Caterpillars on oak trees are rarely a serious problem. Oaks are acorn-bearing trees that grow in most habitats. Sometimes they suffer infestations of caterpillars during spring and summer, but usually they recover and grow new leaves without any help from gardeners. Caterpillars are the young forms moths, butterflies and other insects.
Despite their name, California oakworms (Phryganidia californica) are actually caterpillars. When newly hatched, they are yellowish-green with dark-brown stripes along their sides and dark-brown heads. They grow up to 1 inch long and mature into dark-colored caterpillars with olive or yellow lengthwise stripes. Coast live oak trees (Quercus agrifolia) are their main source of food. Heavy infestations of these caterpillars on trees usually are followed by light infestations, giving trees time to recover.
Fruittree leafroller caterpillars (Archips argyrospila) are green, about 3/4 to 1 inch long and have a black or brown head. They feed on oak buds and developing leaves, drawing them together with silken threads to form a protective casing. When disturbed, a fruittree leafroller often wriggles and drops, suspending itself by a thread. Mature fruittree leafroller caterpillars eat entire leaves, leaving only leaf skeletons. A heavy infestation of these caterpillars can eat the leaves of an entire tree.
Western Tussock Moths
The young of tussock moths are hairy caterpillars, and it’s best never to touch them because their hair is very irritating. The Western tussock moth (Orgyia vetusta) caterpillars have four white tufts that stick up and red spots on their sides. They eat the young growth of oak trees for about six to eight weeks before spinning cocoons that also contain irritating hairs. These caterpillars grow from 1/2 to 1 inch long.
Oak caterpillars have several predators, including parasitic wasps and flies, birds, spiders and yellow jackets. Viruses and fungi also attack them. Healthy oaks usually survive caterpillar infestations easily, but trees suffering from drought or overfertilizing are vulnerable. Broad-spectrum sprays aren’t suitable to control the caterpillars because they often kill beneficial insects. A microbial insecticidal spray such as Bacillus thuringiensis (Bt) can be used to control oakworms, according to the University of California Integrated Pest Management Program. Bt causes the caterpillars to stop feeding and die.
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