Corn Earworm (a.k.a Podworm)
Corn earworm damage to soybeans occurs from August through September. Significant damage may occur when large larvae feed on pods consuming the developing seeds. Fields should be checked for small worms as flowering begins. Control measures should be implemented when an average of one small worm per foot of row is detected.
Source of information
J.P. Michaud: http://entomology.k-state.edu/extension/insect-information/crop-pests/soybeans/cornearworm.html
Pest photo source
Wendy Johnson, KSU Entomology