Soybean aphids beginning to colonize soybean fields

 By Bob Koch (Extension Entomologist)

Soybean aphids are beginning to colonize soybean fields in Minnesota. Over the past week, we found aphids in fields across southern Minnesota, including St. Paul and Lamberton. There are likely other crop-related concerns on your minds now, but this update is being shared to remind you that as June progresses you should start thinking about scouting your fields. Currently, infestations are far below levels that would require insecticide treatment to protect yield. 

Image: Soybean leaf infested with soybean aphids found in St. Paul on June 7, 2023. Arrows indicate "aphid mummies," which are the remains of aphids killed by parasitic wasps. Photo credit: Bob Koch, UMN.

The "success" of these aphids on soybean and the resulting size of their infestations will depend on several factors, such as weather conditions, quality of the soybean plants (which is affected by plant growth stage, moisture, other pests and diseases, etc.), and natural enemies (predators, parasitic wasps and diseases). Considering all these factors, we cannot accurately predict if 2023 in general will be a big year for soybean aphids in Minnesota, let alone how populations will develop in individual fields. Therefore, scouting your fields remains essential for knowing if and when insecticide applications are required to protect soybean yields from soybean aphid.

Early in the season (starting in mid to late June), scouting can often be focused on higher-risk "indicator" fields. Such indicator fields, where aphids generally tend to show up first year after year, are often early planted, smaller fields with wooded borders containing buckthorn. As the season progresses, you will want to transition to scouting all your fields on a regular basis (ideally every 7 to 10 days). To determine when to apply insecticides for soybean aphid, use the threshold of 250 aphids per plant, with most of the plants infested, and aphid populations increasing. Remember, this threshold is not the point at which yield loss starts to occur. It is instead the trigger point to start lining up an insecticide application to prevent the infestation from increasing to higher levels that will cause economically significant yield loss. 

As you consider management options for soybean aphid this summer, keep in mind that many populations of soybean aphid are resistant to pyrethroid (group 3A) insecticides. Also, remember that many of the insecticides we use to control soybean aphid will also kill predatory insects and parasitic wasps, like those that caused the aphid mummies in the image above. Interestingly, the presence of aphid "mummies" indicates that parasitic wasps found these early colonizing aphids before we did.  When these wasps kill the aphids, they leave behind the dried remains of the aphids called mummies. These mummies are conveniently color-coded (tan vs. black) to help us know which types of wasps are attacking the aphids.