Different approaches to labelling parasitoids using strontium

Labelling parasitoids with trace elements is a potentially powerful technique for studying dispersal and trophic interactions in these usually small insects. Laboratory experiments were conducted to investigate the feasibility and efficiency of different methods for trace element labelling of the hymenopteran parasitoid Cotesia glomerata. We concentrated on Sr as a marker and in some relevant aspects also compared its labelling efficiency to that of Rb, which is the trace element commonly used to label insects. Laboratory-reared wasps had a mean background level of 0.43±0.26 (SD) μg g-1 for strontium (Sr) and 0.51±0.25 (SD) μg g-1 for rubidium (Rb), which was much lower than that for seven other common trace elements (i.e. B, Mg, Al, Ca, Fe, Cu, and Zn). Cotesia glomerata could be effectively labelled with Sr by: (1) feeding adults on sucrose solution spiked with Sr; (2) rearing larvae from Pieris brassicae fed the cabbage plant (Brassica oleracea) soil-drenched with aqueous Sr; or (3) feeding adults on extrafloral nectar from a plant (Vicia faba) soil-drenched with aqueous Sr. Although Sr content in labelled wasps varied with the concentration and the method applied, it did not decline significantly with age. Labelled wasps could be unequivocally distinguished from unlabelled counterparts even 16 days after they were denied access to the Sr-enriched food sources. Labelling with Sr did not seem to have any negative effect on the parasitoid fitness. Thus, Sr is an ideal internal marker to label C. glomerata and other hymenopteran parasitoids for investigations of bi- and tri-trophic interactions