Abstract BACKGROUND Climate warming poses a critical challenge to ectotherm survival. While endosymbionts are known to influence host thermal tolerance, the fitness consequences of this symbiosis under sustained warming, particularly for parasitoids used in biological control, remain unclear. Here, we investigated this phenomenon using the key endoparasitoid Tetrastichus planipennisi of the emerald ash borer – a destructive wood‐boring pest across Asia, North America, and Europe. RESULTS The study of four T. planipennisi populations (Jilin, Liaoning, Xinjiang, and Beijing) revealed the Jilin population exhibited better heat tolerance with 100% endosymbiont infestation. At 40 °C, endosymbiont‐negative (E − ) wasps showed 3‐h shorter survival than endosymbiont‐positive (E + ) counterparts, with weaker antioxidant capacity. Critically, elevated temperatures diminished parasitoid reproductive output and severely compromised vertical endosymbiont transmission efficiency. CONCLUSION While endosymbionts provide measurable thermal protection, their heat sensitivity creates an ecological trade‐off that may disadvantage parasitoids under climate warming. These findings advance understanding of endosymbiont‐mediated thermal adaptation in parasitoids and have important implications for optimizing biological control strategies in warming environments. © 2026 Society of Chemical Industry.
Dang et al. (Tue,) studied this question.