Tracing prey origins, proportions and feeding periods for predatory beetles from agricultural systems using carbon and nitrogen stable isotope analyses

Predatory beetles are an important component of the natural enemy complex that preys on insect pests such as aphids within agroecosystems. Tracing diet origins and movement of natural enemies aids understanding their role in the food web and informs strategies for their effective conservation. Field sampling and laboratory experiments were carried out to examine the changes of carbon and nitrogen stable isotope ratios (delta C-13 and delta N-15) among crops (cotton and maize), pests (cotton and maize aphids), and between wing and abdomen of predatory beetles, Propylea japonica, and to test the hypothesis that prey origins, proportions and feeding periods of the predatory beetles can be deduced by this stable isotope analysis. Results showed that the delta C-13 values both in wing and abdomen of adult P. japonica were changing from a C-3- to a C-4-based diet of aphids reared on maize or cotton, respectively; the isotope ratio of their new C-4 substrates were detectable within 7 days and the delta N-15 values began to reflect their new C-4 substrates within 3 days. The relationship between delta C-13 and delta N-15 values of P. japonica adults in wing or abdomen and diets of aphids from a C-3-based resource transitioning to a C-4-based resource were described best in linear or quadratic equations. Results suggest that integrative analysis of delta C-13 and delta N-15 values can be regarded as a useful method for quantifying to trace prey origins, proportions of diets and feeding periods of natural enemies. The results can provide quantifying techniques for habitat management of natural enemies. Crown Copyright (C) 2014 Published by Elsevier Inc. All rights reserved.