In interactions between plants, insect herbivores and natural enemies, resistant plant varieties and soil abiotic/biotic factors can affect natural enemies through changes in the plant or host insect. Interactions can be negative, neutral or positive in relation to herbivore populations. The majority of studies have used above-ground plant–insect–parasitoid systems and relatively few studies have included below-ground effects on above-ground tritrophic interactions. The aim of the present project was to understand how soil-based stress factors (nutrient availability and root pathogens) influence multitrophic interactions in a potato-aphid (Myzus persicae)-parasitoid (Aphidius colemani) system. Specific objectives were: to assess aphid performance on different potato cultivars under laboratory and field conditions; use aphid resistant and susceptible cultivars to assess the effects of soil nutrition and a plant pathogen (root-knot nematode) on aphid performance; conduct multifactorial ditrophic (aphid performance) and tritrophic (parasitism success) experiments with selected combinations of soil treatments; use olfactometry to determine whether plant volatiles are involved where significant tritrophic interactions occur between treatments. Potato cv. Anya, a cv derived from Desiree and Pink Fir Apple, was found to be consistently resistant to M. persicae compared with the three other cultivars tested (Desiree, Santé and Pink Fir Apple) in both laboratory experiments (UK) and field trials (Pakistan and UK). For Anya (resistant) and Desiree and Pink Fir Apple (susceptible), high levels of soil nutrients enhanced aphid performance, their parasitism by A. colemani, and adult parasitoid emergence. Root-knot nematodes had a negative effect on aphid performance at both low and high soil nutrition but had a positive effect on parasitism at low soil nutrition. Olfactometry showed Desiree to be more attractive to M. persicae compared with Anya. The work is discussed in relation to the development of pest management strategies for the control of M. persicae on potato.