Autumn larval cold tolerance does not predict the northern range limit of a widespread butterfly species

Climate change is driving range shifts, and a lack of cold tolerance is hypothesized to constrain insect range expansion at poleward latitudes. However, few, if any, studies have tested this hypothesis during autumn when organisms are subjected to sporadic low temperature exposure but may not have become cold tolerant yet. In this study, we integrated organismal thermal tolerance measures into species distribution models for larvae of the Giant Swallowtail butterfly, Papilio cresphontes, living at the northern edge of its actively expanding range. Cold hardiness of field-collected larvae was determined using three common metrics of cold-induced physiological thresholds: the supercooling point (SCP), critical thermal minimum (CTmin), and survival following cold exposure. P. cresphontes larvae in autumn have a CTmin of 2.14{degrees}C, and were determined to be tolerant of chilling. These larvae have a SCP of -6.6{degrees}C and can survive prolonged exposure to -2{degrees}C. They generally die, however, at temperatures below their SCP (-8{degrees}C), suggesting they are chill tolerant or modestly freeze avoidant. Using this information, we examined the importance of low temperatures at a broad scale, by comparing species distribution models of P. cresphontes based only on environmental data derived from other sources to models that also included the cold tolerance parameters generated experimentally. Our modelling revealed that growing degree-days and precipitation best predicted the distribution of P. cresphontes, while the cold tolerance variables did not explain much variation in habitat suitability. As such, the modelling results were consistent with our experimental results: low temperatures in autumn are unlikely to limit the distribution of P. cresphontes. Further investigation into the ecological relevance of the physiological thresholds determined here will help determine how climate limits the distribution of P. cresphontes. Understanding the factors that limit species distributions is key to predicting how climate change will drive species range shifts.