In the Santa Monica Mountains, we see outbreaks like this every time we have a wet spring, but following the seven-year drought, sycamore trees in the wildlands and creeks were especially vulnerable. California sycamores are generally found where they can sink their roots into the water table and, in doing so, provide important habitat along many of our creeks.

The beautiful specimen tree located two miles north from PCH on Topanga Canyon Blvd. is a beacon of home to many residents of Topanga. This sycamore survived the 1980 flood that destroyed most of the road, and holds up the bank, providing shade to the creek below.

However, when anthracnose delays the regrowth of the canopy, the pools below warm up in the sunshine and breeding conditions for native frogs deteriorate.

Unfortunately, anthracnose is not the only threat to our native sycamores, especially as temperatures rise during the summer. As the drought progressed, trees along the creeks became stressed and 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 dieback 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.

California sycamores are a preferred reproductive host for ISHB, although this pest has attacked over 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 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 iNaturalist, 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 (rcdsmm.org/resources/caring-for-your-oak-trees) and take the full training provided by the University of California Cooperative Extension (campus.extension.org/enrol/index.php?id=1704).

The Los Angeles Agricultural Commissioner is starting 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 (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)622-0431.

In-person volunteer training has been postponed until fall due to covid-19 restrictions, but the beetles are already reproducing as the temperatures rise.

Check out the RCDSMM website for updates and other opportunities.
Resources: https://ucanr.edu/sites/pshb/;
https://www.rcdsmm.org/resources/caring-for-your-oak-trees/