Today while scouting a commercial field north of Ames, I was looking for soybean aphid. I didn't see one aphid (sniff, sniff), but I did see a handful of thistle caterpillars. The caterpillars web together soybean leaves and feed inside the temporary home. They also produce copious amounts of dark brown frass pellets inside the webbed leaves. Thistle caterpillars are most commonly seen in the early vegetative stages but rarely build up to economic populations. Read more about thistle caterpillars in this ICM Article.
People don't like to see defoliators in field crops, but do like to see butterflies. In this case, the thistle caterpillar will turn into the painted lady butterfly.