The victims -- thousands of stately California live oaks and their less glamorous tanoak cousins -- are dying in epidemic proportions in localized areas of Marin County, the Santa Cruz Mountains and Big Sur. Scientists are doing their best to unravel the mystery surrounding the sudden deaths while offering advice on how to protect the majestic trees.

Two obvious culprits are the western oak bark beetle and oak ambrosia beetles, which bore through a tree's cambium, the living tissue just underneath the bark, several inches deep into heartwood. The beetles, a normal part of coastal ecology, usually only attack sick trees. But in recent years they've been attacking apparently healthy trees in high numbers. Something is out of whack.

"There are many theories as to why some areas have beetle epidemics that eventually kill so many California live oaks," says Steve Tjosvold, an environmental horticulturist with University of California Cooperative Extension. "The relatively old age of coastal oak woodlands is one factor. Fire suppression leading to denser forests and greater competition for water, nutrients and sunlight is another. The tanoak infestation may also be spilling over into the live oaks, but the relationship is not completely understood."

Although beetles play an important role in the death of California live oaks, there are many other factors that could contribute to the decline of trees. So Tjosvold and fellow UC Environmental Horticulture Advisor Pavel Svihra compiled advice for landowners with oaks in their leaflet, Beetles Kill Stately California Live Oaks: A Synopsis of the Problem and Practical Guide for Landowners.

Until the cause of the epidemic is better understood, they suggest landowners do what they can to maintain a healthy environment around oak trees. Particularly important is root health. For instance, avoid foot and vehicle traffic that can compact soil and reduce water, air and nutrient movement. Pesticides may be applied to trees exhibiting signs of infestation -- black oozing sap and reddish-brown boring dust. February and March may be the best time to make a pesticide application. They also suggest cutting down and removing dead tanoaks and California live oaks as soon as possible and have advice on how to do so without spreading the infestation.

Aerial surveys are planned to determine the extent of the epidemics. Research is also under way to more accurately assess beetle emergence dates and the effectiveness of different sprays.

To obtain a copy of Beetles Kill Stately California Live Oaks: A Synopsis of the Problem and Practical Guide for Landowners, contact a UC Master Gardener at (831) 763-8007.