Scout for soybean aphid every 7- 10 days from bloom (R1) through seed set (R5). Count the numbers of aphids on 30 plants. Choose plants at random and try to sample different areas of the field. Estimate the number of aphids per plant. As the density increases, sample more frequently. The economic threshold, or action threshold, for soybean aphid is 250 aphids per plant with increasing populations on 80% of the plants. In other words, aphid populations must be increasing on most of the plants in order to justify an insecticide application.
Insecticide applications made after R5 have not always proven to be worth the control costs.
If you an insecticide application is warranted, use sufficient volume and coverage to make contact with aphids on the undersides of leaves and in the lower canopy. Leave a check strip so that you can evaluate product efficacy at harvest. After an application, check the field again 3-4 days later to verify a sufficient application was made. Continue scouting fields to monitor for winged summer migrants.