The Polyphagous Shot Hole Borer
By Amina Khan
Akif Eskalen steps through the dense, damp leaves in a wooded neighborhood, scrutinizing the branches around him. He’s looking for evidence of an attack: tiny wounds piercing the bark and sap dried around them like bloodstains.
The victims are box elders, sycamores and coast live oaks, all in some state of suffering. Eskalen approaches a tree riddled with 1-millimeter holes, as if someone used it for miniature target practice. It’s time to nab a perp. He selects a hole, pulls out a large knife and expertly levers out a chunk of wood.
There, in his hand, is a glossy beetle no larger than a sesame seed: the polyphagous shot hole borer.
Though small and sluggish, its appetites are wide and its spread is relentless. It attacks forest trees, city trees and key agricultural trees. It has defied all conventional and chemical weapons. No one seems to have a way to stop it.
This cul-de-sac, in the foothill community of Pasadena Glen, has been particularly hard-hit. One giant tree was so weak that one of its trunks toppled from one resident’s yard across a creek and crashed into a neighbor’s. Another tree was filled with green jelly, its insides completely digested.
Anxious residents cluster around as Eskalen examines a wounded sycamore.
“We’ve got a canyon full of highly educated people who want answers,” said longtime resident Linda Williams, a retired business owner.
Eskalen, a plant pathologist at UC Riverside, wants to contain this invasive bug before it spreads throughout Southern California. Already the beetle has been sighted as far south as San Diego, as far west as Santa Monica, as far east as the Riverside County city of Eastvale.
Eskalen tips the beetle into a glass vial. He detaches a pink spray bottle from his backpack and administers a few lethal squirts of ethanol before twisting the vial shut.
These beetles have a strange M.O. They don’t eat wood, like termites; instead, they drill circular tunnels toward the heart of the tree. They carry fungal spores in their mouths and sow them like seeds as they go. Then they harvest the fungus to feed their larvae. It’s a deadly partnership: The beetles attack, but the fungus also helps to kill, colonizing the wood tissue and spreading through the plant.
The beetles have easily evaded the authorities. Inside the tree, they’re well protected from pesticide sprays. The incestuous offspring mate with their siblings inside the trunk, so sex pheromones do not lure them out.
“If we can’t control them,” Eskalen said, “they are going to wipe out all our trees.”
Eskalen’s first contact with these devious bugs was in 2012, triggered by a desperate email from South Gate resident Chelo Ghaly. The real estate closer’s avocado tree had been oozing white spots all over its trunk.
Local gardening authorities were of no use. Suggestions to try fungal sprays failed. Frustrated, she scoured the UC Riverside website and found Eskalen, who studies avocado diseases. She sent him pictures of the damage.
Eskalen said he looked at the strange symptoms and grabbed his car keys.
After examining the fungus in Ghaly’s tree, he took his findings to the California Avocado Commission. To date, they’ve given UC Riverside scientists a total of $800,000 to broaden his investigation into this mysterious species of ambrosia beetle. His survey came to a head when he reached the Los Angeles County Arboretum and the Huntington Botanical Gardens — two repositories of healthy, well-kept trees.
They’re not so healthy anymore.
At the Huntington in San Marino, Eskalen spied a diseased specimen. He hopped up, grabbed a high branch with both hands and bounced until it snapped off.
“Ach, Akif, really?” said Tim Thibault, the Huntington’s curator of woody collections. It didn’t matter: The plant was probably a goner.
Such pests typically feast on a small group of plants. But this one doesn’t seem to discriminate.
When Eskalen and his colleagues surveyed the 335 species at the Huntington and the Arboretum, in Arcadia, they found the beetle had attacked 207 of them and 54% of these victims were infected with fungus. Nearly two dozen of the trees were being used as reproductive hosts — places where the beetles can raise their brood.
The consequences of a wide-ranging infestation could be enormous. Common city trees, such as American sweetgum and maple, would become public branch-dropping hazards. Native trees such as the California sycamore and the coast live oak have started to succumb, creating a fire risk in the form of dead, dry tinder. Avocados and other crops could face huge financial losses.
Like any good forensic investigator, Eskalen is using DNA to crack the case. He started by sequencing the fungi from diseased trees and found there were three different species — two that may serve as food and one that may act as protection by fighting off competing plant pathogens.
He also enlisted UC Riverside entomologist Richard Stouthamer to examine the DNA of the beetles themselves. Stouthamer’s genetic analysis traced the bugs to Vietnam.
Experts aren’t sure how the beetle made its way to the U.S. Invasive species around the world are moved on human cargo, in clothes, in wooden shipping pallets and in boats’ ballast water.
The beetles are not as much of a pest in Asia — perhaps because some other critter keeps them in check. Maybe that natural enemy could be brought to California to fight the infestation, a practice known as biological control.
In March, Eskalen, Stouthamer and Thibault spent two weeks in Vietnam, searching forests and fields for these natural vigilantes. They collected a host of possible allies, whose DNA is being analyzed in the lab.
“I am very hopeful that we are going to find some solutions to control this fungus,” Eskalen said. “We have to.”
Thus far, what little is known about the polyphagous shot hole borer doesn’t place it in the alarm-raising category of the Mediterranean fruit fly or the Japanese beetle, which some say spread more quickly. The U.S. Department of Agriculture hasn’t imposed any restrictions to contain this beetle.
Still, Eskalen and others fear a worst-case scenario if not enough is done to contain the pest. The number of tree species attacked by the beetle now stands at 286.
Even as the Vietnam-related infestation spreads, Stouthamer’s genetic fingerprinting linked the incursion in San Diego to a different population in Taiwan.
While scientists try to get a handle on the situation, residents like Ghaly are struggling with the pest on their own.
Ghaly had been keeping the tree alive at Eskalen’s request. But as her tree worsened, she thought about her elderly mother cleaning up after the failing plant. There were also signs that the infestation in her neighborhood was spreading.
She couldn’t wait any longer. Early this year, she had it chopped down.
