Oak Leaf Skeletonizer
A leaf attacked by oak leaf skeletonizer (OLS) caterpillars is reduced to a veiny skeleton held in place by thin tissue of the upper leaf surface (Figure 1).
Skull-like empty head capsules littering the lower leaf surface bear mute testimony to molt of these small caterpillars as they grow. Accumulations of black pepper-like waste pellets cling to the silk shrouds (Figure 2) under which the ghostly pale caterpillars retreat to molt or to pupate.
OLS caterpillars are pale yellow green and about ¼-inch long when fully grown. When disturbed, they drop from leaves on silken threads.
OLS occurs over much of the eastern U.S., but its activity usually goes unnoticed. Most of the time populations are held in check by weather or natural enemies. A recent sample from south central Kentucky showed extensive damage by this species and could mean population levels are up in other locations. Feeding by the first of two generations is nearing completion, but a second generation occurs during July and August.
Control of this insect may be warranted on small, newly established landscape trees, but several consecutive defoliations would be necessary to affect the health of larger, established trees. General tree and shrub insecticides that list caterpillars on the label can be used if control is necessary and practical. Support tree health by watering if needed and fertilizing as recommended.
Oak Slug Sawfly Larvae
Oak slug sawfly larvae cause similar skeletonization of leaves at about the same times that the two OLS generations are present, but there are some noticeable differences. Slug sawflies chew their way across the underside of leaves in a shoulder to shoulder wave (Figure 4)
while OLS caterpillars feed in isolated patches that can coalesce. In addition, there are no silk pads on the undersurface of infested leaves where sawflies have fed. This can be an important distinction because sawflies are larvae of plant feeding wasps and are not affected by Bt sprays that might be selected for caterpillar control. Infestations usually are confined to scattered leaves.
By Lee Townsend, Extension Entomologist