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Unusual weather conditions make oak trees vulnerable
By Tony Tomeo
During the last few years, many of my colleagues have been noticing an unusually high mortality rate among the native tanbark oaks in the Santa Cruz Mountains. In the wild, it is not unusual to see stressed oak trees suddenly die from infestation of boring beetles. Elimination of weak trees is actually beneficial to surviving healthy trees, which may better utilize the formerly occupied space.
However, when seemingly healthy trees are attacked, there is a definite concern. Even more of a concern is the recent similar increase in mortality rate among the coast live oak. Because the coast live oak population is so vast, this epidemic will likely severely affect both the natural ecosystem and urban landscapes.
Although the three species of beetles causing all the problems are native to this area, their population is normally kept to a minimum. With normal weather conditions, the forest supplies enough stressed trees to sustain the beetles, which are happily employed eliminating the weakling trees in favor of healthy trees.
Unfortunately, recent weather conditions have caused most of the oak population to experience at least some degree of stress. First we had an unusually wet winter, then an unusually dry winter followed by a cool spring and summer. Rather than work overtime, the beetles increase their population to be certain that as many of the stressed oaks can be infected as possible. With so many beetles flying around, some mistakes are made, and healthy trees may be infected, causing stress, which attracts heavier infestation. Even as weather conditions become more favorable to the oaks, it will be quite some time before the beetle population reaches equilibrium.
The majority of us who live in suburban environments are not exempt from the threat of this "sudden death" of oak. Tanbark oaks may not be common in refined landscapes; but coast live oaks are. Other species of oaks which are more commonly planted and not normally susceptible may be afflicted during this current epidemic. Urban oaks generally have the advantage of distance from other oaks, but also the disadvantage of more stressful conditions.
Live oaks are particularly sensitive to normal landscape conditions and irrigation, making them very attractive to boring beetles. Holly oaks and pin oaks, though not favorites of the borers, also find the Silicon Valley lifestyle to be stressful and may be infected. Red oaks do better here, but are also at risk. Cork oaks, bur oaks, blue oaks, black oaks and white oaks (valley oaks) are only rarely attacked. (Wasn't it the Beatles who composed "Norwegian Wood"?)
The western oak bark beetle is usually the first to attack, leaving dark granules (frass) and bleeding entrance holes. Bleeding may be indicated by lightly stained bark or by a thick burgundy syrup. These symptoms attract oak ambrosia beetles and minor oak ambrosia beetles, which occupy the already infested trees and bore into the heartwood, producing fine blonde sawdust on the trunk. Before these beetles deposit their eggs in their new galleries, a symbiotic fungus is introduced to feed their larvae as they develop.
This fungus spreads through the galleries and into the tree's vascular system, which is quickly clogged, killing the tree. There is no known remedy for infection. Prompt removal of dead trees is advisable; but where infected oaks are numerous, removal will not likely affect the beetle population. Application of permethrin to the lower four feet of trunk in late September and March, when beetles are most active, is performed on highly valued trees, but it's not very effective. The beetles can be active at any time between applications, and those already in the wood or bark are not affected. Some of my colleagues have suggested experimenting with applications of any sticky substance to trap the beetles as they try to bore into the bark, but logic tells me that they would only enter higher on the trunk.
The best protection is to keep oaks healthy and unappetizing to the beetles. Slight changes in irrigation can be stressful to oaks; increases are more stressful than decreases. Oaks that are not irrigated should be left without water. Planting or excavating around oaks should be postponed. Light pruning may improve vigor; but improper cuts or other wounds may be perceived by ambrosia beetles as entrance wounds made by oak bark beetles. Because of the severity of this epidemic, I'll try to stay updated with any new information, which I will relay as it becomes available.
Tree of the Week: Coast Live Oak
Back at the nursery, the most abundant tree species in the 100 or so acres not being cultivated is coast live oak, Quercus agrifolia. (You can understand my direct concern.) Because this tree is native to the area, many of the old trees can still be seen where urban development has sprawled around them. In fact, my Los Gatos neighborhood, Live Oak Manor, is named for the very impressive oaks towering over my neighbors' homes. Another impressive grove that was adopted by Saratoga Tree Service is maintained for the benefit of the trees. Unlike typical parks, there is an absence of landscaping and irrigation so the trees can live as they would without all the problems associated with urban living. Because of the coast live oak's intolerance of typical landscape conditions, it is not commonly planted and certainly not recommended for irrigated areas.
Readers can contact horticulturist Tony Tomeo at 358-2574.
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Los Gatos, California Since 1881
