A chance discovery by an Epsom man has sent shockwaves through the world of butterfly conservation, revealing a previously unknown population of one of Britain's most elusive species in Surrey. Gareth Tilley stumbled upon a group of black hairstreak butterflies during a walk in 2020, defying experts' expectations that the insect was confined to woodlands between Oxford and Peterborough.
The black hairstreak butterfly (Satyrium pruni) is notoriously tricky to track due to its clever camouflage: caterpillars resemble leaves, chrysalises look like bird droppings, and adults tend to stay high up in tree canopies. Since 2002, the species' UK distribution has plummeted by a third, largely attributed to habitat loss – making Tilley's find all the more significant for conservation efforts.
Tilley's innovative approach to monitoring came when he applied research from the US on glowing caterpillars under UV light to his own work. In May 2022, armed with a UV torch, he discovered that black hairstreak caterpillars fluoresced brightly under ultraviolet light – a game-changer for surveying these elusive creatures.
Collaborating with Butterfly Conservation, the local council, and the University of Sussex, Tilley's findings have been confirmed, establishing Surrey as a new habitat for the black hairstreak. Steven Lofting, South East Conservation Manager at Butterfly Conservation, praised Tilley's work as "a huge boost" to UK butterfly conservation.
The exact reason why the caterpillars glow under UV light remains unknown – but its practical application is already making waves. The technique has been adopted by conservation groups across the country, with volunteers now able to contribute to the species' monitoring in a new and engaging way.