Last week, we saw soybean aphid populations that were still relatively low. However, this week’s scouting revealed dramatic population increases in the same fields. Fields near Brookings, SD had soybean aphid populations that averaged from 50 to 200 aphids per plant. This indicates that the 250 aphid per plant threshold on 80% of the plants will probably be reached relatively soon in the Brookings area. If these fields had been speed scouted (i.e., looking for 40 soybean aphids per plant for a positive plant) the decision would have been to spray. With speed scouting, remember that it is highly recommended that you re-scout the field in 3-4 days to confirm a spray decision. The fields that were scouted near South Shore, SD and near Beresford, SD still were well below the threshold.
Since pyrethroid resistant soybean aphid populations were found last year in South Dakota, using these products may result in an incomplete population reduction. Although we aren’t sure how widespread these resistant soybean aphids are, there is a possibility that they are widely distributed in South Dakota. We are not recommending the use of pyrethroid class insecticides this year for soybean aphid management. However, if a pyrethroid insecticide is used for management, make sure to re-scout the field within one week of treatment to ensure that populations were effectively reduced. If they weren’t, make sure to use a different class of insecticide for retreatment.
Refer to this fact sheet for a product list of pyrethroid alternative insecticides.