show Abstracthide AbstractHigh temperature events can disrupt species interactions, including those among hosts, symbionts, and natural enemies. Understanding the genetic and physiological processes underlying these disruptions is a critical scientific challenge in this era of anthropogenic climate change. We explore how high temperatures disrupt the interactions among an herbivorous insect host, Manduca sexta, its insect parasitoid, Cotesia congregata, and the parasitoid's symbiotic virus. In this system, high temperatures kill developing parasitoids, but not hosts. We evaluated the physiological and transcriptomic causes of thermal mismatch in ecological interactions using parasitoid egg in vitro experiments, immunological assays, and RNAseq. We found that high temperatures disrupt the capacity of the parasitoid's symbiotic virus to immunosuppress the host insect, resulting in thermal mismatch and death of the parasitoid. At the transcriptomic level, key viral genes involved in suppressing host immune pathways showed reduced expression, driven by the virus's circular genomic structure. This work is among the first to demonstrate the genetic and physiological mechanisms by which a symbiont limits the ecological functioning of host-parasite dynamics, and provides a framework for understanding how molecular processes give rise to ecological outcomes in response to high temperature events caused by climate change. Overall design: This experiment tested the impact of a heat shock at 40C on the interactions between a host insect (Manduca sexta) and a parasitoid's symbiotic virus (Cotesia congregata bracovirus). The experiment consisted of 2 treatment types (parasitism, temperature) each with 2 levels (parasitized or control; 25C or 40C). Third instar M. sexta were either parasitized by C. congregata wasps (parasitized), or injected with sephadex beads as a control (control), then several hours later placed at either the heat shock temperature (40C) or control (25C, same as rearing temperature) for 24 hours. Insects were then returned to 25C for an additional 24 hours to mitigate acute effects of temperature stress. After this recovery period, insects were flash frozen for transcriptomics. The experiment includes a lab and field population of M. sexta to investigate population effects (indicated as LAB or FLD in the sample name). Because the lab and field populations were run at different times, some lab population individuals were run alongside the field individuals as a control (indicated by LABINFIELD in sample name)