Walnut and Yellow Necked Caterpillars Walnut and Yellow Necked Caterpillars are similar to one another except that the walnut caterpillar feeds on walnut. Yellownecked Caterpillar feeds on most deciduous trees. White egg masses, each with up to 100 eggs are easily visible on leaf undersides. Young caterpillars will first skeletonize leaves, and then strip them of foliage as they mature. Younger larvae are brick-brown with yellow stripes along the body.
Older larvae are black with yellow stripes and up to two inches long.
Late season defoliation does not significantly harm tree health, but can be unsightly. After spending the winter as pupae, adults emerge in June and July and lay egg masses on leaf undersides. These eggs hatch in late July into caterpillars that feed in groups through August. Large caterpillars pupate in the soil where they spend the winter. Look for young caterpillars in July and treat before caterpillars become large and cause major damage.