What kind of caterpillars eat leaves from birch trees?

The walnut caterpillar is closely related to the yellownecked caterpillar. Its favorite food trees are pecan, hickory, walnut, and butternut. Walnut caterpillars, like the yellownecked, feed in colonies, moving from branch to branch. Young larvae are red to purplish with white longitudinal stripes; fully grown larvae are black, 2 to 2¼ inches long, and covered with long white hairs. Two caterpillar broods occur each year, one in late May and June, and one in late summer (mid-July through September).