Gareth Tilley found 46 black hairstreak caterpillars glowing under UV light in an Epsom woodland in a single evening, more than traditional surveys had ever recorded in the county. Butterfly Conservation is now training volunteers nationwide to use the method.
In May 2022, Gareth Tilley pointed an ultraviolet torch at the foliage of an Epsom woodland and counted 46 caterpillars glowing back at him. Each one was a black hairstreak, one of Britain's rarest butterflies, a species so difficult to monitor that traditional survey methods had never recorded it in Surrey at all.
The find was not supposed to happen. Tilley, a hobbyist butterfly observer, was retracing a lockdown walk he had taken in 2020, when he first noticed a black hairstreak along a Surrey hedgerow. Black hairstreaks had previously been recorded only in woodlands running between Oxford and Peterborough. The species' United Kingdom range has contracted by 33 percent since 2002, according to Butterfly Conservation, the national charity that tracks British butterflies, largely because of habitat loss. The charity itself had no Surrey records. BBC News reports on Tilley's lockdown find and the 33 percent range decline, attributed to Butterfly Conservation.
Tilley suspected the lockdown sighting was either a stray or evidence of a small, unnoticed population. He had read United States research showing that some caterpillars fluoresce under ultraviolet light, a quirk of biology that turns their otherwise leaf-like bodies into brief, bright signals. In May 2022, he bought a UV torch and walked back to the same stretch of hedgerow. In a single evening he counted 46 caterpillars. The number was not just a personal triumph; it suggested a population that conventional methods, which rely on spotting adults high in the canopy and on larvae and chrysalises that resemble leaves and bird droppings, had been missing entirely.
Locals initially suspected the Surrey population was an illegal release. Tilley worked with Butterfly Conservation, Surrey County Council, and the University of Sussex to confirm that it was a natural occurrence and to log the first county record for the species.
The biological function of the caterpillars' fluorescence is still unknown. Tilley himself has speculated that it may help deter predators, or have no direct function at all. BBC News reporting on Tilley's discovery notes the same uncertainty about the fluorescence's purpose.
What is now reportable is the institutional response. Steven Lofting, Butterfly Conservation's south east conservation manager, said the charity recognized the significance of Tilley's method and is now promoting UV surveying for hairstreaks nationwide. "UV surveys are finding much higher numbers of hairstreaks," Lofting said, calling it a "new, fun way to engage volunteers." BBC News quotes Lofting on the nationwide rollout of UV surveying.
The technique is also leaving Britain. Tilley has shared the UV-survey method with conservation groups in Australia, a sign that what began as a single amateur's field test in a Surrey wood may end up reshaping how multiple countries count their most cryptic caterpillars.
The news peg here is methodological, not heroic. UV surveying is a counting tool, not a habitat-restoration tool. The 33 percent range decline since 2002 is a separate problem, driven by the loss of the dense hedgerows and well-managed woodland edges the species depends on, and one amateur's proof-of-concept is not a peer-reviewed method. What the technique does is let surveyors see what was already there. The open question is whether the lost third of the range can be re-found, or whether the hedgerows the species depends on are gone for good.