Evaluating the use of UV photoluminescence for surveying the immature stages of rare butterflies: a case study using the Black Hairstreak (Satyrium pruni)

Conservation of rare and elusive butterflies can benefit from the use of contrasting survey techniques to confirm their presence and to understand the lifecycle in more detail. This case study trialed different survey techniques for all stages of the lifecycle (ova, larva, pupa and adult) to assess their value for monitoring Black Hairstreak (Satyrium pruni) populations in Britain. Daytime surveys for well camouflaged ova, larvae and pupae yielded little success. However, the novel use of UV flashlights in nocturnal surveys for photoluminescent larvae was found to be an effective technique when compared to surveying adult butterflies. Critically, combined surveying for larvae and adults provides unique insights into the behaviour of the species at different stages in the lifecycle and, when used in combination, gives a more holistic understanding of the true nature of a colony of Black Hairstreak butterflies. Implications for insect conservation: our results show that the novel use of nocturnal surveys for larvae with UV flashlights is a valuable technique for surveying for Black Hairstreak butterflies. It therefore has potential to be used in conjunction with other survey techniques for assessing the health of a colony and supporting conservation at different stages of the lifecycle of the butterfly.