Flexible trophic position of polyphagous wireworms (Coleoptera, Elateridae): A stable isotope study in the steppe belt of Russia

Abstract

Larvae of Elateridae beetles, known as wireworms, are widespread pest species damaging various crop plants. In spite of a general morphological similarity, wireworms demonstrate a wide spectrum of trophic modes ranging from phytophagy to predation. In many wireworm species, facultative carnivory ensures the acquisition of proteins necessary for metamorphosis, whereas feeding on living plant tissues (especially juicy roots and tubers) can be stimulated by water deficiency. We therefore hypothesized that polyphagous wireworm species inhabiting three types of steppe (grassland) ecosystems should be more phytophagous in dry conditions. To assess the trophic position of wireworms, we compared their isotopic signatures with those of plants and the soil, as well as of reference species including carnivorous centipedes and Diptera larvae, herbivorous weevils and saprophagous earthworms. Larvae of Agriotes obscurus, Agriotes lineatus, Selatosomus aeneus and Selatosomus latus inhabiting well-drained soils were 3–4‰ enriched in 15N compared to larvae inhabiting wet floodplains, suggesting a difference in at least one trophic level. A comparison with soil animals with known trophic positions indicates that omnivorous wireworms tend to be phytophagous and saprophagous in floodplain, but carnivorous in well-drained habitats. The capability of changing the diet is confirmed by the age-related shift in the trophic position of some species. Elder A. obscurus larvae are significantly depleted in 13C and enriched in 15N, likely indicating a switch from saprophagy to carnivory. Overall, our data suggest a considerable flexibility in the feeding behavior of elaterid larvae in steppe habitats.