Microbial symbiont performance under thermal stress does not predict vector competence

Abstract Insect–microbe symbioses are critical to host development, immunity and ecological interactions, including pathogen vectoring. The squash bug Anasa tristis is an agricultural pest and vector of Serratia ureilytica, the causal agent of cucurbit yellow vine disease. This insect hosts an environmentally acquired Caballeronia symbiont that aids development and suppresses pathogen load. While different Caballeronia strains appear functionally equivalent under benign conditions, they differ in thermal tolerance. It is unknown whether host thermal stress influences symbiont-mediated suppression of S. ureilytica. We evaluated whether elevated temperature affects the ability of different symbiont strains to influence pathogen titre and host performance. We measured development, adult weight, survival and S. ureilytica counts in A. tristis reared with different symbiont strains from 24°C to 36°C. Thermal stress affected host performance in a strain-dependent manner, with some symbionts conferring greater survival at high temperatures. Surprisingly, pathogen suppression did not vary with symbiont strain or host rearing temperature. These results suggest that while symbiont identity modulates host performance under thermal stress, these effects do not alter pathogen load or transmission potential. Instead, the symbiont may initiate some host immune responses. This work underscores the context-dependent nature of symbiont–pathogen interactions and the importance of disentangling environmental effects on vector competence.