While scouting fields for gall midge this week I have been noticing a lot of holes in soybean leaves. The majority of the defoliation is due to the second-generation bean leaf beetle adults. However, there are also fields that have large green cloverworm caterpillar populations present in them. A few other defoliators that I have also been seeing include grasshoppers, colaspis beetles, yellowstriped armyworms, and also yellow woolly bear caterpillars.
The defoliation threshold for this time of the year is an average of 20% defoliation for the entire plant (Figure 1). The majority of the fields I have been looking at are well below that threshold, and with current soybean prices an insecticide application for these pests might not be economical. However, in fields where more than one defoliator is present or bean leaf beetles and/or grasshoppers are feeding on pods an insecticide application may be necessary to prevent yield loss.
For soybean, it’s estimated that at 20% defoliation yield losses will vary from 3-7%. According to the USDA National Agricultural Statistics Service the average soybean yield in South Dakota is 43 bushels per acre, which means 3-7% yield loss is approximately 1-3 bushels. However, in some areas of the state, soybean yields are going to be higher and some are going to be lower. Based on 43 bushels per acre with current cash bids of $7.30 to $7.82 per bushel the average loss from 20% defoliation is between $7.30-$23.46 per acre. For areas that have received adequate precipitation this summer and yields are likely going to be above the state average, defoliation should be monitored more closely.