Wednesday I found Bean Thrips (Caliothrips phaseoli) in low numbers of less than 1 per leaflet on the leaves of soybean plants. The top 2 pictures show minimal feeding injury along the leaf veins. Bean Thrips can be found on the lower half of the plant first, and will move up the plant as populations increase. They tend to be found on the upper leaf surface more than the lower surface.
I have seen thrips have been a pest of soybeans in the past during dry years. In 2009, dry conditions resulted in very low soybean yields. Many fields were also infested with bean thrips. These fields had significant defoliation at populations above 10 thrips per leaflet.
My economic threshold is 10 thrips per leaflet or 30 per trifoliate, but I think a lower number may be justified. Maybe 20 per trifoliate.
Below are some pictures from 2009 soybeans:
Below are reports from the 2009 research trials:
Most soybean fields are blooming and will soon begin to form pods (R3). This is the beginning of the damage window for stink bugs and caterpillars. We will be scouting fields with a sweep net to find stink bugs. Treatment is justified with 36 of the stink bugs per 100 sweeps. However, for red banded stink bugs the threshold is much lower: 12 bugs per 100 sweeps. I count nymphs and adults equally.