Parasitism is expected to change in a warmer future, but whether or not warming leads to substantial increases in parasitism remains unclear. Understanding how warming effects on parasitism in individual hosts (e.g., parasite load) translates to effects on population-level parasitism (e.g., prevalence, R0) remains a major knowledge gap. We analyzed the temperature dependence of parasitism at both host and population levels in eight empirical mosquito-borne host-parasite systems and found a very strong positive correlation between the thermal optima of individual- and population-level parasitism. By contrast, we did not find a significant correlation in five empirical environmentally-transmitted host-parasite systems. Likewise, parasitism thermal optima were close to host performance thermal optima in mosquito-borne systems but not in environmentally-transmitted systems. We then adapted and simulated simple models for both transmission modes and found a similar pattern to the empirical systems: thermal optima in mosquito-borne systems were more strongly correlated across scales compared to environmentally-transmitted systems. Generally, our results suggest that information on the temperature-dependence, and specifically the thermal optimum, at either the individual- or population-level should provide a useful--though not quantitatively exact--baseline for predicting temperature dependence at the other level in mosquito-borne parasite systems. By contrast, environmentally-transmitted parasitism may operate by a different set of rules, suggesting the need for trait-based studies of temperature-dependence at individual and population levels in these systems.
