Tracing spatial distribution of parasitism in fields with flowering plant strips using stable isotope marking

Flowering plant strips providing nectar food sources at field edges might influence habitat exploitation by parasitoids, and are therefore of great interest for biological control. The present field study examines the host-foraging movement of the parasitoid, Cotesia glomerata (L.) (Hymenoptera: Braconidae), and the resulting distribution pattern of parasitism in Pieris brassicae (L.) (Lepidoptera: Pieridae) larvae in cabbage fields in the presence of floral nectar plants. Parasitoids were released in the center of 35 × 35 m field patches with flowering dill plants, Anethum graveolens L. (Apiaceae), arranged at two diagonally opposing corners, and non-flowering dill plants at the other two corners. The movement of released parasitoids marked with the stable isotope 44Ca was assessed based on parasitized host larvae on trap cabbage plants. This isotope marker is transferred from the parasitoid to host caterpillars through oviposition, and tracking of the marked wasps in the field was feasible in the presence of a native population by analyzing the sedentary host caterpillars. Data from two field trials with two trial areas each consistently showed that the parasitoid females dispersed over the entire cabbage field patches within 3 days and that the caterpillars parasitized by the released wasps were randomly distributed in space. Parasitism rates in sectors with and without flowering dill plants at the edges were not significantly different. In conclusion, C. glomerata females are able to cover the cabbage field patches of 1225 m2 within 3 days, foraging for hosts, while food sources at field edges remain within their reach.