By Vincent Lazaneo.
Two small exotic beetles, the polyphagous shot hole borer (PSHB) and the closely related Kuroshio shot hole borer (KSHB), are taking a big bite out of the arboreal canopy that shades both developed and undeveloped areas in southern California. The beetles physically damage trees when they bore through the bark and create reproductive galleries in the wood. Serious systemic damage also occurs on reproductive host plants when the beetles infect them with fungi that provide a source of food for their larvae. The fungi colonize the beetles’ galleries and invade the tree’s vascular tissue, thus blocking the flow of water and nutrients within the tree. This causes branch dieback, canopy loss, and potential death of the host tree.
How or when the beetles first arrived in Los Angeles County is not known, but by 2012, pathologists knew that PSHB was transmitting a fatal fungal disease dubbed fusarium dieback (FD) to 19 tree and shrub species. Since then, researchers have identified 33 additional reproductive host plants. The list is expected to grow in the next few years.
Some popular cultivated reproductive host plants susceptible to FD currently on the list include: avocado, fig, London plane, coral tree, Palo Verde, camellia, liquid amber, carrot wood, and king palm. Native plants on the list include some beloved species: sycamore (California and Mexican), oak (coast live, Engelmann, valley, and canyon live), cottonwood, and white alder.
Thousands of trees have already been killed and as many as 27 million trees could be lost to PSHB alone in Los Angeles, Orange, Riverside, and San Bernardino counties based on a survey of the region’s trees conducted by Greg McPherson, a research forester with the US Forest Service. That’s about 38% of the 71 million trees in the 4,200 square mile urban region. The survey did not include San Diego County, but we are also likely to lose a substantial portion of our tree canopy.
The loss of so many trees will have a large economic and ecological impact. The most severe damage may occur in native habitat. Last year, KSHB infested more than 440,000 willows in the Tijuana River Valley Regional Park. Native sycamores are also dying, and according to Akif Eskalen, plant pathologist at UC Riverside, “PSHB could kill all the sycamores in the state unless we find a way to control it.” Research is being conducted on biocontrol and chemical management strategies for the new pest/disease complex.
KSHB has been found throughout San Diego County and PSHB is expected to arrive soon. Be prepared and learn more about the new pests at pshb.org. If you think a plant on your property is infested, submit a sample for identification to the County Department of Agriculture (sandiegocounty.gov/awm/entomology.html).
Early detection of PSHB/KSHB and removal of infested branches will help reduce the pest population and slow the spread of FD. Disinfect pruning tools to avoid spreading the fungal pathogen. Chemical control of the beetles is very difficult and the chance of saving a moderate to heavily infested tree is very low. Promptly transport infested wood to a landfill or green waste processing site for disposal. Limbs cut for firewood should be securely sealed under a tarp to keep emerging beetles from escaping while the wood dries. Firewood from infested trees should never be transported to other areas.
Vincent Lazaneo is UC Urban Horticulture Advisor Emeritus. He has a master’s degree in horticulture and a teaching credential in vocational agriculture from UC Davis. In 1983, Vince began the Master Gardener program in San Diego. Vince frequently contributes to the San Diego Union-Tribune and other publications and he enjoys growing specialty plants in his home garden, reading, hiking, and fishing.