Populations remain quite tolerable in the northwest region. The exception might be in heavily wooded areas and with smaller fields such as in Ottertail County near Underwood, a location which persistently has problems with the aphids. According to Doug Holen, Regional Educator for Crops located in Fergus Falls, treating for aphids is getting underway this week in Underwood and surrounding communities.

Everyone should begin watching smaller fields near wooded areas (e.g., forested waterways, shelterbelts, tree claims/homesteads). These areas may have buckthorn and could have been earlier sources of spring migrants moving into nearby fields in June. Those few aphids may now be building to noticeable numbers. These wooded areas may also serve as windbreaks that allow winged aphids (figure 1) to settle out of the southern winds that are currently transporting migrants from infested fields to the south. Look for these winged aphids on field edges. You may also find winged aphids settling on field edges near roads or other areas where there is a distinct contrast in color such as from brown to green or black to green.

Fields are also beginning to advance rapidly through the reproductive growth stages at this time of year. Most fields are in the R3 stage now and will reach R4 shortly. The R5 stage becomes the critical point of deciding when to leave aphids alone. Field trials indicate that there may still be a yield response at R5 when treating aphids that reach the 250 aphid per plant threshold. However, this response is not as predictable, and tends to be a smaller yield benefit compared to treating threshold level populations in the R1 to R4 stages. Also, the probability of a yield response is greater when treatments to control threshold level populations occur in early R5 versus late R5 stage soybeans.

For information sheet on the soybean growth stages from R1 to R6