Two invasive beetles are killing trees across California — and researchers can’t predict where they’ll strike next.
The kuroshio and polyphagous shot-hole borers attack by drilling into trunks and branches, spreading a fungus that cuts off a tree’s ability to survive.
They target a wide range of trees — from backyard shade and ornamental species to fruit crops such as apple, cherry, pear, and plum. The damage can be widespread: In south San Diego County, about 120,000 native willows died within three years from a single infestation.
Here’s what to know about the tree-killing pests.
What trees are at risk from shot-hole borers?
University of California Scientists say some trees are more vulnerable than others. The University of California has identified 15 tree species at high risk and dozens more that can also be attacked, including many common California natives and widely planted landscape trees.
Among the most vulnerable trees in California:
Researchers say maples and sycamores are among the landscape trees most frequently killed in urban settings, while willows and cottonwoods have suffered widespread die‑offs along rivers and floodplains in Southern California.
Shot‑hole borers also attack fruit and orchard trees, including avocado, apple, cherry, pear, plum, peach, and other deciduous crops, as well as stressed or damaged shade trees planted in yards, parks, and along streets.
University researchers say tree stress is a major factor. Drought, sunburn, flooding, construction damage, and disease can make trees more attractive to beetles, which spread a fungus that blocks water and nutrient flow and can quickly lead to branch dieback or death.
UC scientists have identified more than 60 reproductive host trees in California, meaning the beetles can both infest the tree and reproduce inside it — and hundreds of additional species that can still be attacked, even if the insects don’t successfully reproduce.
How are shot-hole borers tracked?
Tracking the beetles remains difficult. Infestations don’t spread in neat waves; instead, they appear in scattered pockets and hit some areas much harder than others, researchers found in a study of the Tijuana River Valley, one of the regions most affected by the pests.
That randomness has long made it nearly impossible to explain — or predict — where shot‑hole borers will show up next, the researchers wrote.
Now, scientists say pollution may help explain the pattern.
The Tijuana River Valley — one of the hardest‑hit areas — is also among the most polluted in Southern California. Sewage runoff has acted as fertilizer for plants, altering soil and water conditions and stressing trees, researchers said. That environmental stress may make trees more vulnerable to invasion, creating favorable conditions for shot‑hole borers to take hold, according to the study.
Shot-hole borers are common’ in the Central Valley
The shot-hole borer is “really a common pest” that “has been around forever,” said Tom Tucker, Tulare County agricultural commissioner and sealer.
The beetle can pose the greatest threat to young, newly planted trees—especially deciduous varieties such as peaches and plums—when it bores into the bud or graft union, Tucker said.
“If it gets in there … it’ll actually kill the trees,” he said.
The pest is widespread in areas where deciduous trees are grown and is also known to target older trees that have been sunburned or otherwise damaged, Tucker said.
He emphasized that proper irrigation is key to prevention. “Farmers everywhere, especially here in the Valley, know how important it is to make sure that their trees are not water-stressed,” he said, noting that maintaining tree health is critical to preventing borers and other diseases.
E-commerce a growing threat spreading pests in California
E-commerce has become an emerging threat to pest prevention in California. Many websites offer plants and soil that often conceal invasive pests.
“Major marketplaces like eBay, Amazon, and Etsy have made efforts to ensure these restrictions are clear to consumers, but further outreach is required to ensure that other businesses comply with state regulations,” according to the Comprehensive Pest Prevention Program Analysis.
Shot-hole borers were first reported in the northwestern U.S. in the early 1900s. Introduced from Europe, they have since spread to most of the U.S. and southern Canada. The adult shot-hole borer is brownish black, about 1/12 inch long, and has brown legs, antennae, and a stubby snout.
Researchers advise property owners to keep their trees healthy, as shot-hole borers are attracted to injured or stressed trees, and chemical controls have not been effective.
“The best investment we can make in our invasive pest system is to prevent infestations from happening in the first place,” CDFA Secretary Karen Ross said. “We have a robust and comprehensive approach to detect and eradicate infestations when they do make it into our state and into our fields, but it’s important to limit the need for these more costly measures by keeping pests out when we can.”
This article originally appeared on Visalia Times-Delta: Why shot‑hole borers are killing California trees — and hard to stop
Reporting by Steve Pastis, USA TODAY NETWORK / Visalia Times-Delta
USA TODAY Network via Reuters Connect
