No. 21 Article 2/August 14, 2009

Soybean Aphid Densities Remain Well Below Economic Threshold

The good news continues regarding the very low densities of soybean aphids throughout Illinois this season. A review of the sentinel plot data through early August reveals no aphids in many of the plots and very low numbers in several. On August 10, Jim Morrison, crop systems extension educator, Rockford Extension Center, reported that a late-planted soybean field (V8 stage) in Boone County had an average density of 30 aphids per plant. He also observed an average of 20 aphids per plant in a soybean field (R4 stage) in Jo Daviess County. As we move beyond the R5 (beginning seed) stage of development in some areas of the state, many soybean fields will become less susceptible to this insect pest. I urge growers not to treat soybean fields that are below the economic threshold (250 aphids per plant, 80% of plants infested) because of the questionable yield benefit and the certain negative effects an insecticide application will have on natural enemies. If fields in your area of the state begin to exceed the economic threshold, please share these observations with me and I will inform readers.--Mike Gray

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