Today’s caterpillar is a little confusing in a a couple of ways. This individual is either Erynnis juvenalis (Juvenal’s duskywing) or E. horatius (Horace’s duskywing). Both species look pretty much identical as larva and both use several oak species as host plants. I’m no expert but it seems that these are best identified to species in their adult form.
The second confusing thing is that I found this individual while beat-sheeting a small ash tree (Fraxinus sp.) in mid-July at Onondaga Cave State Park in Crawford County, MO. From my understanding, both species are obligate oak feeders. This might mean it had fallen from a higher oak and landed in the ash tree, or it might be pre-pupal and looking for a place to pupate. Most often Caterpillars move from their host plants to pupate in a neighboring tree or plant.
If you have any thoughts or tips on how I might discern this guy to species with just a couple of simple photos and the information I supplied, please let me know.
A Thousand Acres of Silphiums 