White-letter Hairstreak - Life Cycle and Foodplants

Life Cycle and Foodplants

Eggs are laid singly, usually on the girdle scars near the terminal buds of Elm trees. Wych elm Ulmus glabra is preferred, but English Elm Ulmus procera and Smooth-leaved Elm Ulmus minor var. minor are also used, as are some exotic species, notably Japanese Elm, and ornamental hybrid cultivars such as 'Sapporo Autumn Gold'. The butterfly normally breeds on mature trees, but there is some evidence to suggest that it may have adapted to the suckers arising from the roots of dead elms. Unlike other hairstreak eggs, White-letter Hairstreak eggs are harder to find during the winter months as they turn brown with age and thus far less distinct on the twigs.

The caterpillar hatches in March as the elm begins to flower. It feeds on the flowers and then seeds before moving onto the emergent leaves as the season progresses. Pupation takes place underneath a leaf or twig; there seems to be little association with ants. Adults are on the wing from late June until the middle of August in the UK, where there is one brood a year. The butterflies occasionally fly down from the canopies to nectar from flowers when honeydew is unavailable, notably after heavy rains have washed it from the leaves. The much-preferred flower is creeping thistle, but bramble and others are used. In France, the butterfly has been observed nectaring on Buddleja davidii.