There are several caterpillars that feed on oaks including the California oak worm (Phryganidia californica) and tussock moth (Orgyia spp.). Each of these caterpillars feed on oak leaves, primarily in the spring, but can be active as second generation insects in late summer or early fall. Leaves will appear skeletonized in early stages and then completely consumed as larvae mature. Feeding is usually not noticed until frass or fecal pellets fall onto underlying patio furniture or cars where they are considered a nuisance.
Oak worms are smooth, small, yellow-green caterpillars with brown heads and dark stripes down their sides. They can range from 1/10 to 1 inch in length throughout their development. Tussock moth larvae are very distinctly hairy with three prominent creamcolored dots towards the head capsule.
In California, oak worm is most commonly found on coastal live oak in San Francisco, Monterey Bay, Santa Barbara, and other areas close to water sources. Tussock moth is common in San Francisco but can also be found along the Central Coast.
Healthy oaks affected by oak worm experience defoliation in the spring and throughout the summer from one or more of these pests. Damage may appear sporadically or thoughout the entire canopy. Many leaves appear partially chewed (skeletonized) and will turn brown and die, while other leaves may be completely eaten. Oaks that are experiencing other stresses, such as drought, can decline from oak worm infestation at a quicker rate. Look for signs of leaf feeding or fecal pellets around the base of the tree for activity or worms raining down on you.
Info Source: Arborjet
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