The interactive effects of heat stress, parasitism and host plant quality in a host–parasitoid system

Species interactions are expected to change in myriad ways as the frequency and magnitude of extreme temperature events increase with anthropogenic climate change. The relationships between endosymbionts, parasites and their hosts are particularly sensitive to thermal stress, which can have cascading effects on other trophic levels. We investigate the interactive effects of heat stress and parasitism on a terrestrial tritrophic system consisting of two host plants (one common, high‐quality plant and one novel, low‐quality plant), a caterpillar herbivore and a specialist parasitoid wasp. We used a fully factorial experiment to determine the bottom‐up effects of the novel host plant on both the caterpillars' life history traits and the wasps' survival, and the top‐down effects of parasitism and heat shock on caterpillar developmental outcomes and herbivory levels. Host plant identity interacted with thermal stress to affect wasp success, with wasps performing better on the low‐quality host plant under constant temperatures but worse under heat‐shock conditions. Surprisingly, caterpillars consumed less leaf material from the low‐quality host plant to reach the same final mass across developmental outcomes. In parasitized caterpillars, heat shock reduced parasitoid survival and increased both caterpillar final mass and development time on both host plants. These findings highlight the importance of studying community‐level responses to climate change from a holistic and integrative perspective and provide insight into potential substantial interactions between thermal stress and diet quality in plant–insect systems.