• By Sherby Sanborn | April 2, 2012

Oak Pit Scale - a new pest?

Oak pit scale is a non-native oak pest introduced into the United States from Europe, now well established throughout California. As a Consulting Arborist, I have observed populations of this pest to be increasing in many parts of Sonoma County, particularly Sonoma Valley and may be on the increase 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 donut shaped cratering on the bark tissue of twigs and small branches. All of our native oaks can be a host to this insect, however, Oak pit scale seems to be most prevalent on valley oak, Quercus lobata, and this is the tree I have observed the most significant damage. Blue oak, Quercus douglasii, and white oak hybrids can also be heavily infested.


When oak pit scale is present on a twig or small branch, their feeding can cause the infested part to die. When twigs die in the summer, the leaves that are present also die but they don’t drop off in the fall like otherwise healthy leaves.


However, I also see similar symptoms on oaks with numerous small branch cankers caused by Diplodia quericna, another fungal disease.


During the winter, potentially infested valley oaks are easy to spot because there often have areas of the crown that still retain brown dead leaves. I’ve seen individual valley oaks so severely infested by oak pit scale that they exhibit signs of severe decline. If left untreated, these oaks may continue to decline and die.


Clients often call believing there oaks are dying but upon inspection, I discover the primary reason for the die-back is oak pit scale. If left untreated, these oaks may continue to decline.


The good news is that this pest can easily be controlled. There are systemic insecticides available that when applied to the soil or through trunk injections will kill oak pit scale. Treatments may have to be applied for several years to assure complete control and before the tree to show 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 the oak to decline. Additionally, there are human activities that can cause tree decline. Any stress factor that weakens a tree may make it more susceptible to pests and diseases.


When oak pit scale is present on a tree, it may not mean it is the primary cause of decline. It is important to get an accurate diagnosis and identify all the factors affecting a trees health before undertaking treatments. Improper diagnosis can lead to a partial or complete failure to control the problem as well as unnecessarily introducing potentially


For more information on this pest and treatment options please see
University of California IPM Pest Notes