DAMAGE—Ugly holes and spots caused by the shot hole borer. Courtesy photo
An ecosystem is a delicate balance, and when a foreign species is introduced into the mix the results can be disastrous.
Take the case of the Polyphagous shot hole borer beetle, a species native to Vietnam that has been aggressively infesting and killing local trees. With no native predator in place to stop it, the infestation is spreading.
Rosie Dagit has been studying the beetle since 2012. Dagit is a senior conservation biologist for the Resource Conservation District of the Santa Monica Mountains. She doesn’t know how or when the beetle arrived in Southern California.
PEST—Right, the Polyphagous shot hole borer beetle. Above, damage caused by the bug.
“It was initially affecting avocado trees, so the avocado orchard folks were all concerned. At that time, we didn’t realize what its preferences were,” Dagit said. “I think at this point they’re known to infest up to 300 species of trees, they can actively reproduce in, I think over 75 species of trees.”
The beetle bores a tiny hole into a tree and fills the trunk with fungus, which it uses to feed its larvae. Once it’s in the tree it can cause leaves and branches to die. Infected trees also show dark or discolored bark or can have a powdery white substance on the trunk. Depending on how strong the tree is, the beetle can kill it in just a few months.
Courtesy photos
John Luker is on the board of the Santa Susana Mountain Park Association. He worked with Dagit when she was conducting a study of the shot hole borer beetle.
“The scientists we’ve been working with feel that between the effects of drought, climate change, the shot hole borer and other boring pests, probably 100 percent of the sycamore trees in the Santa Monica Mountains are going to die, probably around 60 percent of the Oak Trees,” Luker said. “We’re sort of confirming that as we walk around these areas collecting acorns and studying the effects of these (beetles) on the oak groves around here, it’s just terrible.”
Luker and fellow board member Wendi Gladstone have been giving out oak saplings and organizing community planting events. The beetle only infests fully matured trees, so they hope that if they can plant enough saplings it will give researchers a window to find a solution while the trees mature.
“We’ve also been working with the Jet Propulsion Lab and NASA in mapping the areas that the borer has affected most. I can say that they are around the West San Fernando Valley and there are many in other areas—Calabasas is pretty hard hit with the shot hole borer,” Gladstone said.
Gladstone conducted a study of the environment around Calabasas Lake and started documenting the beetle’s impact on the trees in the area once she started working with Dagit.
Calabasas resident and former mayor Dennis Washburn has been helping spread the word about the problematic beetle. He worked with Gladstone and Dagit in studying the beetle. He’s also on the board of the Resource Conservation District of the Santa Monica Mountains. As part of the study on the impact of the shot hole borer, he put traps in trees at his home in Calabasas.
“We found them in my yard. They’re actually at the intersection of Old Topanga (Canyon Road) and Mulholland (Drive), at Headwaters Corner and in the area where the fire burned us out last year or the year before,” Washburn said. “The last couple weeks I’ve been going around with seedlings, local coast live oaks. They’ve been sprouted by some of the folks who are doing research and we’re trying to convince people that they need to start planting seedling and saplings to replace the trees that we’re ultimately going to lose. That’s probably the best thing we can do until there’s some systemic or other kind of means of combatting the problem.”
Normally the ecosystem balances the harmful impact of the shot hole borer beetle—natural predators keep the insect’s population in check. But because the beetle is non-native, there are no safeguards in Southern California. Researchers are trying to find a means to keep the beetle from spreading but so far haven’t had any luck.
Dagit said before the beetle had made a serious impact it was classified as a low priority pest. Consequently, limited funds are available to fight it.
“The only way to remove the beetle is to remove the tree, which is horrifying and nobody likes to do. But the reality is that none of the injections or trunk drenches or chemicals do anything because the beetles are burrowed into the trunk,” Dagit said. “If people do have an infected tree removed, they shouldn’t give the firewood away because that’s how this beetle is most easily spread. When you take down a tree that’s infested you have to take care of that tree—at this point Calabasas Landfill is our best bet locally, because they chip things down to a very small size and use the material as landfill cover, so it’s not going out and being spread.