Are those moths? What are they doing to my oaks? Why are they out in the daytime? If you live in the Santa Cruz area (or much of the Bay Area) and have coast live oak or other oak species on your property, you have probably been wondering these exact questions.
Bird? Plane? No-- California oak moth. You will likely see oak moths flying at the ends of branches of coast live oak trees for the next month or so in parts of Santa Cruz County.
2024 has brought another "outbreak" of the California oak moth/worm (Phryganidia californica). As a native species, we see explosions of oak moths in and around coast live oak trees every few years. While the adult moths (pictured below) are most visible right now, you may have seen them as caterpillars just a few months ago. The adult moths are now flying about, laying eggs which will again hatch and become caterpillars this year (the moths have two generations per year), and the process of defoliation will continue into the late summer.
In 2022, I watched closely as the oak moth caterpillars devoured leaves of coast live oak throughout the Carmel Valley - leaving behind defoliated trees that could be seen in vast swaths across the hills of the Valley. Luckily, just a few months later, coast live oak, as resilient as they are, were leafing out vibrant green foliage to replace those leaves in the canopy consumed by the caterpillars. And now, in 2024, I have observed the same phenomenon just outside my back door in Aptos.
Oak moths are not typically seen as a threat to healthy oak trees, so no action is generally the recommendation for property owners and land managers. Rather than considering eradication efforts, I would encourage you to allocate those funds to another forest stewardship project and watch closely and enjoy this natural phenomenon unfold -- the green of the fresh leaves in the Fall will be well worth it.
Further Reading
The Essig Museum of Entomology at UC Berkeley has a great website discussing recent outbreaks and displaying the defoliation and recovery of individual oak trees: https://essig.berkeley.edu/research/oakmoth/
KQED did a fascinating piece on oak moths in 2021: https://www.kqed.org/science/1972082/these-silk-swinging-caterpillars-will-ruin-your-picnic
UC IPM has more details on the ecology and managmenet of oak months, discussing when and how managmenet may be necessary in further depth: https://ipm.ucanr.edu/PMG/PESTNOTES/pn7422.html?src=AR16