As if the mysterious and unidentified "Sudden Oak Death Syndrome" disease killing tan bark oaks and some coast live oaks in West Marin is not enough, oaks throughout the area are now being defoliated by the so-called California Oak worm (Phryganidia californica). As you can probably guess from the name this is the native oak moth once again at a high year in its cyclical population. There are millions of the caterpillars out there eating oak leaves, soon to become clouds of moths.

Residents throughout the area are saddened looking at their beloved but leafless trees and wondering what to do. Fortunately healthy oaks will generally grow new leaves after a single defoliation and my recommendation is not to be too terribly concerned—but realize that it will take a tremendous amount of a tree’s stored energy to do this. If the second crop of leaves is again defoliated by worms then it is time to become concerned, as three years of defoliation will generally kill an oak tree. For those worried about oak trees around their houses Bolinas Arborist Ray Moritz notes that the oak worms "generally do not do any significant damage to the trees, but they do weaken them. There are several low-cost treatments. For stressed trees try application of insecticidal soap such as ‘Safer’ or pyrethrin based (non-toxic) insecticides." Bacillus thuringiensis (Bt), developed to combat the voracious gypsy moth in northeastern US, is a natural biological agent which is effective and, except to moths and butterfly larvae, is non-toxic. 

A totally defolitated live oak

Tom Kent, arborist of Inverness, is not too concerned about the worms, except to note that many of our oak trees are already weakened by the Sudden Oak Death Syndrome, which results in lethal infestations of wood boring Ambrosia beetles and bark beetles. This oak mortality is now showing up in several areas of coastal California and the cause is still unknown. There are no effective chemical treatments. Kent feels "it is more important to remove the dead tan oaks than it is to spray live oaks. Remove dead tan oaks and render them into small chips to fragment the (Ambrosia beetle) population. Buy rolls of clear plastic 20’ wide and carefully seal in the beetles so the sun will cook them" rather than allowing new generations of insects to infest nearby trees.

Partial defoliation in live oak—caterpillars are still active

As with all tree problems, think in terms of overall health. All trees need undisturbed root systems in well aerated soils to support well structured stems and crowns. Favorable environmental conditions, sufficient nutrients and plenty of space and light are necessary for trees to ward off insects and disease. Although consuming a lot of soil moisture established oak trees normally survive best without summer watering, which tends to spread damaging root fungi. Your trees naturally harbor a great variety of insects and fungi, and they have mutually beneficial relationships with many of them. If your trees show chronic signs of stress you might want to fertilize with an N-P-K fertilizer and mulch lightly within the "drip zone". If you are concerned about a particular tree use common sense in diagnosing the problem or call Cooperative Extension or other experts for advice.

Like people, all trees do eventually die. Standing dead trees and fallen woody materials in native forest settings provide valuable habitat for insects and decay organisms and forage for a great variety of wildlife—although in some forest types contributing significantly to fuel loading which increases fire hazard. In fire prone areas like Inverness homeowners must be sure to maintain fire safe zone 30-100 feet around dwellings.

Tan bark oak suffering from defoliation and threatened by Sudden Death Syndrome
 
 

Remember, these trees are part of our native forest. They survived millenia without any human intervention and then adapted well to native American influences such as regular burning. The biggest killers of trees around houses are humans who compact soils, disrupt natural conditions, spray chemicals and injure trees by improper pruning and other mechanical injuries. In forested areas we have increased tree crowding and reduced tree vigor by disrupting the cycle of periodic fire and injecting our homes into previously undisturbed ecosystems. If you can help your native tree by providing a "natural" place to live, it will probably be okay.