The influence of thermal ecology on the distribution of three nymphalid butterflies.
Studies have shown that many adult and immature insects are able to maintain body temperature well above, and often independently of, ambient temperature in the presence of direct solar radiation. They may do so directly by basking, or indirectly via microhabitat choice. The implications for this are often ignored in development models that serve to predict species' responses to climate change. To investigate the difference in development times attributable to solar input, field development of Aglais urticae, Inachis io and Polygonia c-album larvae was followed both in a natural open situation and in an artificially shaded environment. Each species completed development more quickly in the open, equivalent to a 20, 10 and 15% reduction in development time, respectively. Observed development times in the artificially shaded environment were used to compare the techniques of rate summation and degree-day modelling. Rate summation models described development in the shade best for A. urticae and I. io, although a degree-day model performed best for P. c-album, possible reasons for which are discussed. The best performing models for each species were modified to include larval thermoregulation data, assuming a linear relationship between body and ambient temperatures during the measured sunshine hours each day, and tested against observed development times in the open situation. Times for 50% adult emergence were predicted exactly for A. urticae, and to an accuracy of 1 day for I. io and 5 days for P. c-album. The models were tested further using climate data from 128 UK Meteorological Office weather stations across England, Wales and Scotland. Thermoregulation model predictions matched observed UK distribution and voltinism better than predictions made by the standard unmodified models. It was estimated that larval thermoregulation allows A. urticae and I. io populations to persist approximately 200 km further north than would otherwise be possible, and that the extent of bivoltinism may be shifted northwards by around 300 km. 5. These results have significant implications for predicting the effects of global warming on insects' geographical ranges, the potential distributions of invasive species, and the phenology and voltinism of introduced biological control agents as one component of their likely success.