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Soybean aphid infestations are developing: Recommendations for scouting and managing insecticide resistance

by Robert Koch (Extension Entomologist), Bruce Potter (Extension IPM Specialist), Phil Glogoza (Regional Extension Educator)

Scouting for soybean aphids in Minnesota soybean fields should be underway. While aphid populations are low in most fields, we have received reports of increasing soybean aphid numbers. In parts of central Minnesota, some of these fields are nearing threshold levels and will likely require insecticide treatment soon to protect yield. Early-planted soybean in areas with moderate rainfall this year might see significant aphid populations first. However, aphids are now spreading to other fields putting them at risk too. Below, we provide an overview of scouting recommendations and updates on insecticide resistant soybean aphids.

Scouting and threshold: 
How will you know if any of your soybean fields are at economic risk from soybean aphid? The decision to apply insecticide for soybean aphid should be based on scouting and the economic threshold. Briefly, you need to get into the field to count or estimate aphid numbers on plants. Count the number of aphids on plants selected from throughout the field. Compare the average number of aphids from those plants to the economic threshold. The economic threshold for soybean aphid is an average of 250 aphids per plant with more than 80 percent of the plants infested with aphids, and the aphid population appearing to be increasing. See our soybean aphid scouting guide, if you need a refresher on scouting tips and techniques for this pest.

Remember, the economic threshold is the trigger point for scheduling an insecticide application to prevent populations from reaching an economically damaging level, which is called the economic injury level. A more detailed discussion of these concepts can be found in our Review of the Biology and Economics of Soybean Aphid Management Recommendations.

Alternatively, treatment decisions for soybean aphid can be made using Speed Scouting, which is a scouting and decision-making procedure that doesn’t require counting large numbers of aphids. Speed scouting can provide a more efficient option for some users to make treatment decisions. Validation research has shown this methods to be somewhat conservative (slightly over-recommending treatment).

Apply full labeled rates of insecticides for soybean aphid management and follow the instructions on product labels.

Insecticide resistance:
Populations of soybean aphid resistant to pyrethroid insecticides, such as bifenthrin and lambda-cyhalothrin (Group 3 insecticides), have been detected in Minnesota over the last three years. This year, preliminary results from laboratory bioassays with soybean aphids collected earlier this week from a field near Willmar once again suggest some level of soybean aphid resistance to pyrethroids. Because of this recent history of issues with pyrethroids for management of soybean aphid, we currently discourage the use of products containing only a pyrethroid insecticide. Generally speaking, products containing a single active ingredient from a labeled insecticide group other than Group 3 are preferred over mixtures when resistance management is a goal. However, with the limited number of insecticide groups available for soybean aphid, mixtures of insecticides will likely play a role in alternations. After treating a field, be sure to scout the field after 3-5 days to ensure efficacy of the treatment. If a field needs to be retreated, alternate to a product containing a different insecticide group for the follow-up treatment. Check product labels for insecticide group numbers. For further information, see our recent publication on managing insecticide resistant soybean aphids

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