Influence of presence and spatial arrangement of belowground insects on host-plant selection of aboveground insects: a field study

1. Several studies have shown that above- and belowground insects can interact by influencing each others growth, development, and survival when they feed on the same host-plant. In natural systems, however, insects can make choices on which plants to oviposit and feed. A field experiment was carried out to determine if root-feeding insects can influence feeding and oviposition preferences and decisions of naturally colonising foliar-feeding insects. 2. Using the wild cruciferous plant Brassica nigra and larvae of the cabbage root fly Delia radicum as the belowground root-feeding insect, naturally colonising populations of foliar-feeding insects were monitored over the course of a summer season. 3. Groups of root-infested and root-uninfested B. nigra plants were placed in a meadow during June, July, and August of 2006 for periods of 3 days. The root-infested and the root-uninfested plants were either dispersed evenly or placed in clusters. Once daily, all leaves of each plant were carefully inspected and insects were removed and collected for identification. 4. The flea beetles Phyllotreta spp. and the aphid Brevicoryne brassicae were significantly more abundant on root-uninfested (control) than on root-infested plants. However, for B. brassicae this was only apparent when the plants were placed in clusters. Host-plant selection by the generalist aphid M. persicae and oviposition preference by the specialist butterfly P. rapae, however, were not significantly influenced by root herbivory. 5. The results of this study show that the presence of root-feeding insects can affect feeding and oviposition preferences of foliar-feeding insects, even under natural conditions where many other interactions occur simultaneously. The results suggest that root-feeding insects play a role in the structuring of aboveground communities of insects, but these effects depend on the insect species as well as on the spatial distribution of the root-feeding insects.