Caterpillars on oak trees are rarely a significant problem. Oaks are acorn-bearing trees that grow in most habitats. Occasionally they endure infestations of caterpillars during summer and spring, but usually they recover and develop new leaves with no help from gardeners. Caterpillars are the young types moths, butterflies and other insects.
California Oakworms
Despite their name, California oakworms (Phryganidia californica) are actually caterpillars. When recently hatched, they are yellowish-green with dark-brown stripes along their sides and dark-brown heads. They climb up to 1 inch long and mature to dark-colored caterpillars with olive or yellow lengthwise stripes. Coast live oak trees (Quercus agrifolia) are their principal source of food. Heavy infestations of these caterpillars on trees typically are followed by mild infestations, giving trees period to recover.
Fruittree Leafrollers
Fruittree leafroller caterpillars (Archips argyrospila) are green, about 3/4 to 1 inch long and have a dark or brown mind. They feed on oak buds and developing leaves, drawing them together with silken threads to form a protective casing. When disturbed, a fruittree leafroller frequently wriggles and falls, suspending itself by a thread. Mature fruittree leafroller caterpillars eat whole leaves, leaving just leaf skeletons. A heavy infestation of these caterpillars can eat the leaves of an whole tree.
Western Tussock Moths
The young of tussock moths are hairy caterpillars, and it is best never to touch them since their hair is quite irritating. The Western tussock moth (Orgyia vetusta) caterpillars have four white tufts that stick and red spots on their sides. They consume the young growth of oak trees for about six to eight weeks before spinning cocoons that also contain irritating hairs. All these caterpillars develop from 1/2 to 1 inch long.
Control
Oak caterpillars have a lot of predators, including parasitic wasps and flies, birds, spiders and yellow coats. Viruses and fungi also attack them. Healthy oaks typically survive caterpillar infestations readily, but trees afflicted by drought or overfertilizing are vulnerable. Broad-spectrum sprays aren’t acceptable to control the caterpillars since they frequently kill beneficial insects. A microbial insecticidal spray such as Bacillus thuringiensis (Bt) is used to control oakworms, according to the University of California Integrated Pest Management Program. Bt causes the caterpillars to stop feeding and die.