Saving a Rare Species – White-letter hairstreak

binoculars focussed on elm treesFollowing on from the good work of the East Dartmoor NNR volunteers to find and plot the wych elm trees in Rudge and Hisley Woods, a group of conservationists got together to learn more about a rare butterfly. The white-letter hairstreak is dependent on various species of elm as a food plant and the wych elms of the Bovey Valley Woods were believed to support a colony of this specialist tree canopy species. Though there have only been a few recently confirmed records of the white-letter hairstreak, a sunny July morning spent searching for them would hopefully reap a rich reward. (wych elm identification)

Wych elm leaves
Wych elm leaves

Anticipating a successful day, the group met at the entrance to Pullabrook Wood, kitted out with binoculars and studying a map of where the elm trees stood. Albert Knott of Natural England introduced the training day explaining that “we manage all the habitats in the valley as one big ecosystem. During our walk today we’ll be looking for empty white-letter hairstreak eggs and, hopefully, the adult butterflies too, but the question is, are they still there?”

Starting the walk from the car park and along the lane, binoculars were trained on the roadside elm tree tops. Jenny Plackett from Butterfly Conservation described how to “look up from underneath and you might see the shadow of a butterfly”. Moving further into the woods where a cluster of elms stood, there were no signs of white-letter hairstreaks in flight or at rest on the trees. Other woodland species were making the most of a bright summer spell and the silver washed fritillary, white admiral and the ringlet all made appearances along the grassy woodland edge.

 

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While keen eyes scanned the canopy, Jenny took the opportunity to explain some more about the life cycle of this scarce species. “The female is larger than the male but both have a defined white “W” on the underside of the wings. They are subject to a massive 96% decline in numbers, associated with the loss of elm trees and a 45% decline in distribution. Since 1970 a number of sites are now extinct but even a single elm can support a population”. The elm trees standing in this valley are clearly becoming a higher conservation priority and the efforts to protect, propagate and plant them, all the more pressing.

Jenny Plackett Butterfly Conservation
Jenny explains the life cycle of the white-letter hairstreak

With a collection of photos and diagrams, Jenny continued to describe more about the species. “The adult flies in summer and, once mated, they lay their eggs at the point on a twig where new growth meets last year’s growth or around a bud. A positive identification of an egg on a twig would be enough to confirm the presence of the species but this takes a lot of experience”.

WLH egg Pete Eeles 2
Very small eggs are difficult to find – note the leaf scar and bud nearby [photo: Pete Eeles]
The larva, after leaving the egg, goes through various growth stages, or instars. Each time, these match the appearance of the spring time development of tree flowers, buds and leaves and each time the caterpillar uses fantastic camouflage. While Jenny talked, it became clear just how dependent the white-letter hairstreak as a species is upon the elm trees, when every stage of their life cycle is linked so closely to the development of the tree at that particular time of the season.

WLH larva Pete Eeles
Every stage of the larval development mimics the concurrent emergence of buds, seeds and leaves [photo: Pete Eeles]
Once the leaves have opened, the white-letter hairstreak larva will provide further clues of its existence by feeding on the foliage of the elm with distinctive patterns. “When you look up into the canopy with binoculars, you will get really excited if you see these patterns. It is indicative of their presence, but it’s not proof” enthused Jenny. “One of the classic feeding patterns is the neat hole in the leaf, between the veins”.

white letter hairstreak feeding
Feeding patterns of the later instar caterpillars

As the group continued to survey the tree tops, they watched around five purple hairstreaks; a similar species associated with oak trees that haven’t suffered the same drastic reduction in numbers, but still a treat to see. After a few hours and a few hints and clues to the presence of the white-letter hairstreak, the group left the woods, walked along the lane and, just before reaching the car park where it all started, one keen-eyed spotter called out “I can see one … maybe two!” And there they were, angling their wings to catch the sun. In the top of a small elm tree were two confirmed sightings, making it all worthwhile. Beautiful butterflies!

White-letter hairstreak on a wych elm twig
White-letter hairstreak on a wych elm twig

Use the links below to find out more …

Butterfly Conservation WLH booklet

UK Butterflies white-letter hairstreak information page

by Matt Parkins

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