Is it too late to Spray Soybean Aphids?

Soybean aphid is still the most important insect pest of soybean in Iowa, despite low populations throughout the state in recent years. This summer, we have seen high populations throughout Iowa, with hundreds of aphids per plant, honeydew, and sooty mold development on leaves. We have also heard various reports about performance issues with pyrethroid applications. This has prompted many questions about when to spray soybean aphids and which insecticides to use.

Scouting and Insecticide Timing

Soybean aphid populations fluctuate throughout the growing season, so it is ideal to begin scouting in June to monitor how things are progressing. However, if you are just now starting to scout for soybean aphids, use the Speed Scouting method to make management decisions. Speed Scouting makes a treatment decision by counting up to 40 aphids on each plant, then using a decision tree to determine whether to treat, not treat, or continue scouting. Alternatively, using the economic threshold for soybean aphid is 250 aphids per plant with 80% of plants infested and populations increasing is a proven scouting method.

If your scouting determines that a treatment is necessary, consider the plant's growth stage before deciding to spray. Many years of research have consistently shown that there is no yield benefit to spraying for soybean aphids after seed set is complete (R6 growth stage and beyond).

Insecticide Options and Resistance Concerns

Foliar insecticides, mostly pyrethroids and organophosphates, have been the primary tactic used for soybean aphids in Iowa since 2001. Recently, some new products and modes of action have become available for farmers to use, though pyrethroids are still the dominant choice because of their low cost compared to newer chemistries. Pyrethroid resistance in soybean aphid has been a growing problem over the past decade in the Upper Midwest, and it should be assumed that there is some level of resistance in every population at this point. Use this regional factsheet to understand effective management strategies better, and make sure to double-check that listed products are labeled for use in Iowa.

Resistance situation in Iowa: A graduate student in the ISU Soybean Research Laboratory has been collecting aphids before and after pyrethroid sprays in northern Iowa and screening them for mutations that confer resistance to pyrethroids. Every aphid had at least one mutation after they sprayed with a pyrethroid. We assume soybean aphid populations are not totally susceptible to pyrethroids anymore.

Recommendations:

• Speed Scout for soybean aphid to determine whether it is economical to spray an insecticide.
• Late-season aphid infestations may not impact yield, and applying insecticides after R5 may not result in a return on investment.
• Use high volume and pressure to ensure droplets make contact with aphids on the undersides of leaves.
• Pyrethroid resistance is an emerging issue in Iowa. Do not assume that a pyrethroid will work to manage soybean aphids.
• After any insecticide application, revisit the field three to five days later to assess product efficacy. Consider leaving a check strip when applying to compare aphid populations in a treated vs. untreated area.
• If a field is to have multiple insecticide applications in a single year (for soybean aphid or a combination of pests), alternate the mode of action used. Repeated exposures will eventually lead to resistance development in the population.
• Be aware of pre-harvest intervals (PHI) for applications made late in the growing season.