By James Duncan
Woods Mill Engagement Officer
The Brown Hairstreak, Thecla betulae, can be a particularly difficult butterfly to spot. This is owing to the adults' preference for basking in areas we just can't see - high up in the trees. It's the largest of the UK's five native Hairstreak species and lives in small sedentary colonies that breed in the same area year upon year. The first adults (imago) are generally on the wing by late July or early August and it's the males that typically fly first. Their numbers normally peak late August, making this the perfect time to head on down to Woods Mill to spot them. As with many UK butterfly species, they love a warm day and in the event of cool (sub 20°c) overcast weather, they'll rarely be spotted. Similarly, if it's too early in the morning (before 10am) or too late in the afternoon (after 4pm) it's unlikely they'll fly. This is owing to their extensive use of thermoregulation. They'll often be seen soaking up the sun's rays by holding their wings open at 180° - the dusky colouration of the scales help to ensure maximum heat absorption. However, if it's just too hot they'll keep those wings firmly shut, their shiny underside and white body hairs helping to reflect the sunlight.
In the world of the Brown Hairstreak, a 'master' tree is key to their reproductive success. The males congregate in the canopy of this tree, typically an Ash, and a tall one at that. Here they'll be busy feeding almost exclusively on sugar-rich honeydew, secreted by aphids. They will, on occasion, feed on nectar from a variety of flowering plants, namely Hemp Agrimony, Common Fleabane, Ragwort and Bramble, though this tends to be when honeydew is scarce. Freshly emerged females will head directly for this master tree, wasting no time in their rush to mate. Whilst in the canopy her eggs will mature, typically taking between six and ten days, at which point she'll start her search for suitable egg-laying sites - a sheltered woodland edge or hedgerow. The females are incredibly selective in their preferences and their search for suitable sites will take them far and wide, frequently more than a few kilometres from the breeding area. The rather wonderful sea-urchin shaped eggs are laid singly in the fork of a branch, close to the ground, on new shoots of the larval foodplant - Blackthorn. The larvae overwinter in egg (ovum) form, ready to emerge and feed in spring, passing through four caterpillar (instar) stages prior to crawling to the ground to pupate in late June or early July.
Female Brown Hairstreak © Derek Middleton
There's little doubt the Brown Hairstreak is a highly attractive butterfly, no doubt enhanced by its rarity and lack of visibility owing to a somewhat secluded lifecycle. The females are particularly beautiful, displaying obvious orange patches on the forewing, though both sexes display intricate patterning and vibrant colouration on the underwings. In the early 18th century the females were even considered to be that of a different species, the so-called 'Golden Hairstreak.'
Though the Brown Hairstreak is widely distributed across central Europe, it's no longer so across the UK. Long-term data suggests a severe decline in both range and abundance and it's now just locally distributed across parts of Southern England and SW Wales. The catastrophic loss of hedgerows since 1945 and declines in traditional woodland practices (such as layering and coppicing) have led to it being classified as a UK Biodiversity Action Plan Priority Species and listed as Vulnerable on the new Butterfly Red List. Unfortunately it's one of many examples of a species struggling in a modern agricultural landscape.