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A healthy tree is the best deterrent for California oakworm

The California oakworm has chewing mouthparts that make distinct holes in leaves.
The California oakworm has chewing mouthparts that make distinct holes in leaves.

Q: There are worms hanging out of my oak trees, and the leaves on the trees look terrible. What’s happening? — Nelda Amspacher, Arroyo Grande

A: One of the most striking features of our lovely Central Coast landscape is the abundance of magnificent oak trees. Unfortunately, there are a variety of threats to these precious woodlands. The most likely cause of the problem you describe is the California oakworm, the most important oakfeeding insect in our area. Outbreaks consist of two to three generations per year and vary greatly in severity, being very serious some years and virtually absent in others. Infestations occur every five to 10 years and generally last about two years.

The cycle begins (and ends) with an adult oak moth, tan or gray, with a 1-¼ inch wing span with prominent wing veins. It lays clumps of tiny round eggs on the underside of leaves. The eggs hatch into small caterpillars with large heads and prominent lengthwise stripes. When grown, they enter the pupal stage, hanging suspended in a colorful case, or chrysalis. When mature, they emerge as moths.

The damage done by the caterpillars appears more severe as they grow. Young caterpillars skeletonize the leaves while older ones eat right through and eventually kill leaves. Once an oak has been completely defoliated the caterpillars may drop from the trees and crawl away, looking for food sources and occasionally even entering houses. Despite the general “yuckiness” of having hordes of caterpillars climbing up your drapes, this is simply a nuisance and not harmful. Eventually, without oak leaves to eat, the caterpillars will die.

A word about spraying chemicals to control oakworms don’t.

Although affected oaks can look pretty dreadful, healthy trees can survive this exfoliation and will not be seriously harmed. The best management, then, is maintaining a healthy tree. Any control measures should be based on good cultural practices and encouraging the health of the oakworm’s natural predators such as birds, spiders and many other insects.

Inappropriate irrigation, trunk or root damage and changes in soil are much more damaging to a tree than the oakworm. Spraying is almost never appropriate, and if absolutely necessary should be handled by professionals using the least toxic products available.

For information on proper care of oak trees and more about management of California oakworm, see University of California ANR Publications website for a free download. Go to http://anrcatalog.ucdavis.edu and type in publication No. 7422. If you need a printed version or need more information, as always, the Master Gardeners are happy to answer your questions.