At the suggestion of Beatriz Moisset, I started going through the Unidentified Sawfly Larvae
gallery looking for host plants that I (as a botanist) might recognize. I did spot several plants that I could identify, including dogwood, raspberry, dandelion, aster, and grasses. Of special note was the following series of images occurring on grass:
(Actually the last one on the right is dandelion, but it often occurs in lawns and fields with grass where the larva may have climbed up the "wrong" plant. My reason for including it comes next.)
The images come from Washington, Oregon, Michigan, Pennsylvania, West Virginia, and Maryland and each one features a different species of grass, but when I compiled them together, the larvae appeared to strongly resemble each other. Are they possibly all the same widespread species?