Extension crop specialist reports from across the state indicate that for the third week of May, bean leaf beetles were causing the most injury to soybean in central, south central, southwestern, and western Iowa. Very few problems were reported from the rest of the state. Early-emerging soybean attracted large populations of the overwintered beetles and their feeding was heavily scarring the cotyledons and defoliating the young leaves.
Many questions have been asked about the wisdom and economics of treating soybean for these early-season bean leaf beetles. Even though the leaf feeding may look terrible, a more important concern is the possible transmission of bean pod mottle virus by the beetles. As I mentioned in the May 14, 2001, ICM newsletter (pages 73-74 ), research into this problem is continuing but currently we don't have answers to many of the questions. My recommendations for deciding whether to use an early-season insecticide application should be based on answering "yes" to the following three questions before spraying:
- Are bean leaf beetles present in the field?
- Are plants in very early growth stages (VC-V2)?
- Were bean pod mottle virus symptoms confirmed or strongly suspected (i.e., green stems at harvest) in your soybean fields last fall?
Some of you will want to wait and spray the beetles at the same time that you apply a burndown herbicide to soybean. If soybean seedlings are beyond the V2 leaf stage, an insecticide application will probably be too late to be effective.
|Bean leaf beetles on early-emerging soybean.|
|Heavy feeding by bean leaf beetles causes scarred cotyledons, defoliated leaves, and stunted soybean seedlings.|
This article originally appeared on pages 87-88 of the IC-486(11) -- May 28, 2001 issue.