The secondary symbiont Arsenophonus improves thermal tolerance of Aphis gossypii (Hemiptera: Aphididae) exposed to extreme heat stress.

Bacterial endosymbionts are ubiquitous in insects and play a critical role in host ecology, including adaptability to thermal extremes. The cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae), is a major agricultural pest in China that harbours diverse microbial symbionts. However, the contribution of Arsenophonus, a key secondary symbiont in A. gossypii, to the host's thermal tolerance remains poorly understood. In this study, we used antibiotics to eliminate Arsenophonus from A. gossypii and evaluated the effects on host thermal tolerance by comparing the life-history traits of an Arsenophonus-infected line (A-infected) and an antibiotic-cured, Arsenophonus-deleted line (A-deleted) across a temperature gradient (26-35 °C). Our results revealed that while host performance declined for both lines as temperatures increased, the magnitude of these fitness costs was significantly modulated by infection status. No significant differences in fitness parameters were observed at 26 °C or 29 °C. Furthermore, at 32 °C, the A-infected line exhibited significantly extended adult longevity compared with the A-deleted line. Under extreme heat stress (35 °C), the infected line surpassed the cured line in both longevity and fecundity. These results confirm the dependency of A. gossypii on Arsenophonus for a conditional fitness advantage at high temperatures, effectively broadening its thermal niche. Ultimately, this symbiont-mediated heat tolerance provides new insight regarding the ecological resilience and population stability of this pest in warming agricultural environments.