POPULATION ECOLOGY Relationship of Soybean Aphid (Hemiptera: Aphididae) to Soybean Plant Nutrients, Landscape Structure, and Natural Enemies

In the north central United States, populations of the exotic soybean aphid, Aphis glycinesMatsumura (Hemiptera: Aphididae), are highly variable across space, complicating effective aphid management. In this study we examined relationships of plant nutrients, landscape structure, andnatural enemieswith soybeanaphid abundance across Iowa,Michigan,Minnesota, andWisconsin, representing the range of conditions where soybean aphid outbreaks have occurred since its introduction. We sampled soybean aphid and its natural enemies, quantiÞed vegetation land cover and measured soybean nutrients (potassium [K] and nitrogen [N]) in 26 soybean sites in 2005 and 2006. Multiple regressionmodels found that aphid abundancewas negatively associatedwith leaf K content in 2005, whereas it was negatively associated with habitat diversity (SimpsonÕs index) and positively associated with leaf N content in 2006. These variables accounted for 25 and 27% of aphid variability in 2005 and 2006, respectively, suggesting that other sources of variability are also important. In addition, K content of soybean plants decreasedwith increasing prevalence of corn-soybean cropland in 2005, suggesting that landscapes that have a high intensiÞcation of agriculture (as indexed by increasing corn and soybean) are more likely to have higher aphid numbers. Soybean aphid natural enemies, 26 species of predators and parasitoids, was positively related to aphid abundance; however, enemy-to-aphid abundance ratios were inversely related to aphid density, suggesting that soybean aphids are able to escape control by resident natural enemies. Overall, soybean aphid abundance was most associatedwith soybean leaf chemistry and landscapeheterogeneity.Agronomicoptions that can ameliorate K deÞciency and maintaining heterogeneity in the landscape may reduce aphid risk.