Hi Casey,

From what I can see in the photo, I think those are the caterpillars of the Mourning Cloak butterfly (Nymphalis antiopa).

These are large, attractive butterflies--mostly dark brown/black with a bright yellow band along the wing edge. The wings can take on a purplish tint. It's unusual among butterflies in that the adults are the overwintering stage; they lay eggs encircling small twigs and branches in early spring. The newly hatched caterpillars will feed in groups at first, moving to individual lifestyles when larger. Adults from these caterpillars will be flying in July or August, and they will lay eggs that will develop into a second generation this year, the adults of which will overwinter.

The caterpillars will eat the leaves of your cottonwood, but the trees (unless very new) are unlikely to be affected in the long-term. Natural enemies will likely reduce the number of caterpillars fairly quickly, and an established tree would need multiple years of complete defoliation to be harmed in the long term.

If your trees are small and you're a fan of butterflies, you might consider gently pruning a twig that they're on and moving them to a more mature cottonwood.

I hope this is helpful!

Cordially,

John