Berks, Bucks & Oxon Wildlife Trust (BBOWT) is working with Butterfly Conservation to train volunteers in a brand new skill: hunting for glow-in-the-dark caterpillars.

The technique has never been widely used to collect official statistics before, and experts say it could make surveying butterflies and moths significantly quicker and easier and reveal hidden secrets about their behaviour and evolution.

Steven Lofting, Conservation Manager for Butterfly Conservation, said: "This really could be a game-changer: at the moment we rely on daytime surveys for these rare and endangered species and that often means trying to spot a dark brown butterfly high up in a tree. 

"If we can just shine a light in the bushes and these caterpillars suddenly glow at us like an electric lightbulb it could make it so much easier and quicker to do surveys of these species: it has the potential to change our understanding of their biology, distribution, abundance and ecology. It's really exciting."

Steven led the first training session for volunteers and BBOWT staff on Wednesday night at the Trust's Finemere Wood nature reserve between Bicester and Aylesbury.

On this occasion the group were specifically looking for caterpillars of the UK's five species of hairstreak butterfly: the black hairstreak is endangered, the brown hairstreak and white-letter hairstreak are both vulnerable, and green hairstreak and purple hairstreak have also declined in abundance and distribution since the 1970s.

BBOWT and Butterfly Conservation have surveyed hairstreaks for decades, but the techniques have been slow and painstaking: for brown hairstreak, both charities rely on teams of dedicated volunteers going out in the middle of winter to look for tiny, pinhead-sized eggs on twigs using magnifying glasses.

In the new technique, all volunteers have to do is shine an ultraviolet light (UV) into bushes and trees which makes the luminous larvae gleam as if they are producing their own light - an effect known as photoluminescence.

At the survey, the team of 30 found a total of 32 black hairstreak caterpillars in two hours. Steven said that a team of 30 looking for black hairstreak eggs would be lucky to find one or two in that time, highlighting the dramatic difference the new technique could make.

Caterpillars of many other butterflies and moths also glow under UV light but the technique is so novel that scientists still don't have a full list of which ones. Butterfly Conservation is now asking volunteers across the country to go out with UV lights and start submitting their findings to illuminate this new field of ecology. Guidance on how to do this safely can be found at butterfly-conservation.org/uv.

The training session was part of a landscape-scale conservation partnership project led by BBOWT called Reconnecting Bernwood, Otmoor and the Ray which aims to work with communities, farmers and landowners to restore habitat for threatened species. The project, on which Butterfly Conservation is a key partner, has received £512,182 from The National Lottery Heritage Fund, and BBOWT is applying for a total of £3.6 million.

The volunteers will now go out using UV lights to look for hairstreak caterpillars specifically on private land such as farms which haven't been well surveyed before. The hope is that they will shine a light on the species' true distribution. 

Emily Coulam, RBOR Nature and Greenspaces Officer at BBOWT said: “It is fantastic that we are able to utilise new and innovative techniques to better understand the wildlife of the region through our National Lottery Heritage Fund supported RBOR programme. 

"The data we gather through these volunteer surveys will help us work with Butterfly Conservation, the Ray Farmer Cluster and others to develop projects which hope to increase abundance and range of the hairstreak butterflies through habitat creation and restoration.”

Surveying butterflies is especially important for conservation because they are known as 'indicator species': that means that rises and falls in their populations can predict the health of the wider ecosystem.

Official figures from the UK Butterfly Monitoring Scheme released at the beginning of April revealed that 2024 was the fifth worst year for UK butterflies since the scheme began in 1976, and more than half of species were in long-term decline.

Find out more about glow-in-the-dark caterpillars and how to survey them at Butterfly Conservation’s new online hub –butterfly-conservation.org/uv

Find out more about the RBOR partnership project.