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Soybean aphid populations increasing in some previously treated fields

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by Robert Koch, Extension Entomologist

Soybean aphid populations are increasing in some fields that were previously treated with foliar insecticides for soybean aphid.  Soybean fields should continue to be scouted until the R6.5 growth stage, even if they were previously treated.  This post-treatment scouting will allow you to catch potential resurgence of aphid populations.  When determining whether to treat an aphid infestation at this time of year, pay particular attention to plant growth stage (the economic threshold of 250 aphids per plant is valid through R5, but yield loss can occur into early R6), whether or not the aphids have wings or "wing pads" (some aphids are already moving to buckthorn, but we don't know what fraction of the population will stay on soybean), and pre-harvest intervals of insecticides.  The recent soybean aphid scouting guide provides more information.  Be on the lookout for two-spotted spider mites too.  Certain insecticides applied to control aphid populations can increase (flare) populations of two-spotted spider mites.  The spider mite fact sheet provides more information.  If a field needs to be treated again, it is advisable to switch to a different class (mode of action) of insecticide to prevent or delay the development of pest resistance to insecticides.  For more information, review the guide to insecticide resistance and resistance management.


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