The yellownecked caterpillar is a common site on crab apple, pin oak and birch trees. Other fruit and shade trees may be infested also. This insect is named for the large yellow dot that appears behind the head (where the neck would be, if caterpillars had necks) of mature larvae. 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. Large larvae change to a black-and-white striped pattern and sparse, fine hair on the body becomes apparent. 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. Spraying large larvae that are nearly full grown is similarly futile as the damage has already occurred and the larvae will not be controlled before they quit feeding and move into the soil to pupate.
This article originally appeared in the July 29, 1994 issue, p. 122.