Carrying Regiella insecticola Does Not Impose Detectable Life-History Costs on Dominant Chilean Sitobion avenae Superclones Under Simulated Heat-Wave Conditions

The intensification of extreme thermal events increases the risk of pest outbreaks and disruptions in agroecosystems, making it crucial to understand how heat waves affect pest performance. This study examined the impact of thermal stress on two dominant aphid genotypes of Sitobion avenae, focusing on their symbionts Regiella insecticola and Hamiltonella defensa. R. insecticola is associated with neutrality in performance, while H. defensa shows no thermal tolerance. We evaluated symbiont-infected and cured lineages during a heat wave, measuring life-history traits. Key findings include: the H. defensa strain incurred increased costs under heat, reducing survival, fecundity, and growth; the effects of R. insecticola strain were predominantly neutral and specific to clones; and Buchnera aphidicola titers correlated with host performance, with H. defensa lineages having higher titers regardless of heat exposure. This suggests that the R. insecticola strain present in Chilean genotypes does not contribute to the resilience of certain clones to thermal stress in Chile, whereas the H. defensa strain imposes performance costs during heat waves. The results of this research pertain to specific strains of endosymbionts, rather than to all endosymbionts within these taxa. These findings emphasize the need to consider symbiotic relationships and thermal physiology in aphid pest management strategies.