California oakworm (larvae)
California oakworms are the larval (caterpillar) stage of the oak moth. These caterpillars feed on oak leaves, especially those of the coast live oak (Quercus agrifolia). In some years, populations may become very high and certain trees may become completely defoliated. According to the University of California, healthy oaks can generally survive even heavy infestations, so control measures against these caterpillars are not usually necessary.
Another issue with California oakworms is related to the accumulation of tiny, pellet-like frass (second picture) under and around trees. If you stand near a heavily infested tree and listen carefully, you will hear the gentle sound of the frass as it falls from the tree's foliage onto the ground. Any accumulated frass can simply be swept or washed away from walkways/decks or outdoor furniture.
One resident from Sonoma County brought in a large number of oakworms that had wandered onto her deck. She was concerned that the oakworms would damage the deck and outdoor wooden furniture by boring holes or feeding on the wood. Although the sudden presence of several hundred caterpillars on a deck may be unsettling, these critters are adapted for eating leaves and will not eat or bore through wood. The caterpillars probably left their oak tree either to find a new source of food (another tree) or to find a site for pupation.
Click here to view the University of California's management guidelines for California oakworms.