Microbial symbiont performance under thermal stress does not predict vector competence

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 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 titer and host performance. We measured development,
adult weight, survival, and S. ureilytica counts in A. tristis reared with
different symbiont strains from 24–36ºC. 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.