Different approaches to labelling parasitoids using strontium

Labelling parasitoids with trace elements is a potentially powerful techniq ue for studying dispersal and trophic interactions in these usually small i nsects. Laboratory experiments were conducted to investigate the feasibilit y and efficiency of different methods for trace element labelling of the hy menopteran 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. Laborato ry-reared wasps had a mean background level of 0.43 +/- 0.26 (SD) mug g(-1) for strontium (Sr) and 0.51 +/- 0.25 (SD) mug g(-1) for rubidium (Rb), whi ch 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 labelle d with Sr by: (1) feeding adults on sucrose solution spiked with Sr; (2) re aring larvae from Pieris brassicae fed the cabbage plant (Brassica oleracea ) soil-drenched with aqueous Sr; or (3) feeding adults on extrafloral necta r from a plant (Vicia faba) soil-drenched with aqueous Sr. Although Sr cont ent in labelled wasps varied with the concentration and the method applied, it did not decline significantly with age. Labelled wasps could be unequiv ocally distinguished from unlabelled counterparts even 16 days after they w ere denied access to the Sr-enriched food sources. Labelling with Sr did no t seem to have any negative effect on the parasitoid fitness. Thus, Sr is a n ideal internal marker to label C. glomerata and other hymenopteran parasi toids for investigations of bi- and tri-trophic interactions.