|See those little white spots? Those are called Aphids.|
If you can see those little spots on the leaf in the picture, those are aphids. Aphids are fairly new the Midwest; they showed up in 2000. Originally, they were found in Asia. One of the trickiest tactics by the Aphids is the ability of females to reproduce genetically identical clones.
The Aphids specialty is feeding on the plant by sucking out “plant juices.” Typically, when the soybean plant is already stressed by other factors in the field, such as a drought, the Aphids can really have a big impact on a field. Estimates of damage range from 10 to 15% lower yields. Lower populations of Aphids are harmless, but if the conditions are right and the population goes up, the yields go down.
There are natural enemies of the Aphids, including ladybugs! Unfortunately, higher populations can be too much for natural predators, so they are not always helpful. On the other hand, rain is helpful, because it essentially just washes the Aphids off the leaves. Obviously, they can come back, but it helps. Of course, for a more complete coverage, there are insecticides that can be used if the Aphid populations get too high.
(Sources: Purdue Extension Office, Penn State College of Agricultural Sciences.)