Two closely related Asian beetles are boring their way through Southern California’s trees.
By Anne Raver
Smaller than sesame seeds, two beetle species are spreading through Southern California, killing hundreds of thousands of trees and infecting many thousands more with a pathogenic fungus.
At first, scientists thought the pests were the same species because they look exactly alike, but they carry different pathogenic fungi, and DNA analysis revealed genetic differences. But their damage to trees is so spectacularly similar that the two beetles—the polyphagous shot hole borer and the Kuroshio shot hole borer—are now referred to collectively as the invasive shot hole borer (ISHB).
A 2017 U.S. Forest Service survey estimated that 23 million trees are vulnerable to the ISHB that is working its way through Los Angeles, Orange, Riverside, and San Bernardino Counties—or 33 percent of Southern California’s urban canopy. It’s impossible to know how many trees will die, but the projected losses are catastrophic.
The polyphagous shot hole borer (PSHB) was first caught in a California Department of Food and Agriculture trap in Whittier Narrows in Los Angeles in 2003. Identified as a type of ambrosia beetle, of which there are 3,500 species—most of which partner with fungi to decompose dead trees—the insect didn’t appear to be doing any damage, particularly to economic crops, so it raised no alarms.
But by 2010, hundreds of box elders lining the streets in Long Beach were so injured, or dead, that they were all taken down. The beetle moved on to California sycamores, killing thousands on streets and in yards, public parks, and wilderness areas.
If uncontrolled, the beetle “will kill all the sycamores in California,” says Akif Eskalen, a plant pathologist at the University of California, Riverside. And the ISHB is on the move, northward through Ventura and Santa Barbara Counties, and southward, through San Diego, down to Tijuana.
The Kuroshio shot hole borer (KSHB), though found here and there along the coast, is most concentrated in San Diego County and farther south, where it killed 180,000 willows in the Tijuana River Valley. That’s a critical breeding area and habitat for the Least Bell’s vireo and the southwestern willow flycatcher, both federally listed endangered species. “The river valley is just decimated, these wide swaths of willows all down,” says Shannon Lynch, a forest pathologist. “The trees have fallen down, or you just see dead stands. And because of the dead willows, you can see invasives like castor bean starting to encroach.”
When a key native species crashes, all the benefits it provides—collectively known as ecosystem services—are part of the train wreck. “The animals, insects, birds, and everything else that rely on those plants will suffer,” says Mark Hoddle, the director of the Center for Invasive Species Research at the University of California, Riverside. The loss of these willows shading the river affects all “the aquatic organisms, crawdads, fish,” he says. “The water becomes too hot for them to live in.”
The ISHB has a wide plant palette: More than 360 tree species have been attacked, and it is able to reproduce in at least 62 species. This beetle doesn’t eat wood. It bores into the tree, carrying a number of different fungi that cause Fusarium dieback and, in many cases, death.
It looks identical to Euwallacea fornicatus, or the tea shot hole borer, which has plagued tea plantations in southern India and Sri Lanka for decades. It also looks like an ambrosia beetle that has infested avocado groves in Israel since 2009.
And unlike many ambrosia beetles, which are attracted to dead trees, the ISHB attacks healthy trees because the fungi it carries need water and nutrients to grow.
Ambrosia beetles are fungus farmers, which means that they carry fungi in special mandibular sacs called mycangia as they tunnel into the wood, or xylem, to lay their eggs. As Eskalen explains, the females inoculate the walls of the bored channels, or galleries, with the fungi, which flourish on the nutrients and water flowing up the xylem. Mature beetles and their larvae feast on the fungi, while the host tree is deprived of food and water.
The offspring, most of which are female, mate with their male siblings, and, in about a month, exit the tree pregnant and ready to start the cycle all over again in another tree. The wingless males rarely exit the tree.
Scientists have no evidence of how the beetle first entered the country, but suspect that it came in on wooden packing materials from Vietnam. It took a few years to increase its population. The dead box elders in Long Beach sounded an alarm in 2010.
Two years later, Eskalen, who works for the extension service at UC Riverside, got an e-mail from a resident in South Gate asking about her avocado tree. Eskalen inspected the tree and found some of its upper branches had lost their leaves, and the trunk was peppered with tiny holes. “I didn’t know what it was,” Eskalen says. “I hadn’t seen symptoms like that before.”
He took samples back to his lab, and initially identified the beetle as morphologically identical to Euwallacea fornicatus, the tea shot hole borer. But Richard Stouthamer, an entomologist at the lab, has found differences in the genetic coding that suggest three distinct but closely related species.
A few months after Eskalen was called out to look at the ailing avocado in South Gate, he got a call from the Huntington Botanical Gardens in San Marino. Some of the avocado trees looked sickly, said Tim Thibault, the curator of woody plants, “and we had a large English oak that was declining, but we couldn’t figure out why.” Eskalen examined the trees and saw the telltale symptoms on both species.
Surveying the Huntington’s huge collection, as well as the trees at the Los Angeles County Arboretum & Botanic Garden in Arcadia, he found the PSHB infesting 207 species; now that number has grown to 362 species infested by ISHB in Southern California.
Both public gardens, with their smorgasbords of species, are being closely watched by scientists. They’re studying the ISHB’s preferences, which species allow the fungi to flourish, and which ones show resistance. The beetle is able to reproduce in at least 62 tree species, but is repelled by others. Pines, for instance, block them at the holes with resin; cherry species exude a juice that turns hard as amber.
Other tree species somehow discourage the fungi from growing. Just how or why are key questions for researchers.
The Huntington has tested various chemical treatments, its nursery manager Dan Berry says. “We’ve tried soil drench, basal trunk application, sprays outside, prophylactic injection. From what I’ve seen, it’s all a waste of money.” Once the beetle is in the tree, “nothing will work.” Basically, garden managers are watching their trees, trimming off diseased branches, and taking down the dead ones. “We have probably lost about 40 trees, but have easily a couple thousand infected,” Berry says.
About 10 of the Huntington’s box elders, the beetle’s preferred species, were the first to succumb, then about 10 sycamores, as the beetle moved on. Now it is attacking the oaks, including English oaks (Quercus robur), California live oak (Quercus agrifolia), and valley oak (Quercus lobata).
Berry used to look out his office window onto a canopy of pines, oaks, and other species. “Now, it’s kind of like a picket fence.”
For homeowners, of course, removing a signature tree is a terrible loss, both emotionally and financially. Berry is advising one of the garden’s philanthropists, who has 250 oak trees on his 11-acre property. “Quite a few are infected, but oaks are strong, healthy trees, so we’re not sure of the timeline,” Berry says. “Ultimately, he’ll have to take some down.”
Berry has taken down five oaks at the Huntington, and isn’t optimistic about the rest. “I don’t think they will survive.”
On the other hand, though avocado trees were among the first species to show damage, suffering wilt and branch dieback, they seem to survive and still bear fruit.
Trimming diseased branches, then chipping and grinding the wood, and covering the material to heat up in the sun kills about 98 percent of the beetles, Berry says.
Liability issues push city arborists to cut down trees that a homeowner might simply prune and hope for the best. Orange County, in the epicenter of the attack, has taken down 2,000 trees. The University of California, Irvine, has cut down 700 trees on its own wooded campus. Park West, a landscape construction and maintenance company, which maintains 364 properties, most of them sprawling master plan communities, has removed thousands of trees. “It’s almost an epidemic in some of these areas, and there’s not much we can do once the shot hole borer gets into the tree,” says Cory Gallagher, Park West’s president. “Treatments are not effective.”
Streets once lined with sycamores are now bare, and homeowner associations have not budgeted for the cost of takedown and replacement. “We recommend resistant species to these boards, and they say, ‘Well, we still have some healthy sycamores; let’s replace the dead ones with sycamores,” Gallagher says. It’s illogical, but they love their trees. And he can’t guarantee a different species will resist the beetle.
“Our overall goal is to try to eradicate the pest by removing the trees and planting something that will reject the insect, but the list of host species keeps growing,” Gallagher says.
Shannon Lynch, the plant pathologist, has set up plots all around Los Angeles and in the Tijuana River Valley, studying the borer and the surrounding trees. “It’s not necessarily killing everything, but there’s enough population pressure that if it needs to find a new home, whether it’s going to jump onto a conifer, I don’t know.”
She has taken samples of wood from resistant species, and found some bacteria species that inhibited the growth of the Fusarium. This, she says, “suggests that if the bacteria can prevent the growth of the fungus, then it can inhibit the beetle, because the fungus is its sole food source.” When avocado and sycamore seedlings were inoculated with the bacteria, the Fusarium could not grow.
As Lynch samples a broader range of natives, looking for resistant bacteria, she hopes to develop a biocontrol. “What’s neat about that is they’re locally adapted, they’re already here,” Lynch says.
Stouthamer is working on a couple of insect predators, but testing such biological agents takes years. Meanwhile, the ISHB is spreading unchecked. People cutting down infected trees may not read those warnings from extension agents about grinding up branches and trunks. Maybe they cut up the wood for firewood and sell it to people in another county, or farther north or south, giving the borer a ride. “It’s absolutely an ecological disaster,” says Jerrold Turney, a senior biologist and plant pathologist for the Los Angeles Agricultural Commissioner’s office. “I think it will be an epidemic as significant as Dutch elm disease and chestnut blight. What’s going to stop it? We’re all pushing for the regulation of firewood, but not getting any action on it. Somebody is going to drive to the Sierras or hundreds of miles and move these beetles all over the place.”
Exotic insects can easily come into the country because only a small percentage of containers are inspected. Stouthamer led a team of scientists to Vietnam, the beetle’s place of origin, to bring back possible biological controls. Diseased trees were being cut down to make furniture and wooden containers, which could easily find their way to this country.
Hoddle had just returned from a two-day invasive species summit, where some 40 leaders in conservation, agriculture, and urban forestry brainstormed on how to drive a bill through the state legislature that would deal not just with the shot borer invasion, but many other pests. “We get nine new terrestrial invertebrates a year establishing populations in California, and on average, three become pests,” he says. “The number one impediment to doing anything is no funding. Tens of millions need to be set aside to deal with this problem.”
As the Trump administration dismantles many federal environmental safeguards, Hoddle is undeterred. He’s applied to become a member of the Invasive Species Advisory Committee, administered by the National Invasive Species Council.
Gregory McPherson, who conducted the report for the U.S. Forest Service, lists the dire consequences of the undeterred shot hole borer: hotter cities, more ozone pollution and carbon dioxide, more storm runoff and impaired water quality, and unknown tolls on wildlife and human health.
He cites the deaths from illnesses associated with the loss of 100 million ash trees in the Midwest to the emerald ash borer: 6,113 deaths related to illness of the lower respiratory system and 15,080 cardiovascular-related deaths. He projects the economic losses should the shot hole borer kill half of the 23 million vulnerable trees in the Los Angeles area: $617 million annually in lost ecosystem services, $16 billion to remove and replace dead trees.
Anne Raver has written about plants and the environment for more than 30 years.