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.
