White-letter hairstreak

The white-letter hairstreak, so named because the white lines on its underwing form a W, is the emblem of the Wiltshire Branch of Butterfly Conservation and is the focus of a project to return this butterfly to our countryside.

In 2017, a white-letter hairstreak was identified in the reserve, near the hedge between Sleepers and Cornfield, where there are elm suckers growing from the roots of trees killed by Dutch elm disease. Elm is the sole foodplant of white-letter hairstreak caterpillars and in the 1970s, when Dutch elm disease killed an estimated 60 million mature elm trees, it was feared the white-letter hairstreak would become extinct in the UK.

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[1] The galleries of the elm bark beetle which spreads the fungus that causes Dutch elm disease; [2] dead elms along the lane.

As soon as it was known that there were survivors somewhere in the reserve, the Countryside Team and the Friends set about looking for disease-resistant elms to enlarge and improve potential white-letter habitat. With the help of Wiltshire Butterfly Conservation, a dozen saplings of disease resistant cultivars of Ulmus minor (small-leaved or field elm) were tracked down in 2020 and planted in 2021.

The whole of the white-letter hairstreak’s life cycle depends on elm trees. The eggs are laid at the end of the summer on the scar tissue between twig and elm leaf. The caterpillar, fully developed but still tiny, spends the winter safe inside its egg-case and hatches in the spring when the tree produces flowers and leaves for it to eat. It pupates in the middle of the summer, attaching itself with a silk thread to an leaf or a twig.

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[3] White-letter hairstreak egg on the specialised tissue between leaf and twig; [4] a well disguised white-letter larva; [5] the adults never settle with their wings open.

The adult butterflies emerge after three or four weeks pupation. Their primary food source is the honeydew excreted by the aphids in the tops of their colony’s elm trees so they are rarely seen at ground level. Sometimes, early in the morning or late in the afternoon, and particularly in dry weather, they might come down from the treetops to feed on thistle, ragwort or bramble.

The white letter hairstreak is a small, brown and nondescript butterfly with an erratic flight pattern. When it alights, it always folds its wings so that the W on its underwing is visible. Keep a look out for it; we would love to be able to confirm its continuing presence in the reserve.

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