Oak bone scale is a non-native pest of oak introduced to the United States from Europe, now well established throughout California. As a consulting arborist, I have observed that populations of this pest are increasing in many parts of Sonoma County, particularly Sonoma Valley, and may be increasing in other Bay Area counties. While not a new pest, there has been an increase in both the number of trees affected and the severity of infestations.
Oak pit scale is a small sucking insect that creates doughnut-shaped craters in the bark tissue of twigs and small branches. All of our native oaks can harbor this insect, however oak pit scale appears to be more prevalent on valley oak. Quercus lobata, and this is the tree that I have seen the most significant damage to. blue Oak, Quercus douglasiiand white oak hybrids can also be heavily infested.
When oak scale is present on a small twig or branch, its feeding can cause the death of the infested part. When the twigs die in the summer, the leaves that are present also die but do not fall off in the fall like healthy leaves. However, I also see similar symptoms on oak trees with numerous small branch cankers caused by quericana diplodiaanother fungal disease.
During the winter, potentially infested valley oaks are easy to spot because they often have areas of the crown that still bear brown dead leaves. I have seen individual valley oak trees so severely infested by oak scale that they show signs of severe decay.
Clients often call believing the oaks are dying, but upon inspection, I find that the main reason for the dieback is the scale of the oak pit. If left untreated, these oaks can continue to decline.
The good news is that this pest can be easily controlled. Systemic insecticides are available which, when applied to the soil or by injection into the trunk, kill the oak mealybug. Treatments may need to be applied over several years to ensure complete control and before the tree shows signs of recovery as dead and injured twigs and branches are replaced.
While it may seem easy to diagnose this pest, it’s not always that simple. Many other pests and diseases have similar signs and symptoms, which can cause oak to decline. In addition, there are human activities that can cause the decline of trees. Any stressor that weakens a tree can make it more susceptible to pests and diseases.
When oak bone scale is present on a tree, it may not mean that it is the main cause of decline. It is important to obtain an accurate diagnosis and identify all the factors that affect the health of the trees before carrying out the treatments. An incorrect diagnosis can lead to a partial or complete failure to control the problem, as well as the unnecessary introduction of potentially toxic chemicals into the environment.
For more information on this pest and treatment options, see the University of California IPM Pest Note: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7470.html
Sherby Sanborn Consulting Arborist, April 2, 2012