Tree genotype modulates the effects of water deficit on a plant-manipulating aphid

•We study how poplar genotype mediates water deficit effects on a gall-inducing aphid.•Three irrigation levels were applied to a susceptible and a resistant host genotype.•For both genotypes, water deficit affected plant growth and gall anatomy.•The effect of water deficit on aphid performance differed according to host genotype.•The medium irrigation level favored aphid development on the resistant host.