Considerations for soybean aphid management

by Robert Koch, Extension entomologist, Anthony Hanson, Liz Stahl, Angie Peltier, and Claire LaCanne, Extension educators - crops

This is becoming an “interesting” year for soybean aphid management. After a couple years of limited infestations due to drought, soybean aphid infestations are widespread in 2024. As many people are considering if and how to manage this pest, we provide here some information to consider.

Be thorough when scouting

Aphids are likely distributed throughout the height of the canopy (not just on upper leaves), so examine entire plants.

Scout weekly

Weekly scouting of fields is often recommended, but more frequent scouting may be needed as forecasted weather conditions will be favorable for aphid population growth. Under typical field conditions, the time required for population size to double (doubling time) for soybean aphid has been estimated at 6-7 days. However, aphid populations can double in as little as a few days under more ideal conditions for an aphid, which would include comfortable temperatures in the upper-70s to mid-80s and especially with a lack of natural enemies, such as predatory insects and parasitic wasps. Fields that received early prophylactic insecticide sprays may have few natural enemies to keep colonizing aphids at bay.

Use the 250 aphids per plant threshold

Use the threshold of 250 aphids per plant (with 80% or more of the plants infested, and with aphid populations increasing) to determine when to treat a field with insecticides.

• This threshold (250 aphids per plant) is not when damage occurs to the plant, but indicates that the aphids will likely continue to increase and reach economically damaging levels. Therefore, this threshold is the trigger point when an insecticide application should be lined up within 7 days to prevent the increasing population from reaching a damaging level. That 7-day interval is based on average aphid population growth rates; however, the interval could be shorter if conditions are ideal for aphid population growth (see comments above in item #2).
• Infestations of fewer than 250 aphids per plant are less likely to continue to increase to damaging levels, so an application may be unnecessary (less likelihood of return on investment). Continue scouting those fields with lower infestations to see if they will increase to the threshold or not. By doing so, you let the natural enemies like predators, parasitic wasps, and fungal diseases take their toll on the aphids before you resort to using an insecticide.
• Remember that retail profits, not science, may be driving some of the lower threshold recommendations that you could be hearing.

Think critically about the insecticide product

If you need to apply an insecticide for soybean aphids, think critically about what product to use…

• Avoid products containing only a single pyrethroid or multiple pyrethroids, because of widespread pyrethroid resistance. Recent field data from our colleagues at NDSU is a reminder that resistance is still a problem.
• Some chlorpyrifos-containing products are available again. These are effective, but keep in mind that the human safety profile for organophosphates is worse than some of the other products. As a broad-spectrum insecticide, chlorpyrifos will kill soybean aphids and natural enemies alike.
• There are many formulated mixtures available, but are mixtures really needed?
• Aphids are likely the only threatening insect pest in most fields, so why add a broad-spectrum insecticide like a pyrethroid to one of the newer, more aphid-specific insecticides like sulfoxaflor (Transform) or afidopyropen (Sefina)? Those newer insecticides (Transform, Sefina and Sivanto) are effective against aphids on their own and are less toxic to natural enemies (e.g., lady beetles, etc.), which is a great bonus feature, but price points will need to be taken into consideration. Leaving those natural enemies alive in the field will help to control any aphids that survive or recolonize the field. Adding a broad-spectrum insecticide (like a pyrethroid) won't really increase effectiveness against aphids, but it will surely kill most natural enemies. As an additional benefit, selective insecticides are often more gentle pollinator insects as well.
• For insecticide resistance management in general, the role of insecticide mixtures is nuanced. However, for asexually-reproducing insects like aphids (clonal reproduction; no mating involved during the summer), insecticide mixtures are believed to provide less benefit for resistance management than alternations of products with individual modes of action. This is especially true when the pest is already resistant to one of the active ingredients in the mixture, which is the case for the many mixtures containing pyrethroids. Also, keep in mind that in some mixtures the rate of a given active ingredient may be less than in products containing only that one active ingredient, which might also select for more resistance.
• In addition, using two modes of action in one application reduces the options available for subsequent applications, if you are trying to be cognizant of resistance management and want to alternate modes of action.

Consider other pests

Even though aphids may be the most threatening pest in many soybean fields now, there are multiple other pests present in the fields at low levels. Keep in mind that any insecticide application to the fields is applying selection pressure for these pests to potentially develop resistance too. Avoiding resistance development in other pests is an additional incentive to avoid prophylactic applications to fields.

Assess efficacy of the insecticide application

After an insecticide application, scout the field in several days to assess efficacy. With aphids developing resistance to insecticides, we can't just assume that all our applications will work.

Continue scouting

Continue scouting for aphids on a regular basis through the early R6 (full seed) growth stage. Through the R5 (beginning seed) growth stage, use the threshold of 250 aphids per plant for determining when to apply insecticides. Soybean aphid can cause yield losses in the early R6 growth stage. However, a treatment threshold for early R6 has not been developed. Infestations of soybean aphid in early R6 may require an insecticide application if aphid populations are very large. Regular scouting and timely application of insecticides based on the threshold through the R5 growth stage should eliminate difficult decisions related to treatment of large infestations and pre-harvest intervals (PHI) in the R6 growth stage.