Symbiont‐mediated insect host defense against parasitism: insights from the endosymbiont, Hamiltonella defensa and the insect host, Myzus persicae

Abstract BACKGROUND Sap‐feeding insects like aphids can harbor a complex of bacterial symbionts, including a primary nutritional symbiont and secondary symbionts that may influence various traits such as resistance to parasitoids and entomopathogens as well as fitness. This study explores the presence and impact of the facultative symbiont, Hamiltonella defensa , in a major pest aphid, the green peach aphid Myzus persicae , focusing particularly on its role in aphid parasitoid resistance, an area that has not been previously characterized. RESULTS We detected Buchnera aphidicola and H. defensa endosymbionts in a population of M. persicae collected from Tehran, Iran. Using antibiotic treatments, we mostly removed H. defensa from the aphid and generated a line with only a low level of the symbiont. The parasitism rate of Aphidius matricariae significantly increased in this line compared to untreated controls. Quantitative polymerase chain reaction (qPCR) analysis indicated that the densities of B. aphidicola and H. defensa were affected following parasitism. Twenty‐four hours after parasitism, the density of H. defensa and its phage (APSE, Acyrthosiphon pisum secondary endosymbiont) increased compared to the controls, while the density of B. aphidicola decreased. Reverse transcription PCR (RT‐qPCR) of APSE encoding toxins revealed high transcription levels of the YDp toxin at 24 h post‐parasitism. CONCLUSION These findings indicate that the APSE‐3 bacteriophage is present in H. defensa from M. persicae and likely confers parasitoid resistance in this aphid through the YDp toxin. Overall, these results suggest that Hamiltonella can partly protect M. persicae against parasitism. The results have implications for biological control programs targeting this major insect pest. © 2025 Society of Chemical Industry.