Predatory insects play important roles in regulating and controlling pest insect populations in the agricultural landscape system. Tracing the diet and movements of natural enemies are basic steps for quantitatively evaluating the effectiveness of these species as biological control agents. Stable isotope analyses can be used to determine insects’ dietary sources and movements. In this study, we first introduce the fundamental principles of stable isotope analyses. We then use this technique to determine the prey origins and proportions of the predatory beetle, Propylea japonica (Thunberg), in an agricultural landscape system comprised of cotton and maize crops. Finally, we discuss the potential and caveats of this method.