October 18, 2021

The Oaks are Dying


The beetles have arrived; native trees are threatened.

Around September 3, two new problems for our oaks, sycamores and willows were confirmed in Topanga: the polyphagus shot hole borer (PSHB) that infests many species of native and non-native trees, and the foamy bark canker disease caused by the western oak bark beetle (WOBB) that prefers oaks only.

While each has a unique life history, they both involve a beetle infecting the tree with a fungus as food for their young.


The Oaks are Dying

This oak tree shows foamy canker that pops out like small volcanoes along trunks and limbs that attract swarms of yellow jackets and ants. Tiny holes (the size of a period) ooze a dark red sap from infected trees in response to the fungus, carried by the native Western Oak Bark beetle, that has mutated over time to carry the fungus.

Unfortunately, both result in tree death. Even worse, there are currently no tested treatments that can control the spread. Removing the infested limb and even the whole tree seems to be the only sure way of reducing their population and spread. The only good news is that, so far, no gold spotted oak borers (GSOB) have been documented. Two out of three new pests are surely enough!


Western Oak Bark Beetle (WOBB)—Foamy bark canker was the first to arrive in Topanga with the WOBB. Swarms of yellow jackets and ants attracted to the oaks, feast on the white foam popping out like small volcanoes along the trunks and limbs that is often the first sign of a problem.

Upon closer inspection, you will find tiny holes (the size of a period) often oozing dark red sap. The beetles seem to congregate on the lower trunk or sometimes in large branches and spread from there.

The WOBB is a native insect, one of literally thousands of insects that have lived with oak trees for thousands of years. No one is sure when the mutation took place but, at some point, the native beetle (or its doppleganger) joined forces with a fungus called Geosmithia pallida, which spreads through the phloem (food-conducting tissue of a plant just under the bark) and eventually kills the tree by disrupting the flow of water and nutrients.

The mostly female beetles lay eggs in tunnels excavated just under the bark, the eggs become adults within a few weeks and several generations can be produced every year. The beetles winter under the bark and are most actively flying in search of new trees in spring and fall. They are very small (several adults equal one grain of rice) and are weak flyers that are usually carried by the wind from tree to tree or transported in cut wood that has not been disinfected.

Until it joined forces with this new fungus, WOBB rarely did much damage to healthy non-stressed trees. But after five years of drought, most of the oaks throughout the Santa Monica Mountains are stressed to their limits.

The combination of extreme drought, more hot days and the new fungus seems to be lethal for our oaks.


The Oaks are Dying

Photo of a polyphagous shot hole borer.

Polyphagus Shot Hole Borer (PSHB)—was first observed in avocado trees in California but are native to Vietnam and Thailand where their cousins have long plagued the tea industry. Somehow, these tiny insects were carried around the world and have slowly spread across Southern California and Florida, leaving swaths of dead trees in their wake.

In an effort to control the spread, hundreds of trees have been removed at Huntington Garden, Descanso Garden and Los Angeles County Arboretum. Unlike the WOBB, PSHB will attack both healthy and stressed trees and have been found to reproduce successfully in many of our native tree species, including oaks, sycamore, big leaf maple, white alder and several willow species. Overall, they have been detected in more than 340 species of trees!

Like WOBB, PSHB carries a fungus as it tunnels into the living tissue of the trees under the bark, essentially gardening food to feed the larva as they emerge. Mostly females, these beetles can reproduce every three to four weeks, quickly spreading throughout a tree and disrupting the flow of water and nutrients both through the effects of the fungus as well as the mechanical interruption from their numerous tunnels.

Gold Spotted Oak Borer (GSOB)—Last, but not least, these small insects are transported mostly by firewood cut from infested dying oaks, where they incubate under the decaying bark, then spread to nearby oaks. Larger than either WOBB or PSHB, GSOB make a characteristic D- shaped hole, rather than the tiny pinpoint holes of the others.

All of these beetles are most commonly spread when limbs or whole trees are cut down and the untreated wood is transported to new locations.

Thousands of willow trees were recently lost along the Tijuana River when infested PSHB firewood was stored at a property nearby. The pattern of spread in Topanga and other sites in the Santa Monica Mountains is not that clear.

Dr. Shannon Lynch of UC Riverside identified PSHB in a sycamore tree near the RCD office and on willows behind the Topanga Market center. Oaks on Old Topanga Canyon Road, along Robinson Road, Topanga Skyline Drive and Greenleaf Road have all been infested with WOBB and foamy canker disease, although there is no evidence of moving wood, imported plants or other obvious ways that transport the beetles.

Additionally, the infected trees have often been within groves of many oak trees, with only a few infected. Why is one tree and not another infected, especially when all are equally drought stressed? This is one question among many, remaining to be answered. Despite extremely limited funding, the Eskalen lab and others are trying to identify and test natural “probiotic” agents in the trees that resist infection in hopes of finding ways to stop the devastation.




The Oaks are Dying

Above, five infested trees were removed from this oak grove located on private property in Topanga. Five neighboring oaks that have shown no signs of infection are being protected to prevent further infestation using a combination of orange oil spray on the trunks that are then wrapped with plastic shrink wrap. The stone terrace below was sprayed with orange oil and covered in plastic sheeting to kill any hatchlings from the tree that may have landed on the ground. The oaks on the hillside above the infected area are not considered to be in danger but daily monitoring is ongoing. The tarps and wrap will remain for one month.

Look for the wasps! Pay attention to swarms and take a really close look at your trees, trunk, branches and canopy.

If you see foamy white blobs, staining and bleeding, take a photo, using a pen or ruler for scale, and send it to the Eskalen lab. For handouts with photos of what to look for and more information: http://eskalenlab.ucr.edu/pshb.html.

Dr. Akif Eskalen and his colleagues have compiled the most up-to-date information on what to look for. When in doubt, it is best to send a photo to them first, and they can then determine if sampling is needed.

Also, call your local arborist or tree trimmer, although not all of them are up to speed on these new pests. Definitely communicate with the Eskalen lab and get confirmation of exactly which pest is the problem. Foamy canker and PSHB can have similar manifestations especially on oaks. It is really important to know exactly which problem you are dealing with. It is also critical to track the spread as this problem progresses.


If you get confirmation of foamy canker, you have a few more choices than if you get confirmation of PSHB. Local property owners have recently faced this truly difficult situation. How do you make the decision to remove a well-loved tree? Again, there is a decision tree on the Eskalen lab website to guide you. Sadly, there are few options and none of them good.

Foamy canker—If the lesions are on a branch only, you can remove the branch. While not one of the methods currently undergoing trials, wrapping the trunk and those of surrounding trees with heavy duty Saran or other plastic wrap and sealing the edges may:

1) prevent beetles from getting out and allowing you to see if they are still present on the tree; and

2) provide a barrier to deter the beetles from getting into adjacent trees and allow you to detect them if they spread. It may well be worth the effort.

It is also critical to give these trees a long, deep watering once a month until the rains return. If the trunk is heavily infested, then the tree is becoming what is known as an “amplifier,” promoting a rapid concentration of increased beetle population. At that point, removal is the only way to prevent the WOBB from spreading into the surrounding oaks.

Make sure that your tree company knows how to handle the infested material correctly while taking out the tree or branch and that they take the time to decontaminate their tools. Chainsaws, loppers and hand clippers can spread fungi really well if not treated with rubbing alcohol after a careful cleaning.

Trees infested with PSHB—there are really only three options:

1) If the infestation is new, confined to one side of the tree or branch, and fewer than 30 lesions per square foot, then the use of bark sprays and soil drenches can help control the infestation and reduce tree mortality, or

2) Remove the tree and dispose of the wood and branches responsibly, or

3) Leave the tree to allow further spread of the beetle into other trees.

Dr. John Kabashima of the UC Cooperative Extension notes that “while removal and use of bark sprays have impacts, they are nothing compared to the devastating impact of the PSHB once it becomes established. It is like treating a spot fire with fire retardants to prevent widespread wildfire.”


It is critical to treat any cut material carefully to prevent the spread of beetles! The only treatments that work are:

• Covering the work area with plastic sheeting before removal

• Chip material to less than one inch and either take it to the landfill to use as cover, or use it for hot composting, or solarize for several months

• DO NOT USE OR SELL the wood for firewood as it will spread the problem!

There are handouts on the Eskalen lab website with directions.


Take good care of your trees!

• Long, slow, deep watering with soaker hoses once a month under the canopy drip line but not hitting the trunk (moisture should be visible 6-8 inches deep into the soil).

• Place organic mulch two to four inches deep under the canopy (buy in bags to avoid infested materials) to hold soil moisture and reduce soil temperatures.

• Remove non-native plants and lawns that compete with your trees for water, especially under the canopy.

Pay attention to the root zone and make sure that you are treating it well! Too much water can also be a problem, so be thoughtful with your watering. Prevention is, by far, the best way to reduce risks for your trees.


When faced with such a widespread threat of changes to our environment, it is easy to get discouraged. We cannot change the drought and its impact on our wildlands, but we can change our habits to use water wisely to help our trees and creeks survive. Each of us has the power to keep our eyes open, spread the word on what to look for and report observations to help reduce the impacts of these pests until the rains come and the scientists figure out better treatments.

• Plant more native trees—Fall is the best time to plant oaks and other natives.

• The RCDSMM—will be hosting several oak plantings to continue the ongoing restoration efforts in lower Topanga State Park. Join us from 10 a.m. – noon on Saturday, October 29, and November 19, to help. This is also an opportunity for students to fulfill their community service hours. Extra oak seedlings will be available to take home. Check out the RCDSMM website for more links and details (http://www.rcdsmm.org).

• Los Angeles County Forestry Nursery—also provides free seedlings. It is located at 942 N. Las Virgenes Road Calabasas, CA 91302-2137. Call (818) 222-1108 to make a date to pick them up. (www.fire.lacounty.gov/forestry-division/forestry-contact/y)

• The California Native Plant Society—will hold its annual native plant sale on October 22-23 at the Sepulveda Garden Center, 16633 Magnolia Blvd., Encino, CA 91436-1101; (818) 784-5180.

• Help monitor oak trees in Topanga—Join the RCDSMM and Topanga Canyon Docents on Saturday, September 24, from 12-4 p.m., to assist with the second annual oak tree monitoring effort. Last year we tagged and examined more than 100 oaks in Topanga State Park. Help us keep tabs on how the trees are doing.

• Let the politicians know that trees are important—Dr. Kabashima and other researchers are working hard to find other deterrents and treatments but lack of funding is a real problem. He urges people to “Please contact your local state and assembly representatives and let them know how much you are concerned! We need a groundswell of local effort and concerned stakeholders to make the case for doing the research as soon as possible. We can’t afford to lose more of our ecosystems, like we did in the Tijuana River.”

Rosi Dagit is Senior Conservation Biologist and Certified Arborist, Resource Conservation District of the Santa Monica Mountains (RCDSMM).