Bean leaf beetle thresholds

The first generation of bean leaf beetles is making an impressive appearance in soybean. Last week, Brad Buchanan, Crop Tech Services, reported fields in Henry County in southeastern Iowa with populations of 50-80 beetles per 20 sweeps. In Story County on July 17, I collected an average of 30 beetles per 20 sweeps with one count as high as 49 beetles in 20 sweeps. These numbers suggest that this first generation will contribute to the buildup of a significantly large second generation that will cause economic damage to soybean pods in these fields.

In the July 10 ICM newsletter, I helped write an article that discussed sampling of the first generation to predict the size of the second generation. This concept has created some confusion. Tables 3 and 4 in that article refer to the size of the bean leaf beetle population that is occurring in the field during mid-July. These are first-generation beetles. If this population exceeds the thresholds given in Tables 3 or 4, then the field should not be sprayed now, but instead sprayed later during the second generation of beetles, which will appear sometime in mid-August. Sampling the fields now will help you predict the possibility of economic pod damage at the beginning of pod growth and development a month from now.



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Bean leaf beetle, yellow phase.


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Some bean leaf beetles are red instead of yellow.


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Bean leaf beetle feeding on soybean pod.

This article originally appeared on page 143 of the IC-484(19) -- July 24, 2000 issue.

Updated 07/23/2000 - 1:00pm