Cold Tolerance Differences Between Declining Native and Invasive Bumblebees in Patagonia

Temperature is a key environmental factor influencing the establishment and spread of insect species. Differences in thermal tolerance may thus explain patterns of geographical distribution and dominance during biological invasions. The native Patagonian bumblebee, Bombus dahlbomii, is being displaced by the exotic Bombus terrestris, which has expanded throughout its range. To investigate the role of cold tolerance in this context, we first examined the minimum temperatures likely to be encountered by both species across their South American distribution. Then, we assessed whether these minimum temperatures correlated with their cold tolerance, measured as chill coma onset and recovery temperatures. We found that, despite experiencing similar minimum temperatures in their ranges, B. dahlbomii is more cold tolerant than B. terrestris: the native bumblebee reaches lower ambient and body temperatures before entering chill coma and recovers at lower body temperatures. Exploring possible mechanisms of cold tolerance, we found that the larger body size and longer body hairs of B. dahlbomii compared to B. terrestris may represent traits evolved to prevent heat loss. However, after evaluating the daily foraging window modeled on the species' cold tolerance in northwestern Patagonia, we found it does not appear to confer a substantial temporal advantage in this region. Therefore, the dominance of B. terrestris in Patagonia cannot be explained by cold tolerance. In fact, native bumblebees exhibit greater physiological and morphological adequation to cold. The success of B. terrestris is likely driven by other factors, including heat tolerance, disease resistance, or differences in life history traits such as colony size.