Most commonly found east of the Rockies, these pests are not worms at all; they are the larvae of certain moths. They strip foliage from trees and shrubs, starting at the top of the plant and eating it bare. They weave dangling silken bags as they feed. Later, they retreat to the bags, pupate, and emerge as moths. Females remain within the bags to lay eggs; caterpillars hatch in spring, often blowing from one tree to the next on thin threads.
Bt is the preferred control. Spray when caterpillars are just emerging, since they feed heavily at this stage. If the infestation is light, cut off and burn infected stems and branches.
Chemical controls include acephate, carbaryl, diazinon, and malathion; apply them when you see worm-infested trees.