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Field Guide
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White-letter Hairstreak

Satyrium w-album

Not yet photographed by the community

White "W" on underside; elm-dependent and scarce.

Species Profile

Nationally Scarce B; populations have declined significantly due to Dutch Elm Disease reducing host plant availability.
Lifespan
Approximately 2 weeks as an adult butterfly in the wild
Size & Weight
Wingspan 27–32 mm
Habitat
Woodland edges, scrub, and hedgerows where elm trees are present, typically in warm, sheltered locations.
UK Distribution
Found in southern England, particularly in the Midlands and south-east, with a northern limit around Yorkshire; resident but with fluctuating populations.
Diet
Adults feed on honeydew secreted by aphids and small amounts of nectar from flowers; caterpillars feed exclusively on elm leaves (mainly Ulmus minor and U. procera).
Predators
Small birds, spiders, and parasitic wasps targeting caterpillars and pupae
Mating Season
July to August
Breeding
Females lay single eggs on elm twigs in late summer; eggs overwinter and hatch in spring; single brood per year; caterpillars feed April to June.
Behaviour
Adults are elusive and spend much time in the canopy feeding on aphid honeydew rather than flowers, making them difficult to observe. Males perch on elm branches and dart out to intercept females. They are most active in warm sunshine.
Did You Know?
  • •Named 'w-album' after the white W-shaped mark on the underside of their hindwings
  • •Caterpillars are attended by ants that protect them in exchange for honeydew, similar to other hairstreak species
  • •Dutch Elm Disease has devastated UK elm populations, causing dramatic declines in White-letter Hairstreak numbers since the 1970s
  • •Adults rarely visit flowers, preferring to feed on aphid honeydew high in elm tree canopies
  • •The species has shown some recovery in recent years where elm trees have regrown or resistant varieties are being replanted

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