Using the stable isotope marker 44Ca to study dispersal and host-foraging activity in parasitoids.
Dispersal and host-foraging activity are important behavioural characteristics for parasitoid species used as biological control agents. Appropriate marking techniques are often needed for field investigations into these behaviours because of the small size of parasitoids. This study developed a novel technique using the rare stable calcium isotope 44Ca to mark parasitoids. As demonstrated with a tri-trophic model system consisting of cabbage plant Brassica oleracea, caterpillar Pieris brassicae and gregarious parasitoid Cotesia glomerata, marking parasitoids with the isotope 44Ca is systemic and non-disruptive. Significant uptake and translocation of 44Ca occurred in cabbage plants after being drenched with 50 mMol aqueous isotope solution, and the isotope was transferred to wasps emerging from the parasitized host caterpillars that fed on the isotope-enriched plant. The isotope enrichment did not adversely influence the survivorship of hosts or the development of parasitoids. The 44Ca-enriched female wasps could pass the isotope marker further via oviposition into the parasitized host caterpillars, hence allowing the marker to be transferred across parasitoid generations. Greenhouse release experiments validated the transferability of 44Ca from the enriched wasps to hosts through parasitism. The 95% upper confidence limit for the mean 44Ca/40Ca isotope ratio in the hosts without 44Ca enrichment (control) could be used as a statistical marking criterion to identify reliably the individual parasitized hosts within the first 3 days of parasitism. Synthesis and applications. We have, for the first time, documented the efficient transfer of a stable isotope marker from enriched parasitoids to host insects through parasitism. This isotope technique enables the study of dispersal and host-foraging activity of parasitoids in the field without having to recapture marked individual parasitoids. Information on the dispersal pattern and host-foraging activity of the parasitoid species concerned is obtained from the spatial distribution of the parasitized marked host insects. Furthermore, this isotope technique can be used to quantify the efficacy of parasitism by the parasitoids released in the field.