The yellownecked caterpillar is a common site on crab apple, pin oak and birch trees. Other fruit and shade trees may be infested also. Small caterpillars do not have the characteristic yellow spot though a light colored area may be visible.
Small larvae are purplish, with slender white lengthwise stripes.
Very small larvae feed in a cluster but as they grow they disperse throughout the entire tree. Full grown length is approximately 2 inches. Control of yellownecked caterpillars may be difficult to justify. Small, newly-transplanted or stressed trees would benefit most from protection by spraying with Bt, malathion, Sevin, Orthene or Dursban, particularly while the larvae are small. Spraying large, well-established trees is not usually necessary as occasional defoliation will not be fatal.