The Brown Hairstreak butterfly (Thecla betulae) has not been observed in our parish so far this century.
Late in the evening on 16 May 2025, several larvae of this species were observed and photographed along Bagg’s Bottom, detected using an ultra violet light on the new growth of a number of Blackthorn (Prunus spinosa) bushes. It is known that Brown Hairstreak larvae photoluminesce and our timing fitted in perfectly with this stage of its development. Five suspected larvae were detected and photographed but the images (under UV light) didn’t show enough detail to make a positive identification.
A further ‘UV safari’ was undertaken in late May to confirm the initial observation and to obtain better photos without using UV light. A number of larvae were photographed, confirming beyond any doubt that the Brown Hairstreak has a healthy colony at Bagg’s bottom.
The dome-shaped eggs are usually laid singly on the bark of its blackthorn host plant, where they remain for eight months until the following spring. During the winter months, once the leaves have fallen, the eggs are quite easy to detect, standing out like small white pearls against the dark, bare stems. Regrettably, a lot of eggs are destroyed each year by hedge trimming. The eggs hatch in March/April and after a brief period the newly emerged larvae commence eating the fresh leaves of their host plant. Unfortunately during the larval stage about 80% of these caterpillars succumb to predators such as birds and other insects. They feed at night, making them east to detect with a UV torch.
After several weeks of feeding, the fully fed caterpillar pupates. It is reported that ants attend the chrysalis and that the developing butterfly rubs its legs inside its pupal case causing ‘chirruping’ noises, referred to a substrate-borne vibrations; no-one is sure why they do this. Mice and other small mammals manage to find these pupae amongst the ground litter and a substantial number are devoured each year.
After a month the butterfly emerges from its pupal case. It’s the largest and brightest of the European Hairstreak species, with beautiful orange undersides, and was once referred to as the Golden Hairstreak. It’s an elusive species that spends most of its adult life living in the tree canopy. Butterflies can favour one tree, referred to as a master tree, often an ash, where they can congregate in numbers and only occasionally come down to the ground to feed on nectar-rich flowers. The butterfly has one brood a year, emerging in late July into August. It’s not until late August and September that the females start to lay their eggs, preferably on the new growth of blackthorn bushes; often only a few feet above the ground.
The Joint Nature Conservation Committee oversees species status assessment using accepted international guidelines. The Red List for butterflies was last updated in 2010. There are 62 species of British butterflies; nine are assessed as vulnerable and the Brown Hairstreak is in this group. The assessment for this butterfly species is that its habitat has reduced by 43% on a 25-year trend, with a population decline of between 30-49%.
The Brown Hairstreak has a ‘protected status’ in the UK under the Wildlife and Countryside Act, 1981. It is classed as a Priority Species under the UK Post-2010 Biodiversity Framework and listed in Schedule 5 of this Act, which means that intentionally killing, injuring, or taking this butterfly is illegal. This has implications for how the blackthorn thickets and the adjacent path can be managed along Bagg’s Bottom. Blenheim (the land owners) have been very quick and positive in responding and have come forward with a plan of action, after having walked and discussed details with the authors.
John Sampson & Martin Gascoigne-Pees