Two years ago aphids were so numerous on their winged flight through central Indiana to southern Indiana on a Saturday in mid-September that they were a pest. Even non-farm people were wondering what kind of creature was invading. That same type of scenario may be setting up, although it may happen earlier, if some aphids haven't already migrated southward, according to Purdue University entomologists Christian Krupke and John Obermeyer.
Apparently conditions in early August were ideal for aphid development. Numbers suddenly shot up in northern Indiana. Aphids don't reproduce as rapidly when the temperature is 90 degrees or higher.
Perhaps higher temperatures again last week slowed them down. Nevertheless, the entomologists suggest that anyone with late - planted soybeans, even in southern Indiana, stay alert to this possible threat. Aphids prefer the youngest soybeans they can find. Unfortunately, they may be hitting at the stage when soybeans are most susceptible to any kind of stress and damage.
The entomologists believe it may pay to scout for the aphids. The first step is to go to your fields and begin looking. If you find them, then it's time to do some actual counts so you can determine if there are enough aphids present to justify spraying.
If leaves are already on stems and petioles and soybeans are in the R3 to R4 stage of development, which includes the beginning of podfill, you can assume those beans are over the threshold. Line up spraying as soon as possible.
If you don't see them there, look under the leaves. Find and count the number per plant. Then do that for 20 plants within the field. If the average number of aphids is more than 250 per plant and the beans are still in the R3 to R4 stage, it's economically a benefit to apply a spray for them. Work with your local pesticide applicator of chemical dealer to pick the most appropriate product and method of application.
If your soybeans are already beyond the R6 stage, there is no reason to spray for aphids. The plants are beginning to shut down on their own at this point, and the aphids will affect yield very little, if any. That's why it's important to determine the stage of development of the plants, even if you determine there are 250 aphids per plant.Consult the Purdue University Corn and Soybean pocket field guide for help in staging your soybeans. It should prove most helpful in assisting you as you try to determine if spraying would still benefit the beans, and perhaps return a net profit over the cost of spraying and the application cost.