Intra-and interspecific variation in plant and insect traits can alter the strength and direction of insect-plant interactions,with outcomes modified by soil biotic and abiotic conditions.We used the potato aphid (Macrosiphum euphorbiae Thomas) feeding on cultivated Solanum tuberosum and wild Solanum berthaulti to study the impact of water availability and plant mutualistic arbuscular mycorrhizal (AM) fungi on aphid performance and susceptibility to a parasitoid wasp (Aphidius ervi Haliday).Plants were grown under glass with live or sterile AM fungal spores and supplied with sufficient or reduced water supply.Plants were infested with 1 of 3 genotypes ofM.euphorbiae or maintained as aphidfree controls;aphid abundance was scored after 1 week,after which aphid susceptibility to A.ervi was assayed ex planta.Solanum tuberosum accumulated c.20％ more dry mass than S.berthaultii,and root mass of S.berthaultii was smallest under reduced water supply in the presence of AM fungi.Aphid abundance was lowest on S.berthaultii and highest for genotype "2" aphids;genotype "1" aphid density was particularly reduced on S.berthaultii.Aphid genotype "1" exhibited low susceptibility to parasitism and was attacked less frequently than the other two more susceptible aphid genotypes.Neither AM fungi nor water availability affected insect performance.Our study suggests a fitness trade-off in M.euphorbiae between parasitism resistance and aphid performance on poor quality Solanum hosts that warrants further exploration,and indicates the importance of accounting for genotype identity in determining the outcome of multitrophic interactions.