The most effective way to make treatments decisions for pests with chewing mouthparts (e.g., Japanese beetle, bean leaf beetle, caterpillars, and grasshoppers) is to estimate defoliation. I’m a “lumper” and don’t try too hard to distinguish feeding by species. Replicated data over multiple growing regions consistently shows economic thresholds for soybean are 30% in the vegetative stages and 20% in the reproductive stages. This threshold applies to the entire field. Sometimes these pest cause significant injury along field margins and perimeter treatments may be more cost effective if practical.
In a recent ICM News article, I provide a scouting guideline (Figure 1) to help make accurate defoliation estimates. In general, humans tend to over-estimate defoliation and treat more often than necessary. Practice estimating defoliation with Figure 2. I also recently published a review article for Japanese beetle if you want to learn more about this corn and soybean pest.