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

•We examine the changes of δ13C and δ15N among crops, pests and predators.•δ13C or δ15N values of predators related to proportions of diets with equations.•Values of δ13C or δ15N can trace prey origins, proportions of diets.•Integrative values of δ13C and δ15N can trace feeding period of natural enemies.•Provide quantifying techniques for habitat management of natural enemies.

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 (δ13C and δ15N) 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 δ13C values both in wing and abdomen of adult P. japonica were changing from a C3- to a C4-based diet of aphids reared on maize or cotton, respectively; the isotope ratio of their new C4 substrates were detectable within 7 days and the δ15N values began to reflect their new C4 substrates within 3 days. The relationship between δ13C and δ15N values of P. japonica adults in wing or abdomen and diets of aphids from a C3-based resource transitioning to a C4-based resource were described best in linear or quadratic equations. Results suggest that integrative analysis of δ13C and δ15N 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.

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