Guide to Assessing Percent Defoliation in Soybeans

Treatment thresholds (defined as a point for which you can treat before economic damage is incurred) for foliage feeding insects in soybeans are relatively easy.  The threshold for foliage feeding pests is 30% foliage loss throughout the canopy during the vegetative stages up to two weeks prior to flowering and 15% foliage loss throughout the canopy two weeks prior to flowering through the reproductive stages up to R6-R7.  After this point, foliage loss is likely negligible.

One difficulty in accessing this threshold comes from assessing the foliage loss throughout the canopy.  Foliage feeding insects rarely feed evenly in the upper and lower parts of the plants.  One of the greatest difficulties also comes from our natural inclination to over-assess the percent of tissue defoliated, which the figure below illustrates well.  Use this as a guide when applying the threshold.

Image from Iowa State University Extension.

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