Oakworm moths, the adult version of the California oakworm (Phryganidia californica), are ubiquitous throughout the coast and live oaks seem to be their preferred target, particularly in the Monterey Bay area. Infestations of live oak trees have been described as clouds of pale brown moths and leaves covered in yellow/green worms, with some trees all but defoliated of their leaves.
The adult oakworm moth is uniform tan to gray/silvery and distinguished by its prominent wing veins. The body is about 1/2-inch-long, and the wingspread is about 1-1/4 inches. Unlike females, males have feathery antennae. Young oakworms are small, yellowish green caterpillars with large, brown heads and dark stripes on their sides. Older caterpillars vary in color, commonly dark with prominent, lengthwise yellow or olive stripes.
The University of California advises against any control measures for healthy oaks that can tolerate the defoliation imparted by these insects. Damage will vary wildly in accordance to the yearly population. Stressed trees may need protection because of excessive defoliation, abundant oakworm moths that are deemed not aesthetically tolerable, or oakworm moths that begin to target trees that are not oaks. UC recommends an integrated pest management (IPM) program that relies on biological controls and good cultural practices. Regularly inspect oak foliage for oakworms and, only if absolutely necessary, employ pesticides when caterpillars reach levels warranting treatment.
The following University of California article summarizes these management options and provides an excellent background on the California oakworm life cycle:
UC ANR/UC IPM/Pests in Gardens and Landscapes/California Oakworm
For some context from the northern part of the oakworm’s range, the Marin Independent Journal published an article in May 2022, citing the Marin County Master Gardener Program and the UC Integrated Pest Management website:
https://www.marinij.com/2022/05/13/dealing-with-californias-very-hungry-caterpillar-the-oakworm/
Finally, the University of California Oak Woodland Conservation Group has published an excellent guide on management and preservation of oak woodland habitats: