In addition to soybean aphids feeding on soybeans in late July and early August, other insects may be feeding on the leaves as well. The three most common groups of leaf defoliators will be bean leaf beetles, Japanese beetles, and grasshoppers. Sometimes these insects cannot be found on the leaf or plant, so recognizing the injury they cause can be helpful in identifying the cause of the feeding. Each group has a fairly unique feeding pattern that makes it relatively easy to distinguish one from another. These feeding types are described below.
Bean leaf beetle leaf injury
Adults typically eat round or "oval" holes about the diameter of a pencil, although these holes may occasionally be slightly larger. Often there is still green tissue separating the holes.
Japanese beetle injury
Adults typically eat very small holes in the leaf, which results in a "net-like" appearance. The holes are nearly oval and often connected by a fine network of leaf veins. However, the Japanese beetle does not occur in all Iowa counties but has been "officially" recorded in only 23 counties and mostly near metropolitan areas (see map). Therefore, injury from this insect would not be expected in many soybean growing areas.  
Small grasshopper nymphs will eat very small holes in the leaf, similar to the injury caused by Japanese beetles, but the holes are often jagged and not very oval in shape. Older nymphs and adult grasshoppers will consume much of a leaf so that only the major leaf veins and small amounts of connecting tissue remain.
Defoliation thresholds exist for soybeans, and it should be remembered that soybeans can tolerate a fair amount of defoliation. Typically, 20-25 percent defoliation of all the leaves (not just the leaves in the top of the plant) would be necessary to justify an insecticide application in late season, pod-setting soybeans. 
Marlin E. Rice is a professor of entomology with extension and research responsibilities in field and forage crops.
This article originally appeared on pages 244-245 of the IC-498(21) -- July 30, 2007 issue.