Heatwaves rescue a mosquito host from parasitism across a large geographic gradient

The impacts of increasingly frequent and intense heatwaves on parasitism are an important frontier for understanding disease risk under climate change. These impacts are complex because parasitism arises from multiple interacting host and parasite traits that can vary in thermal sensitivity and among populations adapted to different temperature regimes. Here, we used a lab microcosm experiment to investigate the effects of heatwaves occurring during two different phases of a winter-adapted mosquito host – ciliate parasite interaction, for six populations sourced from two geographic regions with differing histories of winter heat. We found that because heatwaves allowed mosquito larvae to evade infection, they reduced parasitism and increased survival regardless of population source climate, and had stronger effects when they occurred earlier. The results suggest that increasingly frequent heatwaves may accelerate geographic shifts in parasitism; these impacts are predictable from thermal performance curves derived at constant temperatures.