Tracing dietary origins of aphids and the predatory beetle P ropylea japonica in agricultural systems using stable isotope analyses

Tracing dietary origins of the predatory beetle P ropylea japonica ( T hunberg) ( C oleoptera: C occinellidae) aids understanding their roles in the food web and provides information to develop strategies for effective conservation in agroecosystems comprised of wheat [ T riticum aestivum L . ( P oaceae)], cotton [ H irsutum spp. ( M alvaceae)], and maize [ Z ea mays L . ( P oaceae)]. Intrinsic markers of carbon and nitrogen stable isotope ratios (δ 13 C and δ 15 N) in P . japonica need to be developed to ascertain the source(s) of diet. Experiments were carried out to examine the changes of δ 13 C and δ 15 N among the three crops, pests (wheat, cotton, and maize aphids; all H emiptera: A phididae), and P . japonica fed on aphids of each of the three crops. Results indicated that δ 13 C values in P .  japonica fed on wheat, cotton, and maize aphids were −27.2 to −26.5‰, −24.2 to −23.9‰, and −11.0 to −10.7‰, respectively, whereas their δ 15 N values were 1.1 to 2.9‰, 6.0 to 7.4‰, and −0.6 to 0.1‰, respectively. δ 13 C and δ 15 N plots clearly identify the three crops, the dietary origins of the aphids, and the host origins of the aphid prey consumed by the ladybird beetles, as each pathway displays a non‐overlapping pattern. Based on the values of δ 13 C and δ 15 N of the three food webs, dietary origins can be traced in the predatory beetle P . japonica derived from wheat, cotton, and maize crops.