The following is information on the oak skeletonizer, an insect pest of oaks in DE.
The oak skeletonizer is a native insect that has two generations per year in our area and is active from 547--2846 GDD base 50. The second generation is usually sometime during late July to August. The larvae are yellowish green and prefer to eat red oaks, but also feed on chestnuts and other oak trees. Early instars feed as leaf miners, but older larvae feed externally and skeletonize the leaf. Mature larvae drop from leaves on silken threads when disturbed. Populations vary by year, thus some years damage may not be noticed. Severe infestations or repeated attacks by this insect can result in crown thinning, die back, and possibly increase the trees susceptibility to other pests such as borers. Larvae form white ribbed cocoons on the leaves, twigs, or other nearby structures. Control options include B.t., bifenthrin, deltamethrin, cyfluthrin, or cyhalothrin.
Oak skelatonizer larvae, cocoons and defoliation on underside of leaf. Photo from James Solomon, USDA Forest Service, Bugwood.org
Information from Brian Kunkel, Ornamental IPM Specialist, UD