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The shot hole borer doesn’t look or behave like a killer.
Yet the insect — about the size of a sesame seed — could be a lethal threat to Sacramento’s urban forest. It’s already decimated hundreds of thousands of trees across at least six counties in Southern California. Many fear the rest of the state could be next.
Scientists are mobilizing to find ways to slow the shot hole borer’s advance. Fast solutions have not been easy to find. State lawmakers recognized the seriousness of the risk and directed $5 million last year to the Invasive Species Council of California to eliminate the beetle.
In the last decade however, the insect has proven a strong foe and become one of the state’s most unwanted invasive critters. Its progress in Southern California has been steady enough for other regions to be concerned.
“It’s not here yet,” said Ray Tretheway, executive director of the Sacramento Tree Foundation, “but the arborists and urban forest managers always tell us that it’s inevitable that it will be here someday.”
Two variety of the shot hole borer are present in California: the polyphagous, which loosely translates to excessive desire to eat, and the Kuroshio that is more common in San Diego County.
While some pests desire one type of tree, the shot hole borer can survive in at least 64 different kinds — mostly trees that grow near riversides like willows, cottonwoods and sycamores. The Sacramento region, concentrated at the intersection of two rivers and a vast waterfront parkway, could be a prime target.
The beetle prefers to live in sycamores like the spotted gray and flaky-trunked London plane trees, which make up a sizable chunk of the city’s entire tree population.
“If it were to reach our area we would imagine the results would be similar to those in Southern California,” said Kevin Hocker, the city’s urban forester. “There’s a possibility that if things get out of control we could lose 25 percent of our population all at once because they’re all the same type of tree.”
The shot hole borer is not a strong flier so it’s unlikely to move far on its own. It can, however, move faster when hiding in firewood and other green waste or landscaping equipment, said Akif Eskalen, a plant pathologist and professor at UC Davis.
Eskalen is one of many investigators looking for ways to naturally eradicate the beetle. Eskalen’s work focuses on using native plants, but there is an ongoing trial with pesticides and another using natural predators. The studies, he said, “may take several years to complete.”
Native to southeast Asia, the shot hole borer doesn’t eat the trees. The beetle burrows into them and releases spores to create a fungus that it eats exclusively. The fungus restricts the trees’ water distribution, slowly killing it over time.
Once inside a tree, the female produces offspring that mate with each other when they grow up and the death-dealing cycle repeats. This sequence happens as many as four times a year, or more if the weather is hotter, said Beatriz Nobua-Behrmann, a researcher studying the insect for the UC Cooperative Extension in Irvine.
They’re believed to have arrived in wood packing materials made with infested wood from Taiwan and Vietnam, she said. Climate change could make it worse since the beetle thrives in warm weather. Its reproductive cycle accelerates as the temperature rises.
“Knowing that they can travel in wood or in green waste or in equipment that makes it a little bit more dangerous,” Nobua-Behrmann said. “It could easily make it to the northern counties with people moving firewood because they find it for cheap or for free.”
Preliminary data collected by the UC Cooperative Extension shows there were at least 450 infestations detected in 2012, but there were nearly three times that amount detected in 2016. What’s clear in data is the beetle’s preference for sycamores, which accounted for more than one-third of the infestations, a Bee analysis shows.
One of the worst cases recorded has been in south San Diego County. Researchers concluded in a study published in May that the Kuroshio shot hole borer had overrun 30 percent of the native willows, or 120,000 trees, in the Tijuana River Valley.
The first shot hole borer was found in California in 2003 but the insect has a tendency to go under the radar and then emerge in large numbers, Nobua-Behrmann said. The trunk will often start to look sickly but the branches still hold leaves.
“They attack a few trees and they tend to reproduce and stay on the same tree they were born,” Nobua-Behrmann said. “By the time the trees start showing really bad signs like dieback, the tree is filled with thousands of beetles.”
This story is part of Tipping Point, our new project focused on telling the stories of the Sacramento region’s evolution. Read more of our Tipping Point stories here.