Aphid and endosymbiont sensitivity to heatwaves is misaligned across life stages.

Heritable endosymbionts will shape how many insect pests respond to climate warming. Aphids are globally important pests that almost universally depend on the obligate symbiont Buchnera aphidicola, which can constrain host thermal tolerance, while additional secondary symbionts may further modify heat responses. Importantly, aphid-symbiont interactions are developmentally dynamic, with physiological functions and phenotypic effects changing as aphids develop. However, it remains unclear whether aphids and their endosymbionts are most vulnerable to heat stress at the same developmental stages, or whether mismatches in thermal sensitivity shape fitness outcomes. We addressed this gap by testing how heatwaves typical of southern Australia affect the invasive cereal pest, the Russian wheat aphid (Diuraphis noxia), and two endosymbionts (Buchnera aphidicola and Rickettsiella viridis) across developmental stages. Using a factorial design (endosymbiont status × life stage × temperature), we exposed early nymphs or adults to either moderate or extreme heatwave conditions-representative of their invaded range-and quantified survival, post-exposure longevity, fecundity, and symbiont density in treated individuals and their offspring. Heatwaves had a disproportionately strong effect on the nymph stage compared with adults, primarily through reduced fecundity. In contrast, endosymbiont responses were strongest following adult exposure, including Buchnera aphidicola suppression, in adults and/or their offspring. These results reveal a life-stage mismatch in thermal vulnerability between hosts and symbionts, with implications for physiological responses to climate extremes.