Functional responses and intraguild predation of natural enemies of the soybean aphid

The parasitoid Aphelinus certus Yasnosh and the generalist predators Coccinella septempunctata L. and Harmonia axyridis Pallas are key natural enemies of soybean aphid, Aphis glycines Matsumura, in North America. Adult females of A. certus, and third instars, adult females and adult males of both C. septempunctata and H. axyridis all exhibited a Type II functional response toward soybean aphids at 26 ± 1°C. The theoretical maximum daily parasitism rate of aphids by A. certus was predicated to be 22 aphids per female adult. The theoretical maximum daily predation rate of adult aphids by C. septempunctata was 204 per third instar, 277 per female adult and 166 per male adult, and 244, 156 and 73, respectively, for H. axyridis. On V3 soybean plant with the recommended action threshold of 250 soybean aphids per plant, the average parasitism by A. certus was 11.8 aphids per female adult per day. Third instars and females of both ladybeetles consumed significantly more aphids than did adult males on V3 soybean plants with 250 soybean aphids per plant. Intraguild predation by both C. septempunctata and H. axyridis on the parasitoid A. certus was measured in Petri dish with the same number of unparasitized aphids and parastisized aphids by A. certus. Third instars, adult females and adult males of both C. septempunctata and H. axyridis all significantly preferred the unparasitized aphids.