Prior experience indicates many fields begin to see dramatic increases in soybean aphid populations in Mid-July, although some fields have been treated in early July due to extremely high infestations. Soybean producers should start scouting soybean fields by the first week of July to determine the activity of the soybean aphid. They should walk soybean fields at least once a week, preferably twice a week, to determine whether the population is increasing.
Natural enemies (predators and parasitic wasps), temperature, rainfall, planting date, aphid fungal disease, soybean growth stage and perhaps soybean variety all play a role in regulating soybean aphid populations. Regular scouting will help determine how these factors are influencing aphid growth rates under field conditions.
Soybean aphids usually are found on the underside of leaves, on new growth at the top and side branches, and can be distributed on leaf petioles and stems under heavy infestation. The insects are small (1/16 in.), soft-bodied, with or without wings. They are yellow early in the season. Winged adults have a black head and thorax. When scouting, producers should walk in a W-shaped pattern across the field stopping to take whole plant counts on 20-30 plants throughout the field.
Currently, the Midwest soybean aphid action threshold is set at 250 aphids per plant when the population is actively increasing and the soybean plant is in the R1 to R4 growth stages. This threshold incorporates a seven-day lead time so growers have time to schedule treatment, purchase product, or deal with weather delays. A regular scouting interval will help to determine if the population is actively increasing. Beyond the R4 stage, the threshold is likely higher. Midwest entomologists are conducting a common experimental protocol this summer to better determine late season thresholds. Treating beyond R6 (full seed) is not recommended.
Optimum field conditions for the soybean aphid include mild temperatures. University of Minnesota laboratory research indicates that the soybean aphid survives for the longest time at 68º and has the greatest reproductive capability between 68º and 77º. Once constant temperature in laboratory experiments reached 95º, soybean aphids lived for less than five days and no young were produced. Thus, a mild summer would lead to a greater infestation than a hot summer.
To view Extension publication X1134 Reproductive Soybean Development Stages and Soybean Aphid Thresholds to help in staging soybeans, log o to www.planthealth.info/soyaphid/uwexsoystage.pdf. The R1 soybean stage relates to first bloom while the R3-R4 soybean stage relates to beginning pod set.