The adult is called the California oak moth and is a grayish-tan moth about one-half to three-quarter inch long when the wings are folded. The worm, about one inch long at maturity, has a reddish-brown head with a black body with yellow racing stripes down the center of its back. When the worms mature, they often leave the oak tree to pupate on the underside of lawn chairs, picnic tables, house eaves or any other place they can find a little shelter. The second generation moths will emerge in fourteen to twenty days, and they will start laying eggs in June. Oak worms are found all over the Central Coast where native oaks are growing. In some years natural predators and parasites are sufficient to suppress the population below nuisance levels. However, in many years, oak worm populations have exploded to levels where trees are completely defoliated, and the pupating worms become a major annoyance as they drop from trees on their silk threads and fall into pools or pupate all over the house and patio.
The adult moth is attracted to “bug-zapper” lights at night. But if you live in an oak woodland area, you will attract moths from a wide area, which may end up making your problems worse instead of better. If oak worms/moths are a yearly problem, the best treatment is to spray landscape oaks in the spring and in June as soon as worm activity is noted. NOTE: Mark your calendar now to begin making observations in mid-March. Young larvae are most susceptible to Bacillus thuringiensis, but because they only scrape the lower leaf surface, spraying is less effective at that stage unless the underside of leaves is thoroughly treated. Spray treatments are more effective if foliage is thoroughly sprayed when larvae are first observed chewing completely through leaves or chewing at the leaf edges. One more NOTE: If you spray your trees, you should understand you may have the only green (leafy) trees in the landscape. These will be a strong attraction for the second generation moths, so be prepared to make repeat applications.