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Monitor Soybean Fields for Defoliators

August 2, 2002
By early August, soybean fields begin to support a diverse community of insect defoliators. While plants are growing and producing new leaves, soybeans can tolerate considerable defoliation without a yield penalty. But during the early part of the reproductive stage, plants typically become more sensitive to defoliation. Soybean fields should be carefully monitored during pod development. Even at this stage of development (R4 to R6), soybean plants normally can withstand 20% defoliation. Obviously thresholds are not static and are affected by many factors such as the cost of treatment, anticipated market price of soybeans, and the level of stress that plants are under.

Defoliators to pay attention to in August include Japanese beetles, bean leaf beetles, grasshoppers, green cloverworms, and woollybear caterpillars. Defoliation by bean leaf beetles in soybeans appears as small, round holes in the leaves, quite different in appearance from the ragged, edge-of-the-leaf defoliation caused by grasshoppers, green cloverworms, and woollybear caterpillars, and also quite different in appearance from the lacy defoliation caused by Japanese beetles.

When scouting soybean fields for defoliators, don't confine your efforts just to field margins. Folks who walk only into field border rows may overestimate the level of defoliation. Often insects that move into fields will confine their feeding activities to these areas, resulting in a so-called edge effect. We recommend that a range of leaflets be examined for insect injury. Don't base your treatment decision just on leaflets collected in the upper one-third of the canopy. If pressed for time, you can probably focus on the top and middle portions of plants.

Finally, if a treatment appears warranted, based on overall defoliation, make sure that you have correctly identified the insect most responsible for the injury. Different insecticides may be labeled for different insects, and the rates of application also will vary by pest.--Mike Gray and Kevin Steffey

Soybean defoliation levels.

Bean leaf beetle defoliation.

Green cloverworm caterpillar.

Author: Mike Gray

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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