Reminders for soybean aphid management: Scouting, thresholds and insecticide selection

by Robert Koch (Extension Entomologist) & Bruce Potter (Extension IPM Specialist)

Soybean aphids can be found in many fields, but we are aware of only one small field where infestations were large enough to require insecticide applications.  However, we all know that this can change over a short period of time, because this pest’s potential for high reproductive rates. Therefore, you should be scouting your soybean fields for soybean aphids. Here we provide some reminders about scouting, thresholds, and insecticide selection for soybean aphid management.

Scouting

Scouting is required to determine which fields require (or may soon require) insecticide application and which fields do not. An overview of scouting for this pest can be found in Scouting for Soybean Aphid. Here are some key points from this guide:

•        Scouting requires getting into the field and estimating aphid numbers on plants.
•        You may know of soybean fields in your area where aphid populations usually increase first. You can scout these fields early in the season to help gauge how populations are developing.  However, aphid populations can vary from field to field, so each field being managed should be scouted before making an insecticide decision.
•        Estimate aphid numbers from a representative number of plants spread throughout the field.
•        Aphid counts should include winged and wingless aphids, but don't count dead aphids, cast skins, or aphid look-alikes.
•        Fields should be scouted on a regular basis through the R5 growth stage. Scout fields weekly when populations begin to noticeably increase. Fields with rapidly growing aphid populations may need to be scouted more frequently.
•        If aphid populations are large and increasing late in the season, continue scouting until R6.5 (pods and leaves begin to turn yellow).
•        Fields should be scouted after application of a foliar insecticide to monitor for resurgence of the aphid population or outbreak of a secondary pest.

Threshold

Through the R5 soybean growth stage (seeds developing, but pod cavity not filled), the decision to apply a foliar insecticide for soybean aphids should be based on the following economic threshold: average of 250 aphids per plant AND aphids on more than 80% of plants AND aphid populations increasing.

This is not where economic yield loss occurs. Rather, it is the point where the aphid infestation becomes likely to grow to economically damaging levels and an insecticide application becomes more likely to pay. 

As the growing season progresses, note that yield loss due to soybean aphid can occur into early R6 (pod cavity filled by seed). At this stage consider an insecticide application if aphid populations are very high and plants are experiencing other stress. However, late in the season, pay attention to pre-harvest restrictions for products and the estimated number days until R6.5 (yellow pods and leaves) when soybean yields should be minimally impacted by soybean aphids (or most other insect pests).

You may hear recommendations to apply insecticides for soybean aphid at much lower levels of infestation than the above mentioned research-based economic threshold. Such applications of insecticides will likely kill what aphids are in the field, but may not be needed because such low-level infestations often do not reach economically damaging levels.

Furthermore, these sprays can remove biological controls (predators and parasitic wasps) of the soybean aphid, leaving the field vulnerable to aphids arriving after the insecticide is applied. As a result, fields treated early like this sometimes need to be resprayed, which in turn increases costs for the growers, increases the risk for development of insecticide resistance and increases the risk for environmental concerns. Further information about the economic threshold for soybean aphid can be found in Fact-Based Soybean Aphid Insecticide Recommendations.

Insecticides

If soybean aphid infestations in fields reach the economic threshold, they should be treated with a labeled insecticide according to the instructions on the product label. However, remember that we have documented soybean aphid populations resistant to pyrethroid insecticides over multiple years and across many areas of Minnesota. Guidance for selection of insecticides for management of soybean aphid can be found in Insecticide Options for Resistant Soybean Aphid. In addition to this, you can find information about insecticide resistance management for soybean aphid in a multi-state publication, Management of Insecticide-Resistant Soybean Aphids.