Note: This post was originally published in July of 2010, but once again in 2012 there are reports of Japanese beetles feeding on soybeans. In particular it would be worthwhile to go and click on the article below that shows what the leaf feeding insects look like along with threshold values.
There are quite few calls coming in from people who are finding either Japanese beetles or redheaded flea beetles in their soybeans, feeding on the leaves. Though the feeding might be alarming at first, rarely does this defoliation actually reach threshold levels. Most of the defoliation happens at the top of the plant which is why it is easy to overestimate what damage is taking place. Soybeans are great for compensating for the defoliation. Leaves under the ones with holes grow larger to take advantage of the extra sunlight penetrating the canopy thanks to those holes.
Here is an article on assessing defoliation and what the defoliation thresholds are. It also includes pictures of most of the leaf feeding culprits.
Article: Soybean Defoliators and Thresholds