Effect of Tropic Level and Metamorphosis on the Stable Isotope Discrimination of Ectropis grisescens

Abstract Light stable isotopes (δ13C, δ15N, δ2H, and δ18O) of Ectropis grisescens (a leaf-eating pest) were measured at different developmental stages. Isotope values of larval instars, pupae, and adult tissues were determined to understand fractionation patterns at different life stages and to evaluate the tropic shift from food to insect to excrement. The insect’s δ13C tissue values were significantly enriched relative to its diet, whereas insect feces were significantly depleted compared to dietary input. Similarly, δ15N values of the pest tissue were significantly enriched compared to its diet and this enrichment was most likely due to protein quality since both insufficient protein and a high dietary protein intake have the potential to enrich δ15N of bulk body tissues by increasing the protein turnover. The δ2H and δ18O values also showed significant fractionation compared to diet. The δ2H tropic enrichment from plant to larvae and subsequent decrease from larvae to moth is likely due to net enrichment from plant to Ectropis grisescens. Significant correlations between diet, pest tissues and feces were observed for most isotopes. In addition, the metamorphosis of Ectropis grisescens significantly changed the stable isotope (δ13C, δ15N, δ2H, and δ18O) values of the resulting moth.