Issue date: February 02, 2000

So far, oaks here are safe: Beetle infestation has not spread to the Peninsula So far, oaks here are safe: Beetle infestation has not spread to the Peninsula (February 02, 2000)

By ELAINE LEVINE

Extensive damage to live oak trees -- from ambrosia and oak bark beetles -- which emerged last spring in Marin and Santa Cruz counties, has so far not spread to the Peninsula.
Tree specialists are watching for symptoms here, hoping an epidemic is not in the future.
The pests appeared very suddenly last spring, wreaking havoc among the California live oaks in forests and gardens from Sausalito to Novato, reports Dr. Pavel Svihra, University of California Cooperative Extension in Marin.
The beetles are not new pests, but the epidemic is a new phenomenon, says Dr. Larry Costello, UC's tree expert in San Mateo County.
The oak ambrosia beetle and the western oak bark beetle are common throughout California, found ordinarily in firewood and in dead and dying trees. The rise in beetle population and the attacks on apparently healthy trees have not been seen before. So far, tree care companies have not reported unusual numbers in San Mateo County.
The problem was noticed first in tanbark oaks. Several hundred tanbark oaks died suddenly in the Mill Valley, Mount Tamalpais and Inverness area five years ago, leading to a build-up of bark and ambrosia beetles. The beetles then attacked live oaks, even healthy ones. Dr. Costello thinks the tanbark oak and live oak attacks are not necessarily related.
Usually the bark beetles attack severely injured, dying or dead trees. They reproduce in great numbers, primarily in oak firewood, emerging to attack oaks and tanbark oaks in the landscape.
Ambrosia beetles also attack dying, weakened or diseased trees, but most prefer just-killed trees or parts of trees. They often finish the job begun by bark beetles.
Both beetles kill trees by burrowing in the tree and introducing a fungus that plugs the arteries that carry food and water.
Once the beetles are inside the tree, there is no known treatment.
The best preventative is learning the habits of the beetles and proper oak health care, says Dr. Svihra. To care for oaks, he suggests:
**Avoiding heavy intermediate pruning cuts.
**Removing dying, dead and damaged branches.
**Preventing damage by insect defoliators such as oakworm and tent caterpillars, which are on the rise. Spraying with Bacillus thuringlensis (Bt) can provide control.
**Reducing damage to roots and root crown caused by frequent irrigation in summer. Oaks require little or no water in summer unless they have been stressed by drought. In drought years, water in summer only every six weeks, using a soaker hose at right angles to the slope, under the drip line of the tree to a depth of 2 to 3 feet. But not on the trunk.
**Minimize soil compaction to prevent root diseases caused by fungi.
**Spraying trunks of high-value ornamental oaks, oaks whose root system was disturbed by construction, and oaks with initial beetle attacks, with the insecticide permethrin (Astro is the commercial name) in March and the end of September. Dr. Svihra concedes the chemicals are only mildly successful as preventatives, and don't kill beetles already in the tree.
Symptoms of beetle attacks

To look for beetle damage (the insects themselves are only 1.7 to 4 mm. long and unlikely to be seen), check oaks for dark brown to black granules and stains on the bark surface, brown frass (insect droppings) and fine white sawdust streams on the lower trunk. Continue checking regularly from March to September when the insects are active.

Sap flow and bark beetle borings attract oak ambrosia beetles to the same tree. In extremely high numbers they penetrate through the bark deep into the heartwood, producing the white sawdust.

Although tree scientists don't know much about the beetles and their control, Dr. Costello speculates they became epidemics in areas with high rainfall, overloading oak root systems in the spring. He has his fingers crossed that San Mateo County, with lower rainfall than Marin or Santa Cruz, will not be affected.

Should any oak trees become infested here, they should be promptly removed and the wood properly disposed of.

The most important step is covering the firewood immediately with clear plastic. The covering prevents the escape of new beetles to fly to other oaks. Leave the plastic in place for six months.

After the oak is removed, branches and twigs should be chipped, and trunks ground.

Elm project success

Dr. Costello has reason for his concern. A beetle-fungus combination called Dutch Elm Disease has devastated elm trees all across the continent in the past 20 years. For the last eight years, he's been involved in an experiment in San Mateo County to find a resistant elm. So far, so good, he reports. The experimental trees at Holbrook-Palmer Park in Atherton are now the size that might be attacked, and are still healthy.

And tree nurseries are now allowed to grow and sell elms after a ban of several years. Frontier and Prospector varieties are available from Valley Crest Nursery in Sunol and from Oregon nurseries.

An oak pest epidemic would be more critical in California. Oaks are a predominant tree, even the symbol of Menlo Park. They are already threatened by development, and lack of proper care in the landscape. Weakened and stressed trees invite opportunistic pests, which can finish them off.

Information on oak tree care is available from the University of California Cooperative Extension Master Gardener Hotline at (408) 299-2638. Publications are available from the Oak Foundation, 909 12th St., Suite 125, Sacramento, Calif. 95814.