Effects of Grassland Habitat and Plant Nutrients on Soybean Aphid and Natural Enemy Populations

The soybean aphid Aphis glycines Matsumura (Hemiptera: Aphididae) is an invasive economic pest of soybean (Glycine max [L.] Merrill) in the United States. Research has shown the endemic natural enemy community in the United States is capable of suppressing A. glycines below EILs, but this biological control is inconsistent, especially in simple agricultural landscapes. In the course of a 3-yr project (2006-2008) we sought to determine the affects of landscape composition, configuration, and plant nutrients (N, P, and K) on A. glycines and aphidophagous natural enemy abundance. Specifically, we tested whether nearby grasslands contribute to the abundance of natural enemies and the suppression of A. glycines. The study site was located around the Neal Smith National Wildlife Refuge, composed of >2,000 ha of reconstructed prairie, located in Jasper County, IA. We sampled A. glycines, natural enemies, and plant nutrients in 100 soybean fields while characterizing the landscape surrounding each field. A. glycines abundance was lowest in 2006 but reached economically damaging populations in 2007 and 2008. The ratio of natural enemies to A. glycines decreased in each year of our study (2006 > 2007 > 2008). Variation in A. glycines and natural enemies was best explained by year and, to a lesser extent by plant nutrient levels and landscape variables. Results suggest grassland habitat did not significantly contribute to the biological control of A. glycines. Furthermore, yearly decline of natural enemy may have facilitated the colonization of A. glycines leading to outbreaks later in the season.