Vol. 9, Issue 11, Part A (2025)

Aphids (Hemiptera: Aphididae) are a globally important group of herbivores whose ecology and evolutionary trajectories are strongly shaped by intimate associations with bacterial endosymbionts. Their success as pests is not solely due to their high reproductive potential and phenotypic plasticity but is also intricately tied to associations with bacterial endosymbionts. The obligatory symbiont Buchnera aphidicola, which is maintained by all aphids, is vital to host nutrition and thermal physiology because it supplies crucial amino acids that are absent in phloem sap. Furthermore, a large number of species are home to facultative (secondary) symbionts, including Serratia symbiotica, Regiella insecticola, and Hamiltonella defensa. These facultative partners can confer conditional benefits, including enhanced tolerance to heat stress, protection against parasitoid wasps and microbial pathogens, and shifts in host plant utilization. In a rapidly changing environment — driven by warming, altered seasonality, and anthropogenic landscape change — symbiont composition, density, and genotype can alter aphid fitness and population dynamics, sometimes in context-dependent and genotype-specific ways. This review synthesizes current knowledge on how aphid–symbiont partnerships mediate adaptation to abiotic (temperature) and biotic (parasitoids, predators, host plants) stressors, discusses mechanistic bases (metabolic provisioning, toxin production, immune modulation), highlights ecological and evolutionary consequences, and outlines key gaps and future research directions, including implications for pest management.