Molecular gut content and stable isotope analyses reveal the dietary choice of soil-living insect herbivores

Although a significant proportion of plant tissue is located in roots and other below-ground parts of plants, little is known on the dietary choices of root-feeding insects. Assessing trophic interactions under natural conditions, however, is not a simple task. This is especially true for soil food webs where the opaque habitat hampers the direct identification of feeding relationships. Here, we examine how soil insect herbivores trophically exploit below-ground plant diversity. We chose Agriotes wireworms, the larvae of click beetles, as they are generalist plant feeders, abundant in arable soils and a serious pest worldwide. They feed in ecosystems with many co-occurring plant species that are not separated spatially. We employed a combined approach of molecular gut content and stable isotope analyses to identify the larval diet. We present a field experiment where we examined wireworm feeding choices. Maize (Zea mays, C4-plant) was grown in three different treatments: (i) in monoculture, (ii) in combination with wheat (Triticum aestivum; as a companion C3-plant) and (iii) together with a mixture of another five companion plants. First results support our hypothesis that, wireworms preferentially feed on companion plants with increasing plant diversity, thus leading to reduced crop damage and increased yield in the plant mixture treatment. The observed effect of root diversity on soil herbivore feeding choices also will have important implications for soil pest control strategies.