The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) is a significant soybean pest in the north central United States. Insecticidal seed treatments and host plant resistance are two commercially available management tools. Here we investigate the efficacy of both management tools throughout the season. Soybean lines containing the soybean aphid resistance genes Rag1, Rag2, or both Rag1 Rag2 were compared with a near-isogenic aphid-susceptible line. Each line was grown in field plots both with and without thiamethoxam applied to the seed. Individual plants from each plot were caged and infested with soybean aphids to measure the efficacy and potential interaction of aphid resistance and thiamethoxam. Aphid population growth rate was measured for each caged plant for 9–12 d after infestation. New cages were established each week from 34 d after planting (dap) to 92 dap to track seasonal variations in efficacy. Thiamethoxam reduced population growth only at the 42 dap time point and only for the susceptible, Rag1, and Rag2 lines. The lack of an effect of thiamethoxam on the Rag1 Rag2 line was likely because of already high mortality from two resistance genes. Aphid resistance alone reduced population growth compared with the susceptible line at least till 55 dap for single-gene resistance and 63 dap for the two genes combined. Aphid resistance provided suppression of soybean aphid population growth throughout the season unlike the insecticidal seed treatment.