Evaluation of different UV wavelengths and instar for surveying the immature stages of butterflies

Abstract

Nocturnal UV flashlight surveys have been used to study the immature stages of certain butterflies as they offer a novel method of investigating cryptic species. Using the Black Hairstreak butterfly as a model species, we demonstrate how photoluminescence increases with larval stage. Nocturnal UV surveys can be used to show when numbers peak, as larvae become increasingly detectable, before they decrease due to pupation and predation. To investigate the transferability of using UV nocturnal surveys for detecting other species, we compare larval photoluminescence using UV flashlights emitting at 365nm with those emitting at 385-395nm. We find notable differences across six species of butterfly: for example, Small Copper and Purple Hairstreak larvae photoluminesce significantly only under 365nm UV light. As most commercially available UV flashlights emit at 385-395nm, this indicates that there are potentially more species that could be detected using UV flashlights than previously realised. Implications for insect conservation: our results show that researchers wishing to conduct nocturnal flashlight surveys should be aware that, because the photoluminescence of larvae is dependent on the UV wavelength used, monitoring different species will require the adoption of lights emitting the appropriate wavelength. Also, the optimal time for surveying a particular species will be at peak photoluminescence which in turn will be dependent on larval stage.