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Managing late-season soybean aphid infestations: It isn't getting easier.

by Robert Koch (Extension Entomologist)

Aphid populations continue to increase, especially in southern Minnesota where we are seeing more fields surpassing threshold levels and requiring treatment to protect soybean yield. Late-season management of soybean aphids can be challenging. In this article, I provide some considerations for scouting and making management decisions for soybean aphid in the later part of the growing season. How much longer do you need to keep scouting? What densities of aphids should (or should not) be treated with insecticide?  What about later growth stages? What if there are spider mites in the field too?  Can you stop scouting if you already applied an insecticide to a field?  However, before diving into this discussion, be sure you can accurately determine the growth stage of your soybean field (review Soybean growth stages for a refresher).

Scouting: Continue scouting soybean fields on a regular basis through the early R6 growth stage. Some people have the misconception that scouting can automatically stop in mid-August. This would be convenient, but aphid populations at this time of year are responding to plant growth stage, not the calendar. If you stop scouting too early, late-season infestations could increase to damaging levels. At this time of year, later maturing soybean fields are often more attractive to aphids. Keep in mind that fields that were planted with insecticide-treated seed or that received an early foliar insecticide application are still susceptible to infestation. Early-season prophylactic applications of insecticide generally do little to prevent soybean aphid populations from reaching damaging levels later in the season. When inspecting plants for aphids late in the season, be sure to inspect the entire plant because aphids will be spread throughout the soybean plants, and not conveniently located on the upper leaves like early in the season.

Threshold: Through the R5 growth stage, the threshold for determining when to apply insecticides for soybean aphid is 250 aphids per plant with more than 80% of plants infested and aphid populations increasing. Remember, the economic threshold (250 aphids per plant) is the point at which we should begin lining up an insecticide application to prevent the aphid population from increasing to an economically damaging level.

Treating soybean aphid populations below this level is unlikely to provide a return on investment and will increase risk for development of insecticide resistance and other unintended consequences, such as killing beneficial insects (predators, pollinators, etc.). Unbiased university-based research from multiple states and years continues to support the threshold of 250 aphids per plant. Consider other motivating factors (possibly sales profits or other factors) behind the recommendations you might be hearing to apply insecticides for soybean aphids at lower densities of maybe 10, 50 or 100 aphids per plant. My colleagues and I provide a detailed explanation as to why application of insecticide at these lower populations of aphids is not advised in our article Biology and Economics of Recommendations for Insecticide-Based Management of Soybean Aphid.

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 and plants are experiencing other stresses. 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 preharvest intervals (PHI) in the R6 growth stage.

Insecticide selection and application: Apply full labeled rates of insecticides. Pay close attention to pressure, volume and nozzle specifications to ensure good penetration and coverage of the soybean canopy, especially in tall, lush canopies. When choosing an insecticide, keep in mind that populations of soybean aphid with resistance to pyrethroids (such as bifenthrin and lambda-cyhalothrin) have been detected in Minnesota (and some neighboring states/provinces) over the last three years. Preliminary research results from this year indicate resistance to pyrethroids in some, but not all, soybean aphid populations again this year. More detailed information on managing insecticide resistant soybean aphids can be found in Management of Insecticide-resistant Soybean Aphids. As always, read and follow instructions on insecticide labels.

To further complicate things, spider mite populations are beginning to develop in some drier areas of the state. In such situations, keep spider mites in mind when choosing insecticides for soybean aphid. Some insecticides (for example, neonicotinoids and most pyrethroids, except bifenthrin) can increase (flare) a spider mite infestation. An overview of spider mites can be found in Twospotted spider mites on soybean, with some additional information on miticides in an earlier Crop News article.

Keep scouting: If you already applied insecticide to a soybean aphid infestation, you are not done scouting that field for the year. Because of the recent development of insecticide resistance, we cannot assume all insecticide applications will be effective. After an insecticide application, scout the field again after several days to ensure the insecticide provided an adequate level of control. Also, soybean aphid produces a winged form during the summer that can migrate into and rapidly recolonize previously treated soybean fields. If a previous insecticide application killed off the predatory and parasitic insects, such fields can be particularly prone to rapid aphid population growth.
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