Publications

The soybean aphid Aphis glycines Matsumura (Hemiptera: Aphididae), was detected for the first time in North America in Wisconsin during 2000, and it is currently present in 22 states. Since its detection, it has developed outbreak populations that resulted in up to 50% yield reduction during alternating years. I conducted field studies that demonstrated the crucial role of existing generalist natural enemies in suppressing soybean aphid population growth below economic threshold levels. By using predator exclusion cages, I demonstrated that ambient levels of predators exert strong suppression on soybean aphid (3 to 20-fold reduction) in comparison with open controls and sham cages. Through work at the NSF-Long Term Ecological Research site at the Kellogg Biological Station of Michigan State University, I found that this control occurs over a broad range of agricultural production systems, ranging from conventional practices with or without tillage, to organic soybeans. This suppression occurs regardless of initial aphid infestation, with aphid populations being reduced below threshold levels at high, medium and low initial aphid densities. The strength of this suppression results in a trophic cascade, with similar soybean biomass and yield in treatments where aphids were exposed to predation as in treatments completely protected from aphids by aphid exclusion cages. Direct field observations revealed that the coccinellids Harmonia axyridis and Coccinella septempunctata combined provided most of the mortality on soybean aphid, were very effective on a per capita basis, and responded to aphid density by increasing their abundance, time within the aphid patch, and consumption rate. Exposure to predation significantly shifted the aphid’s within-plant distribution towards lower nodes on the plants, suggesting indirect effects of predation.