White-letter Hairstreak Satyrium w-album butterfly and Elm Ulmus

life cycle and identification

Female

Adult Butterfly

(Imago)

Male

The White-letter Hairstreak is about the size of a twenty pence piece and both males and females have tails on their lower hindwings. In females, these tails are more pronounced and longer in length compared to those found on the males.

On their wings, the white markings appear to form the letter "W" when viewed side-on, lending the butterfly its name.

These markings can be reduced or partially absent on males and are bolder and thicker on the female butterfly, which is also lighter in colour.

The wings of newly emerged butterflies have an iridescent quality, which can change from a muted brown colour into a spectrum of bronze, copper and gold hues, especially on the adapted hair-like scales, located towards the base of their hindwings near the body.

The White-letter Hairstreak will take nectar from plants like Small Scabious, Field Scabious, Wild Marjoram and flowering Privet hedges. Such nectar sources become important to the butterfly when there is a lack of honeydew, their main source of sustenance, which they gather from the surface of Elm leaves.

The adult butterfly is thought to live for around four weeks, with the sole purpose of breeding and producing the next generation for the following year.

Where to find them

The White-letter Hairstreak butterfly will only breed on Elm, which is the sole food plant for its caterpillar.

Elm leaves can be identified by a key feature - the occurrence of asymmetrical leaf lobes - as pictured on the following page.

This is where one side of the leaf carries on past the other when meeting the leaf stalk (petiole).

Asymmetrical leaf lobes

Serrated edges and tapered leaf tip

The amount of asymmetry exhibited varies between each species of Elm, but all have leaves with serrated edges and a tapered tip, although the width of the tip again depends upon the variety of Elm in question.

Without close examination, Elm is easily confused with the leaves of Hazel.

Observing the activity of a White-letter Hairstreak colony in the canopy of an Elm tree can be difficult, so to increase the chances of success, it's best to look between mid-June and the third week of July.

Focusing searches in the high canopy of an Elm between 9am and 1pm, during a warm and breeze free day, will also maximise the chances of observing the White-letter Hairstreak butterfly.

Adults typically fly around an area of the canopy on the side of the tree most sheltered from the wind and where the sunlight meets the cusp of shade. Silhouetted against the sky, they resemble "dark triangles" as they perch on the leaves.

In good weather, the butterfly may be seen flying for longer - possibly as late as 5pm.

The White-letter Hairstreak has never been observed opening its wings at rest, or while taking nectar.

In cooler weather, individuals in the canopy may angle their wings directly at the sun, as a means of thermoregulating the temperature of their wing muscles, absorbing as much warmth as possible to sustain bouts of activity.

White-letter Hairstreak caterpillars have an extremely sophisticated camouflage system and adapt to match their host Elm tree, imitating the colouration of the flower, seed, leaf and bark, in that order.

Along the way, they adorn markings which mimic the secondary veins of the leaves (see caterpillar image below).

Egg (Ovum) Young and Mature Caterpillar (Larva) Chrysalis (Pupa)

The adult female butterfly will lay 1mm diameter-sized eggs from late June until early August, coinciding with their flight period.

The eggs are laid on Elm trees in a variety of locations and at all heights, although females tend to favour the south-facing half of the tree. Locations include the growth ''scar'' between the start of current summer growth and the previous year's hard wood, as pictured on above page.

Other prime egg laying locations include the base of leaf buds and when on mature Elm, at the base of flower buds.

The caterpillars, emerge from their eggs in late February or March and head straight for the nearest flower or leaf bud (depending on the maturity of the tree).

They burrow inside for protection and to have their first meal. The caterpillars continue to feed on the flowers once the buds have opened.

Flower Buds

· Predominantly rounded in shape compared to more pointed leaf buds.

· They express a dark red hue and fuller shape.

· Forming on the sides of the twigs.

Flowers

· Anthers are the part of the stamen where pollen is produced. They can be supported by either white (as shown), pale-yellow, green or pink stamen-filaments.

· As the flowers mature, the stigma and styles (having collected pollen) begin to develop the samaras containing the seeds.

Cluster of samaras containing seed

Once pollinated, flowers will soon develop seeds which are contained within a central capsule. This capsule may exhibit a varying saturation of pink (or lack of) and is itself surrounded by delicate wings of tissue called the samara.

Samaras can vary in colouration - from green to a pale-yellow hue - and their size will also vary depending on the Elm variety/cultivar. At the peak of seed development, the samara containing the seed will grow in clusters along each twig, giving some Elm trees the appearance of being in leaf from a distance. The samaras will eventually turn brown before falling from the tree.

White-letter Hairstreak caterpillars and any evidence of feeding damage can be difficult to locate amongst developing flowers, but once the flowers have been pollinated, the first signs to look out for are small, brown-lined circular holes (see images below).

These are made by the caterpillar burrowing its head inside the central capsule to get to the seed. Similar holes can be found on developing leaf buds, which caterpillars may openly rest upon if there is a lack of flowers or seeds nearby.

On young Elm trees which are incapable of producing flowers and seeds, the first signs of caterpillar feeding damage will be on the expanding leaf buds.

Caterpillar feeding damage

Example of caterpillar bore hole into a seed capsule.

Several bore holes can be seen on this leaf bud - the result of a caterpillar trying to access the tender leaves inside.

The typical feeding pattern often displayed on leaves results in a diamond-shaped tip.

White-letter Hairstreak caterpillars feed on Elm leaves in a specific way, creating two semi-circles on either side of the leaf, leaving a diamond-shaped tip (see image above right).

The caterpillars continue feeding until late May or June and can often be seen resting on the underside of leaves. Prior to pupation, the White-letter Hairstreak caterpillar will measure about 1.5cm in length and its green colour will darken.

The caterpillar's decision on where to pupate on its host tree may vary from one Elm species to another. Leaves, twigs or notches in a branch may be used and the caterpillars always attach themselves to their chosen spot by a silken pad and girdle.

The chrysalis will measure around 1cm in length and the adult butterfly will emerge mid to late June. On first appearance, the adult’s wings will appear small and crumpled and the butterfly will sit at rest for some time, its wings expanding as they are pumped with blood.

Once the wings have dried and hardened, the males will ascend to the high canopy of the host Elm tree and stake out a territory to allow the butterfly to form a colony.

Rival males that meet in the canopy can often be seen engaged in an aerial display or “dogfight” - a burst of activity where pairs of butterflies can be seen spiraling skywards in a bid to claim dominance, only ending when one eventually breaks away.

The inconspicuous females emerge slightly later than the males and look to reside at lower levels on their host Elm tree, away from the activity of the high canopy until they are ready to mate.

In favourable years, when a colony size swells, it’s believed females will actively disperse from their host tree, looking to further colonise other Elms within the landscape.

Alternatively, dispersal may happen when their host tree exhibits limited suitability for egg laying, or is in ill-health.

Further reading: www.sussex-butterflies.org.uk/species/white-letter-hairstreak.php

Butterfly Conservation is the UK charity dedicated to saving butterflies, moths and our environment. We provide advice on conserving and restoring habitats. We run programmes for more than 100 threatened species and are involved in conserving thousands of sites and reserves. www.butterfly-conservation.org

The Sussex Branch is run entirely by volunteers. To find out more about our conservation work, please visit: www.sussex-butterflies.org.uk/about

Unless stated on image, all other photos and text © Jamie Burston, White-letter Hairstreak Species Champion, Butterfly Conservation – Sussex Branch.

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