Incorporating natural enemy units into a dynamic action threshold for the soybean aphid,Aphis glycines(Homoptera: Aphididae)

Abstract

Recommended action thresholds for soybean aphid, Aphis glycines, do not adjust for natural enemy impact, although natural enemies contribute important biological control services. Because individual natural enemy species have varied impacts on pest population dynamics, incorporating the impact of a diverse predator guild into an action threshold can be cumbersome. Field surveys identified an aphidophagous natural enemy complex dominated by Orius insidiosus, Coccinella septempunctata, Harmonia axyridis and Aphelinus certus. Functional responses of O. insidiosus were determined in the laboratory, while predation rates of all other natural enemies were obtained from the literature. Natural enemy impacts were normalized using natural enemy units (NEUs), where 1 NEU = 100 aphids consumed or parasitized. A dynamic action threshold (DAT) was developed by combining NEUs with an A. glycines population growth model. With the DAT, an insecticide application was only triggered if natural enemy numbers were insufficient to suppress pest populations. In field experiments, DAT provided equivalent yields to the conventional action threshold and reduced the average number of pesticide applications. The DAT approach has the potential to reduce pesticide use, will help preserve natural enemy populations and can be applied to other pest systems with diverse natural enemy guilds.