Over the last few years, I’ve noticed that the large coast live oak trees along Turnbull Canyon Road appear to have small patches of dead leaves on them, but the trees appear to be healthy otherwise. For a while, I dismissed the dead leaves as a symptom of drought. But as I was made aware of a devastating new pest that is affecting oaks in San Diego County, called the gold-spotted oak borer, I’ve been paying a bit more attention to trees that appear distressed. Although there could be several things causing the dead leaf patches on our oaks, including other insect pests and fungi, I became concerned that these oaks could perhaps be affected by the gold-spotted oak borer. So, I examined some of them closely.
While gold-spotted oak borer attacks can cause thinning of the tree crown and die-back of twigs, none of the trees appeared to exhibit any of the classic symptoms along the trunk, such as weeping or staining, or tiny D-shaped exit holes. What I did find was at the point where the dead twigs met with the live tree branch, under the bark there was a pattern etched in the tissue spiraling around the twig, with dark powdery “frass”inside the tunnels (see photo) – I even found a tiny larva in one! This indicated that the damage could be caused by an insect called the oak twig girdler. It is similar to the gold-spotted oak borer, in that they are related – gold-spotted oak borer’s scientific name is Agrilus coxalis, and the oak twig girdler’s scientific name is Agrilus angelicus. Also, both attack the tree’s tissue just below the bark, causing the growth beyond the attack site to die. However, the gold-spotted oak borer attacks the trunks of trees, whereas the oak twig girdler only attacks the smallest twigs at the ends of branches where new growth is happening. That is why the gold-spotted oak borer is much more of a threat, because when it attacks the trunk tissue, it prevents nutrients from reaching the rest of the tree (branches and leaves).
The larva I collected is being sent to the Los Angeles County Department of Agricultural Weights and Measures for entomologists to confirm whether it is the oak twig girdler. Fortunately, the oak twig girdler is not considered to be a significant impact to tree health. But the presence of these insects may be a sign that our oaks are under some stress, as the oak twig girdler is thought to be attracted to trees weakened by drought.
**UPDATE: The Los Angeles County Entomologist confirmed that this is likely the oak twig girdler, and that it is not a cause for concern. He also mentioned that this is a native species.