The yellownecked caterpillar is a common pest on crab apple, pin oak and birch trees in Iowa. It is also known to feed on elm, linden, honeylocust, maple, and fruit trees. Adult moths lay clusters of eggs on the backside of leaves in July. These eggs hatch into tiny caterpillars that remain in a cluster as they feed on the foliage. Small larvae are purplish with slender white stripes. As the caterpillars mature, they scatter throughout the host tree and feed individually. Full-grown larvae are 2 inches long and are black with white stripes. They have a more-or-less prominent orange-yellow mark behind their head for which the species is named. When disturbed, yellownecked caterpillars raise their head and posterior tip of their bodies making a distinctive "U" shape.
Remedial control of yellownecked caterpillars is not usually warranted. Treatments 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. Birds, predaceous insects and parasitic flies are natural enemies of the yellownecked caterpillar.
The bacterial insecticide Bacillus thuringiensis (Bt) is effective only when sprayed on small caterpillars. The caterpillars must eat the spray residue to be killed. Larger caterpillars can be controlled with sprays of Malathion, permethrin, Orthene or Sevin. Read and follow label directions, especially for applications to fruit trees.
This article originally appeared in the August 10, 2001 issue, p. 103.