Squash cultivation and adaptation in cold tolerance aided the northward range expansion of a solitary bee

Climate change is driving massive range shifts to higher latitudes among terrestrial species. Conversion of landscapes into agricultural land has impeded movement for many species by limiting connectivity between suitable habitat patches. However, in some cases, the expansion of agriculture may aid species' range expansion and facilitate adaptive processes in the thermal limits of organisms experiencing these range shifts. Here we compare the thermal limits between populations of the solitary squash bee (Eucera (Peponapis) pruinosa) across distinct latitudes in North America. This species recently expanded its geographic range northward from Mexico and the southwestern United States to the province of Quebec over the past 5,000 years, tracking the human cultivation of its host plants in the genus Cucurbita (squash and pumpkin). We sampled populations in one region within the xeric, ancestral range of E. pruinosa, and populations in two temperate regions within their recently expanded range. Our results indicate that the critical thermal minimum (CTmin) of E. pruinosa decreases with increasing latitude, tracking colder daily minimum temperatures at higher latitudes. Alternatively, CTmax does not follow a latitudinal trend, though the average CTmax of E. pruinosa in this study (53.1°C ± 3.7°C) provides an ample thermal safety margin compared to the highest temperatures recorded within the sampled range. Adaptation in their lower thermal limit to promote cold tolerance may have enabled the northward range expansion of E. pruinosa following the human cultivation of cucurbits.