The feeding ecology of elaterid larvae in central European arable land: New perspectives based on naturally occurring stable isotopes

To understand soil food webs, empirically generated data on the trophic connections and the feeding ecology of the major below-ground animal taxa are needed. Here we used stable isotope analysis to assess the trophic ecology of wireworms, the larvae of click beetles, in Central European arable land. Wireworms are amongst the major soil macroinvertebrates and are of practical importance in arable soils. Besides feeding on crops, they are thought to feed on weeds, soil organic matter (SOM), and even animal prey, but their feeding ecology is poorly studied under natural conditions. Elaterid larvae and their putative feeding substrates—plant roots, SOM, and litter—were sampled at 17 locations in Austria, Germany, and Italy and their isotope ratios of carbon (12C/13C) and nitrogen (14N/15N) measured to determine the wireworms’ trophic level, the importance of SOM and weeds within the diet of Agriotes larvae, as well as the individual diet variation in Agriotes obscurus larvae. δ15N signatures suggested that Agriotes larvae are predominately herbivorous, whereas the other wireworm species primarily fed on animal prey. In contrast to SOM, weeds were readily eaten by Agriotes larvae: their dietary contribution ranged between 28% and 67% in weedy maize fields. Most A. obscurus larvae fed on a mixed diet of weeds and maize, although ∼15% of the larvae fed primarily on one of the two food sources only. δ15N signatures indicated that ∼10% of the “herbivorous” A. obscurus larvae fed primarily on animal prey, revealing high intraspecific trophic plasticity in these soil insects. Wireworm feeding behaviour is apparently complex at the individual level: the population consists of types A and B generalists, a phenomenon which needs further assessment.