Although warnings about the invasive shot-hole borer (ISHB) beetle first started coming out a few years ago, most local residents didn’t pay much attention, because it hadn’t shown up yet in Malibu. But now it has — in Malibu West and Bonsall Canyon, as well as the neighboring towns of Topanga, Agoura Hills and Calabasas.
The problem is now serious enough that the LA County Department of Agricultural Commissioner Weights & Measures set up an informational booth two weeks ago at an outdoor event in Malibu West to educate residents about the problem and try to keep it from becoming any worse.
The department warns that, “If you live in the Santa Monica Mountains, these pests could be in your backyard.”
The sesame-seed sized insect can bore its way into the trunks of over 100 kinds of mature trees, including everything from native California Sycamores to avocado trees, and kill them by introducing fungi that cause a tree disease.
The message is that if residents aren’t vigilant about learning to identify tree infestations early, the big trees could easily be lost. Heavily treed neighborhoods could look a lot different in the future.
“It’s destroying entire riparian areas… and challenging our ability to sustain natural ecosystems and food systems,” the county wrote. “We know it attacks every native tree, as well as some native shrubs and invasive plant species in the Santa Monica Mountains.”
No effective pesticide or fungicide treatment has been found, but researchers are still testing various combinations of insecticides and fungicides, hoping to find something that works. Repellents and biological controls are also being studied.
“Rapid removal of the infected host tree to reduce the spread is currently the only treatment option. Our goal is to [locate trees infected with the insect] … early enough to respond,” LA County advised.
The invasive shot hole borer beetles (ISHB), native to Vietnam, were first identified in LA County in 2003 after being accidentally shipped in wood-packing materials. The beetles became widespread by 2012, were detected in Ventura County in 2015, the Santa Monica Mountains in 2016; and Malibu around 2017.
Once a tree is invaded by a female ISHB, her eggs hatch and “the offspring mate with each other and continue to create tens of thousands more beetles as they continue to eat the fungus and carve galleries in the tree,” LA County stated. When the tree starts to die, the beetles start to leave, and the cycle repeats itself.
Educating the public involves teaching people how to identify a tree infected with shot-hole borers. LA County says California Sycamores are one of the easiest to observe. The ISHB prefers trees with a diameter of 4 inches or more. Signs of infestation can include a tree trunk with brown stains, gumming, white powdery patches and/or sawdust coming out of an entry hole about the size of a ballpoint pen tip. Contact the county if an infested tree is suspected.
They say if the infestation is caught early enough and isn’t severe, the tree can be pruned and maybe survive. The infested pruned portions have to be disposed of properly.
An “amplifier” tree is one that is so infested that the beetles have already started leaving it. These trees have at least 150 holes, have already started dying back, and will soon start dropping large limbs and then die. These trees have to be removed, or they’ll keep spreading live beetles to other trees around it.
Property owners can help prevent infestations by knowing that firewood, piles of wood and mulch can be infested with ISHBs. Purchase mulch that’s 1-inch diameter or less. Keep trees healthy and inspect every six months.
LA County is doing its part to detect infestations through trapping and visual surveys.
When the Malibu Times last reported on this story, about 14 months ago, the shot-hole borers had begun infecting more and more of the old California Sycamore trees in the Zuma Creek area of Bonsall Canyon.
“The whole of Bonsall’s got it,” resident Raymond Bark said in a phone interview at that time. “All but one family’s sycamore trees in the neighborhood seemed to be in different states of death. Nobody raised alarms.”
For questions, please reach out to pepq@acwm.lacounty.gov