Infection Outcomes are Robust to Thermal Variability in a Bumble Bee Host-Parasite System

Climate change-related increases in thermal variability and rapid temperature shifts will affect organisms in multiple ways, including imposing physiological stress. Furthermore, the effects of temperature may alter the outcome of biotic interactions, such as those with pathogens and parasites. In the context of host-parasite interactions, the beneficial acclimation hypothesis posits that shifts away from acclimation or optimum performance temperatures will impose physiological stress on hosts and will affect their ability to resist parasite infection. We investigated the beneficial acclimation hypothesis in a bumble bee-trypanosome parasite system. Freshly emerged adult worker bumble bees, Bombus impatiens, were acclimated to 21, 26, or 29°C. They were subsequently experimentally exposed to the parasite, Crithidia bombi, and placed in a performance temperature that was the same as the acclimation temperature (constant) or one of the other temperatures (mismatched). Prevalence of parasite transmission was checked 4 and 6 days post-parasite exposure, and infection intensity in the gut was quantified at 8 days post-exposure. Parasite strain, host colony, and host size had significant effects on transmission prevalence and infection load. However, neither transmission nor infection intensity were significantly different between constant and mismatched thermal regimes. Furthermore, acclimation temperature, performance temperature, and the interaction of acclimation and performance temperatures had no significant effects on infection outcomes. These results, counter to predictions of the beneficial acclimation hypothesis, suggest that infection outcomes in this host-parasite system are robust to thermal variation within typically experienced ranges. This could be a consequence of adaptation to commonly experienced natural thermal regimes or a result of individual and colony level heterothermy in bumble bees. However, thermal variability may still have a detrimental effect on more sensitive stages or species, or when extreme climatic events push temperatures outside of the normally experienced range.