Soybean aphids and other insect pests are able to reduce soybean yield until the R6.5 stage (yellow pods begin) stage. You want to pay some attention to soybean insect problems (and identify weed and disease issues) until then. However, this year's aphid scouting efforts should increasingly focus on fields with less mature beans. As the 2016 soybean aphid season begins to wind down, there are several aspects of late-season soybean aphid populations that can influence insecticide decisions.
Aphids will eventually leave soybean for buckthorn. Longer nights, cool weather and maturing soybeans combine to begin aphid movement from soybean to buckthorn. A calendar date is not useful in predicting this timing. For example, aphid movement from soybean to buckthorn has ranged from mid-August to mid-September at the South West Research and Outreach Center. This weekend's cool forecast may start the process, particularly in northern MN. Don't treat soybean aphid populations that are declining.
Pathogens, parasites and predators may be more helpful. Cooler, wetter conditions favor the most common soybean fungal diseases of soybean aphid. Moderate temperatures, heavy dews and persistently wet soybean canopies are more common this time of year. In addition, beneficial insects have had all summer to build populations on aphids in soybean and other plant environments and may be able to move into aphid infested fields. For example, Harmonia (a.k.a. Asian lady beetle) populations are just now becoming abundant in many fields. These natural enemies can contribute to rapid declines in soybean aphid populations.
Base your scouting and decisions on soybean maturity. Be vigilant but don't overreact. Aphids need time and numbers to reduce yield in a soybean field. Low aphid populations this time of year may not be able to spend enough time in early-maturing fields to be a problem.
You may want to treat extremely high soybean aphid populations (1000s of aphids/plant) as late as early R6 stage soybeans (beans fill the pod cavity at one of the upper four nodes) to avoid yield reduction. However, end-of-season treatment anxiety typically occurs as a result of missing a soybean aphid problem earlier in the season.
To ensure that you are not faced with a dilemma on whether or not to treat soybean aphids close to harvest, scout on a regular basis and use the 250 aphid/plant threshold until R6. See the soybean aphid scouting guide for more details on scouting and soybean reproductive stages : http://www.extension.umn.edu/agriculture/soybean/pest/docs/soybean-aphid-scouting.pdf
The pre-harvest Interval (PHI) of insecticides needs to be considered. Always read the label or make sure someone has read it for you. PHIs range from 18 to 30 days for most commonly used soybean aphid insecticides and they influence which, if any, insecticide(s) can be used this late in the season. There are soybean fields within a month of harvest.