Water stress, root herbivory and above-ground multitrophic interactions in a crucifer-aphid system

Plants, insect herbivores and their natural enemies interact in multitrophic food webs that can be influenced by various stress factors. The aim of the present study was to understand the effects of two different below-ground stress factors (drought and root herbivory) on the performance of two above-ground herbivores, a generalist (Myzus persicae) and a specialist (Brevicoryne brassicae) aphid species and two of their natural enemies, a generalist (Aphidius colemani) and a specialist (Diaeretiella rapae) parasitoid species. Preliminary experiments with various Brussels sprout (Brassica oleracea) cultivars (cv) showed that aphid performance was influenced both by host plant on which the parent aphid had been reared and by the host plant on which it was feeding when reproducing. The Brussels sprout cv. Oliver was selected as a consistently good aphid host for subsequent experiments. Root herbivory and drought stress were shown to significantly affect the outcome of plant-mediated interactions between insect herbivores and their parasitoids. High drought stress had a negative impact on the aphid species and their parasitoids and also reduced performance of a root herbivore, the cabbage root fly, Delia radicum. Root herbivory alone or in combination with drought significantly altered foliar herbivore performance. High drought stress combined with high D. radicum density had a negative impact on foliar herbivore performance and parasitoid preference and performance. Plant volatiles and parasitoid preference were shown to decrease with drought stress and root herbivory. Plant foliar glucosinolate, nitrogen and carbon concentrations were significantly affected by drought stress. Relative water content and plant biomass were significantly affected by both stresses. The present study on a Brassica-aphid system underlines the importance of root herbivory and drought stress in mediating interactions between plant, foliar herbivores and their parasitoids. The implications of these findings for multitrophic interactions under potential climate change are discussed with particular reference to pest management