Parasitoid wasps indicate that the marsh fritillary butterfly (Euphydryas aurinia) has persisted on Tiree rather than re-colonised recently

The population of the marsh fritillary butterfly (Euphydryas aurinia) in western Scotland fluctuates greatly and the species is difficult to locate during periods of scarcity. It was recorded on Tiree in 2014 for the first time in 64 years during a period of abundance. The nearest known populations occur 40 km to the east and its discovery represents either a recent colonisation event well beyond its known capability or it has escaped detection for a longer period. Surveys for the larval cocoons of two wasps that are parasitoids of E. aurinia and host-specific were undertaken on Tiree in spring and autumn 2017. The adult wasps are small with limited powers of dispersal and are unlikely to have followed E. aurinia to Tiree since 2014. Cocoons of one species - Cotesia melitaearum – were abundant in all concentrations of E. aurinia in spring 2017 but neither species was found in the autumn. The absence of the second species - C. bignellii - is inconclusive as it was also absent at known locations elsewhere in autumn 2017. The occurrence, distribution and abundance of C. melitaearum on Tiree indicate that E. aurinia had been present for some time before its discovery in 2014. Although continuous presence since 1950 cannot be extrapolated with certainty, the results highlight the propensity of E. aurinia to persist for long periods in numbers that fall below the observation threshold, even in areas of high wildlife awareness, and that it is not always an easy species to record.