By considering the magnitude of isotope enrichment associated with trophic transfers in biologically important elements such as carbon (13C/12C) and nitrogen (15N/14N), it is possible to infer trophic interactions in systems where direct observations are logistically difficult. Several recent reviews have estimated that consumers become enriched in the heavy nitrogen isotope on the order of 2.3 to 3.4‰ with each trophic transfer. Furthermore, these same reviews have estimated that consumers become enriched in the heavy carbon isotope between 0.4 and 0.5‰ per trophic transfer. Although these estimates have been used to infer trophic interactions in a variety of taxa, the applicability of these estimates for studies of arthropod community ecology is poorly understood. Specifically for insect parasitoid communities, estimates of nitrogen and carbon isotope enrichment from a comprehensive study have yet to be published. Here, we present the results of nitrogen and carbon stable isotope analyses for a suite of hymenopteran parasitoids that attack the gall-making midge, Rhopalomyia californica Felt (Diptera: Cecidomyiidae), on coyote bush, Baccharis pilularis (Compositae), in northern California. Mean carbon enrichment for all parasitoids developing on R. californica was considerably higher than expected, based on recent reviews. In fact, discrimination among trophic levels was possible, based on carbon enrichment values alone. Mean nitrogen isotope enrichment was slightly lower than values reported in recent reviews. However, the variation associated with our estimate of nitrogen enrichment falls well within the range of values reported in the reviews. Mechanisms behind the greater than expected enrichment in carbon are currently unknown and will require further investigation.
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Vol. 98 • No. 2