The accurate monitoring of butterfly populations can serve as a valuable indicator of ecosystem health, determine the impacts of environmental changes and contribute towards the conservation of threatened species. However, several species of butterfly are difficult to monitor because certain lifecycle stages are cryptic. This can lead to inaccurate estimates of population densities in monitored colonies or to established colonies being overlooked. Also, where specific lifecycle stages are difficult to monitor, critical information for conservation can be missed (for example, the symbiotic relationship between the larvae of the Large Blue (Phengaris arion) and Myrmica ants, (Thomas, 1980)).
The Black Hairstreak is a rare and elusive butterfly in Britain where it is found only in Blackthorn (Prunus spinosa) scrub on heavy clay soils (Eeles, 2019; Thomas & Lewington, 2014) and has legal protection against being collected (Bourn & Warren, 1998). The species is difficult to observe at all stages of its lifecycle. Adults are rarely observed at ground level, typically residing high in trees such as Oak (Quercus robur) or in Blackthorn scrub where they feed on honeydew secreted by aphids (Fig. 1a). This means that they tend not to be recorded by traditional methods of surveying such as transect walks because recorders focus their attention at lower levels (Pollard & Yates, 1993). The adult is sedentary for long periods when feeding or basking, interspersed between brief periods of flight which are dependent on weather conditions. It is small (c.37mm wingspan), can easily be confused with other similar species such as the White-letter Hairstreak (Satyrium w-album) (Salmon, 2001) and has a relatively short flight period (approximately two weeks). Collectively, these features mean that as an adult, the species can easily be missed in conventional surveys or, where targeted surveys are being undertaken, confidence that the numbers recorded accurately reflects the number present can be low.
Black Hairstreak ova (shown in Fig. 1b) can be extremely difficult to find due to their size and camouflaged colour (Field, 2006; Kosuch, 2004). Thomas observed that “to find two or three an hour is very good going, whereas one might find 100 Brown Hairstreak (Thecla betulae) eggs for the same amount of searching” (Thomas & Lewington, 2014), because the latter are white and have a more distinctive structure. The larval stages of all hairstreak species are also extremely well camouflaged. On hatching, first instar (L1) Black Hairstreak larvae bury themselves in blackthorn buds. At L2 they leave the bud, changing colour through L3 and L4 instars to match the colour of the blackthorn leaves as they mature (Fig. 1c). Pupae mimic bird-droppings for protection from predators (Fig. 1d) and can also be difficult to locate in the wild. The area used in this study is in Surrey, UK, and daytime searches for Black Hairstreak ova, larvae and pupae were undertaken in 2021. Surveys for ova yielded no results in ten hours of surveying, with similar results for pupae. Searches for larvae in late May 2021 resulted in successfully locating only a single L4 caterpillar in 10 hours.
Luminescence of larvae under UV light has previously been reported in certain lycaenid butterflies (Moskowitz, 2017, 2018, 2020), a trait that has been shown to reduce avian predation (Czarnecki et al., 2021). In this study, we show for the first time, that the larvae of Black Hairstreak luminesce under UV light. We also applied a combination of novel nocturnal UV torch surveys and traditional daytime survey techniques in order to compare the effectiveness of different methodologies for surveying Black Hairstreaks. This work aimed to trial improved methodologies for recording, develop a more detailed understanding of cryptic and elusive butterflies, and to support their conservation.