Experts reassuring as moths strip oaks
Swarms cover trees in annual bloom, but no damage likely
Swarms of California oak moths are stretching along the thick stems of oak trees lining the edge of Mosswood Park in Oakland, weaving through the desolate branches, piling on the base of the tree, squeezing into the natural grooves of the oak, and resting on fallen leaves in the yellowed grass.
Thousands of them have hatched this week as part of an unusually large annual bloom, according to experts. And even more moths are on the way. Experts say they will strip the trees — but won’t do any real damage.
Roughly 20 to 40 percent of the moths are still in cocoons clinging to oak trees, said Damon Tighe, a member of the California Center for Natural History, a nonprofit naturalist collective based in the San Francisco area.
“This is the biggest bloom I’ve seen in 10 years,” he said. “We had a big rain season last year, so we had an enormous amount of plant growth, which may have populated the trees to allow moths a place to hatch.”
But the true cause of the huge bloom is mostly unknown.
Jerry Powell, a UC Berkeley graduate professor of systematic entomology, said that California oak moths sporadically hatch in blooms in Palo Alto, Santa Cruz and cities along the California coast, usually in July and August. A second bloom typically occurs in September and
October, he added.
“The reasons for that are not fully understood, but it’s been looked at by experts and students, and they have not met a firm conclusion as to why the buildup occurs and suddenly disappears,” Powell said. “Some populations appear in greater numbers than others.”
Powell, who specializes in the development of small moths, has spent 40 years collecting and studying larvae and adult moths across the state with his students. He said California oak moths hatch primarily on coastal oak and typically remain on or near the oak during their adulthood.
These moths either partially or completely defoliate oak trees, Powell said, but residents near Mosswood Park shouldn’t be concerned about the area’s arboreal health.
“It doesn’t result in the death of trees,” Powell said.
Liam O’Brien, an amateur lepidopterist, has studied endangered butterflies for two decades, and he said he’s seen homeowners call pesticide companies worrying about moths swarming oak trees during recent blooming seasons.
“People always freak out and think the moths will kill the
“People always freak out and think the moths will kill the tree, but they denude the tree exactly when the tree can handle it.” Liam O’Brien, amateur lepidopterist
tree, but they denude the tree exactly when the tree can handle it,” O’Brien said.
Officials with the California Center for Natural History confirmed that the oak trees should be able to recover from defoliation.
At Mosswood Park, hundreds of chrysalides — cocoonlike shells protecting evolving moths inside — still line the smooth bark of a tree towering in front of a gated-off Victorian-style house overlooking the park. Tussock moth cocoons, another species of moth, are also found clinging to the bark along with their California oak neighbors.
Powell said the moth swarms may be more noticeable to locals during blooming season, because they harbor a chemical defense mechanism against predators like birds. As a result, moths are able to feed openly on tree bark and aren’t limited to flying under the security of darkness at night.
“It’s unique and really unusual when you look at the thousands of (moth) species that occur in California,” Powell said, “and this is the only one that has a relationship with its surroundings where it appears and disappears.”