Evaluating seasonal exposure to soybean aphid (Hemiptera: Aphididae) and yield for Rag1 soybeans

Since the discovery of Rag1, a gene that provides resistance to soybean aphid, Aphis glycines (Hemiptera: Aphididae), several resistant soybean cultivars are now commercially available. We hypothesized that Rag1 soybeans would experience lower aphid pressure and have higher yield compared to susceptible soybeans, and in 2010 and 2011, we evaluated seasonal aphid exposure and yield of susceptible and resistant cultivars. Each evaluation included six treatments: 2 Rag1 lines, 2 susceptible lines, and a Rag1 and a susceptible line which were treated with insecticide at the economic threshold of 250 aphids per plant. All treatments were replicated across four Iowa State University Research and Demonstration Farms. In 2010, we sampled insects bi-weekly with non-destructive whole plant sampling and sweep-netting. Due to the low aphid populations (<50 aphids per plant) in 2010, all insecticide applications were performed at a pre-determined spray date in early August. In 2011, we used both non-destructive whole plant counts as well as binomial sequential sampling to compare the precision of these sampling plans. At the end of both seasons, yield data was collected. Results indicate that plants with the Rag1 gene experienced fewer aphids, while yield appeared to be higher in susceptible plants.