PHOTO BY FLAVIA POTENZA Bonnell Park is surrounded by several mature sycamore trees that create shade in the summer and piles of leaves in the fall. Recently, within a week, four branches broke away and fell, slightly damaging one vehicle but no trees. Kimmy and Cris Lilliedahl, who manage the Bonnell Park Association, were quick to put up caution tape and fence netting to warn people away. A County representative, Rachel Burnap (firstname.lastname@example.org) came to take samples for evidence of the Invasive Shothole Borer Beetle now found throughout the San Fernando Valley and surrounding areas that could kill roughly 38% of all trees in the area.
As the drought continues, many of our native trees are showing more signs of stress and are becoming more susceptible to a relatively new invasive beetle, the tiny Invasive Shot Hole Borer (ISHB: Euwallacea sp.).
These invaders from Southeast Asia spend most of their lives inside the tree where they create tunnels throughout the trunk and branches. The entry hole visible on the trunk is about the size of the tip of a ballpoint pen. The females farm pathogenic fungi (Fusarium euwallaceae) on the walls of the galleries to feed their young. Both the tunnels and the pathogen physically disrupt transport of water and nutrients throughout the tree causing die-back and eventual death. Their reproductive cycle from egg to new adults is about 40 days, with multiple broods possible per year. Only the females can fly, so the few males in the brood mate with their sisters inside the tree, then die. Because they are weak fliers, spread is often due to moving infested wood from trees that have died.
Treatments are really challenging because the beetles live mostly inside the trees and are difficult to reach. Pruning out infested branches can help, but we are also finding that some sycamores are able to survive the infestations with just basic good tree care: a well-tended root zone covered with mulch, supplemental deep watering once a month using a soaker hose (slow overnight soak down to four inches in a circle under the canopy), and careful monitoring to make sure things calm down. The most up-to-date information on treatments is found at pshb.org.
California sycamores are a preferred reproductive host for ISHB, and can amplify their populations causing local spread, but they are not the only species at risk. This pest has attacked more than 300 species of trees. Research is ongoing to determine the distribution of these pests in the mountains, but infestations have been documented in Calabasas, Malibu, Ramirez Canyon, Thousand Oaks, and Topanga. With funding from LA County Supervisor Sheila Kuhl, the Resource Conservation District of the Santa Monica Mountains (RCDSMM) has been working to document the extent of the infestation. Thus far, the beetles are taking a toll in hot spots, but are not yet found everywhere. We need to know more, and you can help.
Using the iNaturalist app, you can join the Ventura and LA County Bad Beetle Watch project and contribute your observations of trees in your neighborhood or along your hiking trail. To learn more, you can find training documents on our website at rcdsmm.org/resources/caring-for-your-oak-trees/, and take the full training provided by the University of California Cooperative Extension found at campus.extension.org/enrol/index.php?id=1704.
The Los Angeles Agricultural Commissioner is doing an extensive trapping program this summer, but visual surveys such as those done in your own backyard provide the best level of detection. If you think you see a problem, you can report it to: acwm.lacounty.gov/entomology-and-plant-pathology-laboratories/
For general insect or plant disease problems, or to report suspect trees, contact: Plant Pathology, (562) 622-0433; or Entomology, (562)622-043.
The Oaks at Trippett Ranch will Survive!
People, concerned about the oak trees at Trippett Ranch, have been contacting the RCDSMM since the Palisades Fire burned a good part of the state park and was stopped just short of the Visitors Center.
The black bare skeletons of the many beautiful oak trees within the recent burn zone of the Palisades Fire are heartbreaking to see, but hidden deep under the thick bark and roots lie stored energy reserves that will bring many of them back to life. Spring fires are unusual and often more challenging for the trees because they just used lots of energy pushing out new shoots, catkins, and flowers that are now gone. It will be months before they start pushing out new green shoots and leaves, but it will happen! Be patient and watch the return from the ashes.
You Can Do Something!
If you want to help them survive the heat of the summer, adopt a tree and take it water every time you hike in the park. A few gallons a month spread slowly in a circle about three feet away from the trunk will definitely be much appreciated by the hard working roots. Let the water soak in and not run off on the heated top layers of soil. Be careful not to water the trunk directly, as that will benefit pathogenic fungi more than the tree.
The RCDSMM is working with state parks on a restoration planning effort but any actual planting will need to wait for the cooler late fall early winter to get going when the rains are more likely to help with establishment.
Another way to help is to join the RCDSMM to care for the baby oaks that fortunately survived the fire! We meet at the Trippett Ranch parking lot every other Saturday from 9 a.m.-12:30 p.m. and carry buckets to keep more than 250 baby oaks going. The next dates are June 26, July 7, 10, 24, and August 21. Sign up now at: email@example.com.
These baby trees also benefit from random watering acts of kindness! If you are a regular hiker in the park, or even a sporadic visitor, take along a few extra liters of water and pick a tree to befriend. This supplemental water last summer contributed by more than 30 people who “adopted” baby trees definitely made a difference. Some of those one- and two-year-old trees are now more than four feet tall and almost ready to survive on their own.
The simple act of carrying water to the trees can really help. Thanks for your care and concern.
By Rosi Dagit, Sr. Biologist