Oakworm infestation should pass without killing trees

There seems to be fairly widespread panic among local folks about this year's heavy infestation of California oakworm. Tree authorities are being overwhelmed with inquiries. We are all annoyed as we fight our way through worms on dangling silken threads, continually sweep up thick piles of frass [caterpillar droppings], brush caterpillars off our houses and outdoor furniture and observe trees that have been munched until they have no leaves left.

But these are temporary nuisances that will disappear when the insects soon move into the next phase of their life cycles. What matters in the long run is the health and survival of our trees.

Should we be concerned?

To put things in perspective, here are some facts about the oakworm. It is one of many species of caterpillars that feed on oaks, and is a native insect that evolved with the oaks, having been around for centuries. Its population rises and falls, with heavy outbreaks occurring at about eight- to 10-year intervals.

In our area, damage is most common on our coast live oaks (Quercus agrifolia). The adult is a tan to gray moth with prominent wing veins; males have feathery antennae. Young larvae have brown heads and are yellow-green with dark stripes on their sides. Mature larvae may vary in color but are usually dark with olive or yellow stripes. Pupae are 1/2-inch long, usually white or yellow with black markings, and are seen suspended from branches or leaves, or on other surfaces.

Ordinarily, two generations a year occur in our area. However, in years of unusually warm, dry winters, a third generation may occur. The insect overwinters as young larvae underneath oak leaves. The first generation matures in May or early June here, and we see the adult moths fluttering around the oaks in June and July. These females lay eggs that hatch into the larvae that we are seeing now, chowing down on our trees. The second generation of adults appears in October and November; these females lay eggs on the underside of leaves that will hatch into the overwintering larvae.

The burning question is, of course, "What should I DO about the oakworms?"

Authorities agree: Unless your tree is already stressed by drought or disease, it will survive even a complete defoliation, and will leaf out with abundant fresh new growth in winter or spring — spraying is seldom needed.

In "A Field Guide to Insects and Diseases of California Oaks," by Swiecki and Bernhardt, and published in July 2006 by the U.S. Department of Agriculture, the authors note that when defoliation is severe, the appearance of trees is temporarily damaged and that abundant frass is a nuisance. However, they also say, ""defoliation seldom, if ever, kills affected trees" Go to http://www.ufei.org/files/pubs/psw_gtr197.pdf to read their chapter on the oakworm.)

What you can do yourself is power-wash the tree, which will knock off many of the worms, stop their feeding, and immediately improve the tree's appearance. And keep in mind that population swings are normal, that this year's big population is part of a cycle.

The bright side? The extremely high numbers of worms defoliating our trees now are leaving very little leaf surface for larvae to successfully overwinter, leading to a probable population crash next year. Richard Merrill, former director of the horticulture program at Cabrillo College, taught his students that a healthy oak actually benefits from the worms' "pruning" since it removes old leaves and stimulates vigorous new growth. He also pointed out to his students that the heavy buildup of frass supplies needed nutrients in a readily absorbable form and is nature's own source of fertilizer for the trees.

But, you worry, maybe my particular tree is already stressed and needs help. If your oak showed signs of trouble before the oakworms began feeding this year, or if it is a cherished or heritage tree that is especially valuable, don't guess on whether to spray — get a certified or licensed arborist out to evaluate it.

A tree professional can accurately determine if treatment is warranted and what might be appropriate or effective to use. Most will recommend an IPM (Integrated Pest Management) approach that includes sound cultural practices, protection of natural enemies and possibly spraying of less toxic pesticides, such as BT (Bacillus thuringiensis) or spinosad. There really is no substitute for the investment in the advice of an expert, who can examine your individual tree, and make an educated evaluation of its particular needs.

To learn more, the Web site of UC Davis at www.ipm.ucdavis.edu is an excellent source of information on the life cycle of the oakworm and options for treatment. Also from UC Davis, read the article at www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74125.html on evaluating and hiring a pest-control company. The more you know, the better prepared you will be to protect your oaks, your family, your pets and your home and garden.

-Sharon Hull