Belowground communities can affect interactions between plants and aboveground insect communities. Such belowground–aboveground interactions are known to depend on the composition of belowground communities, as well as on the plant species that mediates these interactions. However, it is largely unknown whether the effect of belowground communities on aboveground plant–insect interactions also depends on genotypic variation within the plant species that mediates the interaction. To assess whether the outcome of belowground–aboveground interactions can be affected by plant genotype, we selected two white cabbage cultivars [Brassica oleracea L. var. capitata (Brassicaceae)]. From previous studies, it is known that these cultivars differ in their chemistry and belowground and aboveground multitrophic interactions. Belowground, we inoculated soils of the cultivars with either nematodes or microorganisms and included a sterilized soil as a control treatment. Aboveground, we quantified aphid [Brevicoryne brassicae (L.) (Hemiptera: Aphididae)] population development and parasitoid [Diaeretiella rapae (McIntosh) (Hymenoptera: Braconidae)] fitness parameters. The cultivar that sustained highest aphid numbers also had the best parasitoid performance. Soil treatment affected aphid population sizes: microorganisms increased aphid population growth. Soil treatments did not affect parasitoid performance. Cultivars differed in their amino acid concentration, leaf relative growth rate, and root, shoot, and phloem glucosinolate composition but showed similar responses of these traits to soil treatments. Consistent with this observation, no interactions were found between cultivar and soil treatment for aphid population growth or parasitoid performance. Overall, the aboveground community was more affected by cultivar, which was associated with glucosinolate profiles, than by soil community.