Rhizobacteria regulate colonising <scp> <i>Sitobion avenae</i> </scp> aphid populations through induced host resistance and alter plant volatiles promoting early parasitoid recruitment on barley ( <scp> <i>Hordeum vulgare</i> </scp> )

BACKGROUND: Soil rhizobacteria can enhance crop resistance to insect herbivores and influence higher trophic interactions, offering potential for sustainable pest management. Aphids are sap-feeding pests that alter plant physiology, reduce yields and transmit plant viruses. Previous work has shown that inoculation with specific rhizobacterial strains can reduce aphid fitness in short-term trials. Here, we extend this approach by testing effects across three plant growth stages in glasshouse experiments, monitoring natural colonisation by aphids and their natural enemies in an outdoor pot study, and conducting host-choice and volatile profile assays. RESULTS: Under glasshouse conditions, rhizobacterial inoculation consistently suppressed aphid populations, although the magnitude of effects varied with plant variety, bacterial strain and aphid genotype. In our experiments, suppression was strongest when aphids were confined to individual plants, consistent with enhanced host resistance. In outdoor trials, inoculated plants were colonised earlier by parasitoid wasps and harboured more parasitised aphids, independent of aphid density. Volatile assays confirmed that bacterial inoculation altered plant volatile profiles, suggesting a mechanism for enhanced parasitoid attraction. One aphid genotype showed a preference for uninoculated plants in host-choice assays, indicating potential genotype-specific avoidance of induced defences. CONCLUSION: These findings demonstrate that rhizobacteria can influence both bottom-up and top-down regulation of aphid populations through plant-mediated mechanisms. By simultaneously enhancing host resistance and natural enemy attraction, rhizobacterial inoculants offer a promising biological tool for integrated pest management. Understanding context dependency among plant, microbe and insect genotypes will be crucial for translating these interactions into effective and predictable field applications. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.