John McClenahan, a fourth-generation arborist and pest-management adviser, and vice president of S.P. McClenahan in Portola Valley, says he has received a lot of calls lately about the brown, one-inch wide moths because they are so visible. But it's actually the caterpillars that cause damage.
He predicts the current batch of eggs will hatch around the end of July. The caterpillars eat oak leaves to the point of defoliation, which, he says, "is a stressor, but one or two bad infestations is not going to kill the tree."
Mr. McClenahan says the trend is not to spray with pesticides. Some clients, however, opt for a preventative treatment, which starts at $125 per tree. Natural predators, such as birds, spiders and yellow jackets, can be effective in cutting back the population, too.