The study investigates the role of facultative bacterial symbionts in aphids, specifically *Acrystosiphon pismum* (the pea aphid), in conferring resistance to the hymenopteran parasitoid *Aphidius ervi*. The researchers used genetically uniform aphid lineages that differed only in their infection status with three types of secondary symbionts (R-, T-, and U-type SS). They found that infection with the R- and T-type SS significantly reduced the success of parasitoid attacks, leading to a 22.5% and 41.5% reduction in mummy formation, respectively, compared to uninfected controls. The resistance was not due to differences in parasitoid acceptance but rather to high mortality of developing parasitoid larvae in infected aphids. This study highlights the potential for symbiotic bacteria to mediate interactions between hosts and natural enemies, providing a mechanism for the persistence and spread of these symbionts. The findings suggest that the prevalence of certain symbionts in aphid populations may be influenced by the presence of specific natural enemies.The study investigates the role of facultative bacterial symbionts in aphids, specifically *Acrystosiphon pismum* (the pea aphid), in conferring resistance to the hymenopteran parasitoid *Aphidius ervi*. The researchers used genetically uniform aphid lineages that differed only in their infection status with three types of secondary symbionts (R-, T-, and U-type SS). They found that infection with the R- and T-type SS significantly reduced the success of parasitoid attacks, leading to a 22.5% and 41.5% reduction in mummy formation, respectively, compared to uninfected controls. The resistance was not due to differences in parasitoid acceptance but rather to high mortality of developing parasitoid larvae in infected aphids. This study highlights the potential for symbiotic bacteria to mediate interactions between hosts and natural enemies, providing a mechanism for the persistence and spread of these symbionts. The findings suggest that the prevalence of certain symbionts in aphid populations may be influenced by the presence of specific natural enemies.