Facultative bacterial symbionts in aphids confer resistance to parasitic wasps Aphids often host symbiotic bacteria that can influence their survival and reproduction. This study investigates how these bacteria affect aphids' vulnerability to parasitoid wasps. The pea aphid (Acyrthosiphon pisum) can host various bacteria, including the primary symbiont Buchnera aphidicola and other secondary symbionts (SS). These SS are transmitted vertically from mother to daughter, but some can also be transferred horizontally. The study found that aphids infected with certain SS, such as the R and T types, were more resistant to parasitoid attack by Aphidius ervi. Infected aphids were as likely to be attacked by wasps but less likely to support wasp development. This resistance was due to high mortality of developing wasp larvae in infected aphids. The research used microinjection to create genetically uniform aphid lineages with different SS infections. These lineages were tested against the parasitoid wasp. The results showed that R and T SS-infected aphids had significantly lower mummy formation compared to uninfected controls, indicating reduced susceptibility to parasitism. The study also found that the presence of SS did not affect the wasp's tendency to oviposit, but did affect the development of wasp larvae. Infected aphids had more moribund or dead larvae, suggesting that SS may interfere with the wasp's development. The findings suggest that symbiotic bacteria can play a significant role in mediating interactions between aphids and their natural enemies. This interaction may contribute to the persistence and spread of these bacteria in aphid populations. The study highlights the importance of considering symbiotic relationships in the context of ecological interactions and the dynamics of host populations.Facultative bacterial symbionts in aphids confer resistance to parasitic wasps Aphids often host symbiotic bacteria that can influence their survival and reproduction. This study investigates how these bacteria affect aphids' vulnerability to parasitoid wasps. The pea aphid (Acyrthosiphon pisum) can host various bacteria, including the primary symbiont Buchnera aphidicola and other secondary symbionts (SS). These SS are transmitted vertically from mother to daughter, but some can also be transferred horizontally. The study found that aphids infected with certain SS, such as the R and T types, were more resistant to parasitoid attack by Aphidius ervi. Infected aphids were as likely to be attacked by wasps but less likely to support wasp development. This resistance was due to high mortality of developing wasp larvae in infected aphids. The research used microinjection to create genetically uniform aphid lineages with different SS infections. These lineages were tested against the parasitoid wasp. The results showed that R and T SS-infected aphids had significantly lower mummy formation compared to uninfected controls, indicating reduced susceptibility to parasitism. The study also found that the presence of SS did not affect the wasp's tendency to oviposit, but did affect the development of wasp larvae. Infected aphids had more moribund or dead larvae, suggesting that SS may interfere with the wasp's development. The findings suggest that symbiotic bacteria can play a significant role in mediating interactions between aphids and their natural enemies. This interaction may contribute to the persistence and spread of these bacteria in aphid populations. The study highlights the importance of considering symbiotic relationships in the context of ecological interactions and the dynamics of host populations.