| Abstract |
- The small tortoiseshell butterfly (Aglais urticae L.) is considered to be a widespread and abundant generalist species in Northern Europe. However, it declined sharply in the U.K. between 2003 and 2008, coinciding with the arrival and spread of a parasitoid, Sturmia bella Meig. (Diptera: Tachinidae), which specialises on nymphalid butterflies.
- Whether the decline in A. urticae is associated with the arrival of S. bella was investigated using data from a large-scale butterfly monitoring scheme, and by collecting larvae to assess parasitoid incidence and parasitism frequency. Similar data were compiled for a related butterfly (Inachis io) which is also parasitised by S. bella but which is not declining.
- Sturmia bella was recorded as far north as north Lincolnshire (53.53°N). Aglais urticae has declined significantly to the south of this latitude, but not to the north.
- Sturmia bella was present in 26% and 15% of the larval groups of A. urticae and I. io, respectively, and now kills more individuals of A. urticae (but not I. io) than any native parasitoid.
- Survival was 25–48% lower in batches of A. urticae larvae where S. bella was present, indicating that S. bella causes host mortality in addition to that caused by native parasitoids.
- Our results suggest that S. bella may be playing a role in the recent decline of A. urticae. However, further research is needed to establish its effects relative to other potential drivers of trends in the abundance of this butterfly.
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