Borer beetles are spreading north from San Diego County into the rest of Southern California and, in the opinion of researchers, comprise a devastating threat to California trees. These beetles (two species of Shot Hole Borers and the Gold-spotted Oak Borer) attack dozens of tree species in Southern California, including commercial avocado groves, common landscape trees, and native species in urban and wildland environments.
The Shot Hole Borers carry a disease called Fusarium Dieback (FD), which is spread by pathogenic fungi. Trees that are FD-susceptible may experience branch dieback, canopy loss, and, in some cases, tree mortality. In San Diego County, Shot Hole Borers have substantially infected and damaged the dominant native trees in the riparian forests of the Tijuana River Valley, and this has led to a drastic alteration in the structure of the canopy of the forests. The loss of canopy is likely to promote the growth and spread of invasive plant species that were relatively inconspicuous in the forests prior to the beetle attack.
One can imagine the overwhelming effects that tree loss might have on birds.
If land managers are to control the spread of the beetles, an effective early detection and rapid response program must be developed. This is one of the tasks of a number of tree pest working groups in which Audubon Starr Ranch staff participate. The annual in person meeting of the SoCal Emerging Tree Pest Education Working Group was held at Starr Ranch in April of this year. University of California Riverside researchers held a “Tree Armageddon Symposium” in May 2017.
Research on these tree pests and diseases is centered at University of California, Riverside. The pathologist, Dr. Akif Eskalen, the entomologist Dr. John Kabashima and their colleagues not only do research on the beetles and diseases but also speak to Homeowners Associations, land managers, and city officials to educate about signs of disease and what the general public and land managers can do.
Although potential biocontrols and other solutions are in development, so far land managers and homeowners have little recourse. Researchers advise against transporting firewood, since firewood is a primary source of beetle movement. There are also excellent detection tools and other resources available at pshb.org. Audubon Starr Ranch interns have developed a protocol for monitoring Bell Canyon, the main riparian corridor on the Ranch, for tree pests and diseases. Our ornithologist also monitors riparian birds. Although the beetles have been detected in nearby parks, so far there is no sign of disease on Starr Ranch. However, we have had alarming tree dieback, which we hypothesize is related to the extended drought.
Sandy DeSimone is Director of Research and Education at the Audubon Starr Ranch Sanctuary in Orange County.