Soybean aphid scouting and management
How to Scout for Soybean Aphid
Identification. Soybean aphids (Aphis glycines), are the only aphid usually found in soybeans in South Dakota. They are pear-shaped, 1/16th inch long, and range from pale yellow to lime green in color. On late-season soybeans, some aphids may be pale and smaller and found on lower leaves of the plant. Adult soybean aphids can occur in either winged or wingless forms. Wingless aphids are adapted to maximize reproduction, and winged aphids are built to disperse and colonize other locations. In South Dakota we often see a ‘second wave’ of aphids in early August as winged migrants colonize new fields. Winged soybean aphids have a dark thorax (central body segment). Soybean aphids have piercing-sucking mouthparts that are used to siphon plant sap; visible damage is subtle or unapparent until plants are heavily infested, but yield can still be reduced before this point.
Scouting. Check fields from mid-June through late August, with more intense scouting from mid-July through August. Walk a broad U or X pattern through the field and examine at least 20 to 30 plants, spread out over the field. Aphids can occur in “hot spots” but treatment decisions should be based on a broad sample of randomly-selected plants. A speed scouting method is also available that can reduce the time needed for sampling. It is less accurate and tends to lead to a ‘treat’ decision earlier than conventional scouting, but can save time when scouting demands are high (see UMN Soybean Aphid Sampling website)
During flowering, pod, and seed fill (R1-R5) the decision threshold for treatment is when populations are 250 aphids per plant when at least 80% of the plants are infested and the population is still increasing. Once soybeans reach the R6 growth stage (full seed, when a pod on one of the four top nodes has green seeds that fill the pod to capacity), soybeans are much less susceptible to yield loss from aphids (and, therefore, many more aphids can be tolerated). Thresholds for R6 soybeans have not been developed because natural aphid infestations in this late growth stage are uncommon, but research to develop R6 treatment guidelines is underway.
Producers who are near but not at threshold should consider checking the field again before treatment (3-4 days after the initial treatment decision is made). If aphid numbers have decreased, or are still near the threshold and have not increased noticeably, or if many natural enemies such as ladybeetles are present, producers may wish to delay treatment, as populations can sometimes decline naturally before reaching damaging levels.