Meeting Abstract

35.4  Jan. 5  Trophic enrichment without change in diet: An ontogenetic shift in &delta¹⁵N during insect metamorphosis TIBBETS, T. M.*; WHEELESS, L. A.; University of Wyoming ttibbets@uwyo.edu

Analyses of nitrogen and carbon stable isotopes are widely used as tools in the study of food web structures and to estimate an animal�s trophic position. Most food web studies assume that an enrichment in ¹⁵N in an animal�s tissues represents a shift in trophic level. We examined the effect of metamorphosis on the ¹⁵N trophic enrichment between the tissues of several species of holometabolous insects and their diet. We measured the &delta¹⁵N of larval, pupal, and adult stages of four Lepidoptera (Bombyx mori, Galleria mollonella, Manduca sexta, and Vanessa cardui), one Diptera (Sarcophaga sp.), and one Coleoptera (Tenebrio molitor). As expected, we found that the tissues of insect larvae were enriched in ¹⁵N relative to their diet. The enrichment in ¹⁵N between larvae and diet was explained by the production of frass significantly depleted in ¹⁵N. Surprisingly, we found that the tissues of adult insects were significantly enriched in ¹⁵N relative to larvae in all but one species (T. molitor). Because we measured the adult tissues immediately after they emerged from pupal cases, the enrichment was not due to a change in diet. Rather, it was due to the excretion of ¹⁵N depleted metabolic waste (�meconium�) that resulted from protein metabolism during metamorphosis. Our results suggest that ¹⁵N enrichment during metamorphosis is significant, that discrimination during metamorphosis is variable across taxa, and that one cannot assume adult holometabolous insects reflect larval or larval diet ¹⁵N values.