The eggs are extremely hardy and easily survive Minnesota winters. The caterpillars have five growth stages, each stage lasting seven to ten days. During the early stages, caterpillars remain in clusters on the leaves. Caterpillars are blue-black with white spots down the back. Long brown hairs sparsely cover the body.
Older caterpillars develop a deep blue velvet coloration with a sparse covering of long brown hairs.
A line of white to cream colored spots runs down the back. These spots often look like footprints or a series of keyholes. As the caterpillars get larger, they consume increasing amounts of leaves and can wander widely in search of more food. They often drop from the defoliated twigs to the ground. Large caterpillars are solitary feeders, but commonly rest in large clusters.