Tracing dietary origins of aphids and the predatory beetlePropylea japonicain agricultural systems using stable isotope analyses

Abstract

Tracing dietary origins of the predatory beetle Propylea japonica (Thunberg) (Coleoptera: Coccinellidae) aids understanding their roles in the food web and provides information to develop strategies for effective conservation in agroecosystems comprised of wheat [Triticum aestivum L. (Poaceae)], cotton [Hirsutum spp. (Malvaceae)], and maize [Zea mays L. (Poaceae)]. Intrinsic markers of carbon and nitrogen stable isotope ratios (δ13C and δ15N) in P. japonica need to be developed to ascertain the source(s) of diet. Experiments were carried out to examine the changes of δ13C and δ15N among the three crops, pests (wheat, cotton, and maize aphids; all Hemiptera: Aphididae), and P. japonica fed on aphids of each of the three crops. Results indicated that δ13C 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 δ15N values were 1.1 to 2.9‰, 6.0 to 7.4‰, and −0.6 to 0.1‰, respectively. δ13C and δ15N 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 δ13C and δ15N of the three food webs, dietary origins can be traced in the predatory beetle P. japonica derived from wheat, cotton, and maize crops.