noneEnter It is well documented that plants can act as vertical communication channels between below- and aboveground plant-feeding insects: root-feeders induce changes in biomass and chemistry of the plant's shoot, and these changes can influence survival, growth and development of leaf-feeders. The main aim of my project was to explore whether and how this interaction between spatially separated insects is restricted to plant feeders (root- and leaf-feeding insects), or can be extended to higher trophic levels, such as insect parasitoids. My model system consisted of the wild cruciferous plant Brassica nigra (first trophic level), belowground the specialist root herbivore Delia radicum (second level) and its parasitoid Trybliographa rapae (third level), and aboveground the specialist leaf-feeder Pieris brassicae (second level) and its parasitoid Cotesia glomerata (third level) plus the hyperparasitoid Lysibia nana (fourth level). The first experiments demonstrated that root- and leaf-feeders negatively affect each other's performances by increasing the levels of toxins in the host plant, and these negative effects were not restricted to the herbivore level, but transmitted to the parasitoid and even the hyperparasitoid. It was remarkable that the parasitoid was the most affected trophic level. I found out that the parasitoid female avoids to parasitise hosts on root-infested plants when hosts feeding on root-uninfested plants were readily available, probably based on changes in the volatile blend of the plant, triggered by the root-feeders. it even appeared that root-feeders can influence the foraging efficiency of parasitoids, even when the leaf-feeders (i.e., the parasitoid's host) and the root-feeders are not feeding on the same plant, but on conspecific plants within the same habitat. Finally, in a field experiment the two most abundant leaf-feeding insects preferred to feed and oviposit on root-uninfested plants. Hence, behavioural decisions in naturally occurring populations of leaf-feeding insects can be influenced by root-feeders under field conditions, where a multitude of interactions take place simultaneously. interaction effects between above- and belowground organisms clearly are not limited to insects directly associated with the plant, but such interactions operate in a complex web of communication among various trophic levels BR Nutrition. Diet. Parasites diseases and disorders. Parasites. Insect parasites. Hosts. Insect hosts. Delia radicum (Anthomyiidae): Host; Trybliographa rapae (Cynipidae): Parasite; Cotesia glomerata (Braconidae): Host, Parasite; Lysibia nana (Ichneumonidae): Parasite; Pieris brassicae (Pieridae): Host. Cotesia glomerata: Hymenopteran parasites, Lysibia nana, Hyperparasitism, plant-mediated multitrophic interactions, Lepidopteran hosts, Pieris brassicae; Delia radicum: Food plants, Brassica nigra, Herbivory, plant-mediated multitrophic interactions, Hymenopteran parasites, Trybliographa rapae, Food plant-mediated multitrophic interactions; Lysibia nana: Hymenopteran hosts, Cotesia glomerata, Hyperparasitism, plant-mediated multitrophic interactions; Pieris brassicae: Food plants, Brassica nigra, Herbivory, plant-mediated multitrophic interactions, Hymenopteran parasites, Cotesia glomerata, Food plant-mediated multitrophic interactions; Trybliographa rapae: Dipteran hosts, Delia radicum, Plant-mediated multitrophic interactions. ST Animalia, Arthropoda, Insecta, Diptera, Brachycera, Cyclorrhapha, Anthomyiidae; Animalia, Arthropoda, Insecta, Hymenoptera, Apocrita, Terebrantia, Cynipoidea, Cynipidae; Animalia, Arthropoda, Insecta, Hymenoptera, Apocrita, Terebrantia, Ichneumonoidea, Braconidae; Animalia, Arthropoda, Insecta, Hymenoptera, Apocrita, Terebrantia, Ichneumonoidea, Ichneumonidae; Animalia, Arthropoda, Insecta, Lepidoptera, Glossata, Heteroneura, Papilionoidea, Pieridae. Arthropods; Hymenopterans; Insects; Invertebrates; Lepidopterans; True Flies.