By Tom Molloy
Our urban and wilderness forests now under attack from two tree borers
Shot Hole Borer
The Shot Hole Borer is an invasive species of beetle affecting over 65 species of trees in southern California, where it has killed tens of thousands of healthy trees.
The beetle was first found in Los Angeles in 2003 and is believed to have arrived from southeast Asia in packaging materials. Agriculture officials and arborists in southern California have been fighting the Shot Hole Borer since then. Sadly, the 65 tree species that shot hole borers attack include our native sycamores, alders and willows, which are generally riparian trees.
The shot hole borer tunnels into trees, preventing the trees from transporting water and nutrients, often killing them in the process. Signs of infection include dead or dying upper branches, dark oozing on the bark, and small entry and exit holes that are round and the size of a ballpoint pen, often surrounded by boring dust.
Although trees arborists will offer chemical treatments, you can also contact your local office of the country agricultural commissioner for direction. Other than that, keeping your trees healthy and free of competition in the root zone and appropriately mulched is the best you can do.
Gold-Spotted Oak Borer
The Gold-Spotted Oak Borer is a beetle native to Arizona. The ecosystem in Arizona is adapted to the beetle and tree mortality from the beetle is low there. It’s believed the Gold-Spotted Oak Borer arrived in San Diego County in firewood carried from Arizona.
The beetle has now killed over 80,000 trees in San Diego County and is moving north. By 2012, it took hold in Riverside County, San Bernardino County, then Orange County. It has been found in L.A. County east of Castaic, and most recently in Santa Clarita. It has been seen just 14 miles from the 600,000 plus oak trees of the Santa Monica Mountains.
Larvae hatched from eggs, which are laid on an oak tree, bore into the tree to reach the cambium layer, and chew through the cambium. The cambium layer carries water and nutrients up and down the tree. Eventually the damage can strangle and kill the tree. An infected oak tree may have a thinning canopy and red or black stains on the trunk. The infection is confirmed upon sight of the D-shaped 1/8 inch holes that the borer makes.
If you think you see a tree in L.A. that is infected, report it to the L.A. County Fire Department at 818-890-5719. Arborists and experts are using systemic pesticides injected into the base of the tree. The results have been expensive but promising. Others are testing a non-toxic lime wash, with results not yet known.
Sadly experts say it’s an issue of when and how quickly not whether the oak borer will spread. In addition to affecting biodiversity, wildlife shelter, food supplies, and the iconic California oak forests that are part of our identify, the hundreds of thousands of dead trees present an extreme fire hazard.
Do not move firewood. That is how the Gold-Spotted Oak Borer spreads.
If you think either one of the borers has infected trees on your property, in addition to calling the numbers above, call a licensed tree arborist who has experience with these borers. If you catch it soon enough, your trees may be saved.
Here is an article about street trees in Los Angeles, published in the March/April 2022 issue of GLS TRACKS. It includes information about tree-boring insects that was current at the time.
