Several states in the Midwest are reporting the presence of aphids in soybean fields, with increasing populations due to cooler weather.

Photo 1. Turn over soybean leaves to estimate soybean aphid density.
Photo 1. Turn over soybean leaves to estimate soybean aphid density.

The Wisconsin Pest Bulletin (WPB) reported that densities of aphid populations had increased to economic levels since late July. Four of six fields sampled in Monroe County had very high average counts of 269 to 587 aphids per plant on 100% of plants. The WPB’s summer aphid survey indicates populations are still low in the majority of Wisconsin soybean fields, but may be rapidly increasing in response to recent mild temperatures.

Michigan State University Extension (MSUE) declared an official soybean aphid outbreak for the Thumb region last month, with populations ranging from 100 to thousands per plant. A report from University of Illinois stated that soybean aphid activity had picked up in northern Illinois.

Photo 2. Mid-seed set (R5.5) have seeds that are expanding in the pod.
Photo 2. Mid-seed set (R5.5) have seeds that are expanding in the pod.

Iowa State University Extension also reported findings of soybean aphids in vegetative soybean at most of the Iowa State University Research Farms in June. Iowa State University Extension entomologist Erin Hodgson offers the following advice for scouting and treating soybean aphids:

• Scout weekly from plant emergence until seed set. Aphids prefer to feed on the undersides of leaves and will establish on the newest leaves. If a large colony develops, they will feed on stems. Initial infestations are patchy and located near field edges, but winged aphids can quickly disperse within and between fields. Commercial fields that have reached uniform infestation should be closely monitored in August.

• Consider a foliar application when the average density exceeds 250 per plant. Populations should be increasing and more than 80% of the plants have to be infested to justify an application. This threshold is appropriate until plants reach mid-seed set (R5.5; Photo 2). Spraying at full seed set (R6) or later has not produced a consistent yield benefit.