Field-level effects of preventative management tactics on soybean aphids (Aphis glycines Matsumura) and their predators

A 2-year field experiment was conducted in northern Illinois to evaluate the effects of host plant resistance and an insecticidal seed treatment (thiamethoxam) on soybean aphids, Aphis glycines Matsumura and their predators. Densities of soybean aphids varied between the 2 years of the experiment. During both years, resistant plants experienced fewer cumulative aphid days than susceptible plants. Populations of soybean aphids on resistant plants rarely exceeded the economic injury level of 250 soybean aphids per plant. The use of thiamethoxam reduced cumulative aphid days in 2007, but not in 2008. Although soybean aphids reached densities that were sufficient to cause yield-loss for untreated and susceptible plants, no yield-benefit was associated with using the two management tactics in either year. This latter finding suggests that densities of soybean aphids need to be greater and sustained for a longer period of time than what we observed if the two management tactics are expected to provide a yield-benefit. Monitoring natural enemies revealed that densities of key aphidophagous predators were relatively unaffected by host plant resistance or thiamethoxam; the effect of these management tactics on densities of predators, as well as the effectiveness of the method used to sample predators, is discussed.