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It is generally thought that soil animals face specific foraging conditions because movement through soil is highly energy consuming. The hypothesis tested here is that root-feeding click beetle larvae (wireworms, Agriotes spp.; Coleoptera: Elateridae), to minimize energy loss, only migrate horizontally through soil when located in food-depleted surroundings. Larvae were placed at either end of a root density gradient created by the grass Holcus lanatus L. (Poaceae), and their position in the gradient was recorded after 12 days of migration. Larval migration depended on the food situation at the starting point, with larvae moving from food-depleted to food-rich areas, but not leaving food-rich areas. Larvae spread further around food-rich areas when together with conspecifics than when being alone, presumably to avoid cannibalism. Food density-dependent migration may have to be taken into account when using trap-crops to control Agriotes larvae. Success may depend on the timing of trap-crop establishment relative to the target crop to generate an effective food gradient.

Soil organisms affect plant growth and chemistry and have subsequent effects on aboveground herbivore performance. However, whether herbivores discriminate between plants exposed to different soil organisms when colonizing their host plants is largely unexplored. In a greenhouse study, Tanacetum vulgare L. (Asteraceae) growing in a ruderal plant community in the presence and absence of arbuscular mycorrhizal fungi (AMF) and earthworms [Aporrectodea spp. (Haplotaxida: Lumbricidae)] was colonized by aphids [Myzus persicae Sulzer (Hemiptera: Aphididae)]. The aphids preferred to colonize plants without earthworms in the soil, and the numbers of aphids remained lower on the plants with earthworms, irrespective of the presence of AMF. Although the N, C, and P concentrations of the shoots were not affected by the soil organisms, AMF increased total aboveground biomass, total N, C, and P content, and photosynthetic activity (measured as electron transport rate) in the leaves under high light intensity. These results suggest that earthworms affect chemical cues that are used by aphids to judge host quality prior to feeding. Discrimination between plants with and without exposure to earthworms by aboveground herbivores is a novel aspect of plant-mediated interactions between below- and aboveground organisms.