Small larvae are purplish with slender white stripes. They grow to 2 inches in length and turn black with white stripes. They have a more-or-less prominent orange-yellow mark behind their head for which the species is named.
Yellownecked caterpillar moths lay clusters of eggs on the backside of leaves during July. These hatch into tiny caterpillars that remain in a cluster as they feed on the foliage. The larger caterpillars scatter throughout the tree and feed individually.
Control of yellownecked caterpillars is not usually warranted. Control late in the season (past mid-August or when caterpillars are longer than 1 1/2 inch), when larvae and damage are most easily noticed, would be particularly difficult to justify. Small, newly-transplanted or stressed trees would benefit most from protection. Young larvae that are still in clusters can be removed by hand, often by pulling off a single leaf or pruning off a single terminal during mid to late July.
Most home landscape insecticides can be used to control yellownecked caterpillars when warranted. Spraying while caterpillars are small and before extensive defoliation has occurred is preferred.