This was found feeding on an ailanthus for last couple of days. Entire plant is infected and has been devoured by these insects. Worrying about ferns underneath. Sprayed with Safer Insecticidal Soap with no luck and also pruned back infected limbs.
We can find no insect matching your photos which feeds on ailanthus. However, the caterpillar looks like a butternut wooly worm. It feeds on walnut trees, whose leaves look very similar to ailanthus leaves. Could the tree have been misidentified?
Butternut woolyworms only feed on walnut and butternut trees. There is no danger to the ferns.
Ailanthus is a non-native invasive species which is damaging to wild and park areas and the natural environment in general. We recommend replacing them with other species of trees.
If you send us photos of the leaves, we can be sure the tree is an ailanthus.