, although it appears to be very sensitive when touched and instantly contracts. This gland is only conspicuous in the last stage of the larva, but whether it is present in any or all of the earlier stages the author was unable to decide. The entire surface is densely covered with white serrated spinous hairs of various lengths, each having a green bulbous pedestal base. On the second and last segments is a medio-dorsal cluster of dark hairs. The spiracles are amber-brown and surrounding them are several lenticles. In most specimens the ground colour is a clear, light greenish-yellow, with the medio-dorsal, oblique side stripes and the lateral ridge pale yellow, ventral surface and claspers green, legs ochreous-green; head shining olive-brown, palest on the crown, eye and mouth markings black and pale yellowish. Some specimens are less green in colour and variegated with rose pink markings on the fourth and ninth segments, consisting usually of a sub-dorsal blotch on fourth segment and medio-dorsal and sub-dorsal and lateral marks on the ninth segment, and a faint brown-pink band bordering on the lateral line and a medio-dorsal stripe on the last segment; the hairs in this form of larva are more or less tipped with amber. When fully grown they gradually turn duller in colour, and finally the yellows and greens are replaced by brown-olive and ochreous hues. The larva spins two or three of the small terminal leaves together, with a good layer of silk spun on the under side of one of the leaves, and thereon pupates. The larva have no cannibalistic propensities whatever during any stage. The first one that spun up on April 22nd (early morning) pupated May 2nd, being ten days undergoing the transformation, an unusually long period." - Frohawk (1924)

The pupa is fastened to a leaf or stem by a silk girdle and the cremaster and is similar in colour to an elm bud. This stage lasts between 3 and 4 weeks.

"The pupa measures 9 mm. long and is stout in proportion. Dorsal view: The head and thorax rounded, the abdomen swollen at the middle and rounded at the anal extremity. Lateral view: The pro-thorax slightly projecting round the anterior edge (somewhat similar to the overlapping first segment of the larva), forming a slight hood over the rounded head; from the anterior edge of the pro-thorax a continuous curve outlines the meso and meta-thorax; it is slightly concave at the waist; the abdomen rises gently and curves round to the anal segment; the ventral surface forms a straight line. The ground colour is a pale ochreous-brown, lightest on the abdomen. The head is purplish-black, the wings are heavily checkered and blotched with dark purplish-brown markings, appearing almost black; a dorsal dusky streak passes over the pro-thorax, then diverging into sub-dorsal bands running over the meso-thorax and meta-thorax, and a medio-dorsal black band extends over the meta-thorax and first two abdominal segments; it then fades into a pale purple-brown band passing over the remaining segments, bordered each side by a paler line than the ground colour; the sides of the abdomen are sprinkled with dark brown specks, the largest forming two sub-dorsal series; the spiracles are swollen and pale ochreous. The whole of the thorax and abdomen is covered with finely pectinated whitish bristles, and the entire surface is covered with a network of reticulations and minute lenticles resembling spiracles; the reticulations on the wings are simple ridges of irregular pattern; those on the thorax and abdomen are raised into circular knobs with radiating keels, many of which sink into the surface before reaching the next projection. The whole of the anal extremity is furnished with cremastral hooks, being very numerous and firmly secured into a layer of silk, also a silken cincture passes round the waist. The first imago emerged May 28th, 1906; others emerged during the following week, remaining twenty-six days in the pupal state." - Frohawk (1924)