Invasive shothole borers are no longer just a Southern California problem. In September 2025, officials confirmed the beetles in two trees in the Tannery area of the City of Santa Cruz, with another heavily infested tree found just upriver in Paradise Park.
That’s concerning because these tiny, non-native ambrosia beetles are among the many pests and diseases that threaten Bay Area trees and can attack healthy trees as well as those already under stress.
Making things worse, they introduce fungi that cause Fusarium dieback when they tunnel into the wood. This disrupts the tree’s ability to transport water, potentially leading to branch dieback or death.
Quick Facts About Invasive Shothole Borers
- Pest type: Tiny, non-native ambrosia beetles in the genus Euwallacea
- Species found in California: Polyphagous shothole borer (Euwallacea fornicatus), Kuroshio shothole borer (Euwallacea kuroshio), and greater shothole borer (Euwallacea interjectus)
- Adult size: About 1/16 to 1/6 inch long, depending on the species and sex
- Associated disease: Fusarium dieback
- How they damage trees: Female beetles tunnel into the wood and introduce fungi that colonize the surrounding tissue and interfere with water movement
- Common signs: Tiny round entry holes, staining or gumming around holes, boring dust, and branch or canopy dieback
- Trees at risk: Dozens of native and ornamental species, including sycamores, maples, oaks, willows, and cottonwoods
- How they spread: Adult females can fly to nearby trees, while infested wood can transport the beetles much farther
The History of Invasive Shothole Borers in California
California’s invasive shothole borer problem dates back to 2003, when the polyphagous shothole borer was first detected at Whittier Narrows in Los Angeles County.
The Kuroshio shothole borer was later detected in San Diego County, and the polyphagous species was eventually detected much farther north, reaching Santa Clara County by 2023..
A different species — the greater shothole borer (Euwallacea interjectus) — was detected in Santa Cruz County in 2024. The September 2025 detection in Paradise Park and the City of Santa Cruz showed that invasive shothole borers remained an active local concern.
Common Name Confusion
Despite the similar name, these invasive Euwallacea beetles should not be confused with the unrelated shothole borer (Scolytus rugulosus), a pest of fruit and other trees.
How Do Invasive Shothole Borers Spread Fusarium Dieback?
Invasive shothole borers don’t actually feed on the wood they tunnel into. Instead, female beetles bore into trunks and branches to create a network of tunnels called galleries, where they lay their eggs and introduce the fungi they rely on for food.
The relationship benefits both the beetles and the fungi. The beetles provide a place for the fungi to grow, while the fungi provide food for both adult beetles and their larvae. When new female beetles leave the tree, they carry fungal spores with them to their next host, allowing the cycle to begin again.
The trees, on the other hand, suffer from this symbiosis.
As the fungi grow, they spread beyond the galleries and into the surrounding wood, interfering with the tree’s ability to transport water. This can lead to wilting, branch dieback, canopy decline, and, in severe cases, tree death.
Which Trees Are at Risk from Shothole Borers?
Invasive shothole borers can attack hundreds of tree species, but not every tree faces the same level of risk. Some trees can also serve as reproductive hosts, meaning the beetles can successfully reproduce and cultivate their associated fungi inside them.
Several California trees are particularly susceptible to serious damage, including:
- California sycamore (Platanus racemosa): This native sycamore can support reproducing beetle populations and may suffer severe dieback or death.
- Bigleaf maple (Acer macrophyllum): Another California native, bigleaf maple is a confirmed reproductive host that can be killed by an infestation.
- Valley oak (Quercus lobata): Valley oaks are susceptible to serious damage and are among the hosts that may be killed.
- Willows: Native species such as arroyo willow (Salix lasiolepis) and red willow (Salix laevigata) can suffer significant damage.
- Cottonwoods: Fremont cottonwood (Populus fremontii) and black cottonwood (Populus trichocarpa) are also vulnerable to the beetle-fungus complex.
Other reproductive hosts are considered less susceptible and are more commonly associated with branch dieback than tree death.
These include coast live oak (Quercus agrifolia), avocado (Persea americana), California buckeye (Aesculus californica), Japanese maple (Acer palmatum), and several acacia species (Acacia spp.).
Signs and Symptoms of Shothole Borer Infestation
Because invasive shothole borers spend most of their lives hidden inside trees, you’re more likely to notice evidence of their activity than the beetles themselves. Symptoms can vary by tree species, but several signs may point to an infestation.
- Tiny, Round Entry Holes: Look for small, perfectly round holes in the trunk or branches. Most invasive shothole borer holes are about the size of the tip of a ballpoint pen, although greater shothole borers create somewhat larger holes.
- Staining or Gumming: Trees may develop dark or wet-looking stains around entry holes. Some species respond to beetle attacks by producing sap or thick gum.
- Frass and Boring Dust: Fine, sawdust-like material may collect around entry holes. On some trees, frass can protrude from the bark in small, toothpick-like strands.
- White, Sugary Buildup: Avocado trees in particular may produce a distinctive white, powdery or crystalline exudate around beetle attacks.
- Wilting and Branch Dieback: As Fusarium dieback progresses, leaves may wilt and branches can begin to die. Advanced infestations can lead to broader canopy decline.
Just note that these symptoms aren’t unique to invasive shothole borers.
Other insects, diseases, and even physical injuries can produce staining, sap, frass, or dieback. The size and shape of the entry holes are an important diagnostic clue, but a professional evaluation can help confirm what’s actually affecting the tree.
Can an Infested Tree Recover from Shothole Borer and Fusarium Dieback?
Fortunately, some trees can recover from beetle infestations and Fusarium dieback. Whether an infested tree can recover depends on the tree species, its overall health, and how far the infestation has progressed.
Trees with light infestations may have more management options, especially when the problem is identified early. But as beetle populations increase and Fusarium dieback spreads through the tree, the chances of recovery decline.
Severely infested trees may eventually require removal, particularly when dieback or structural damage creates a safety concern.
Treatment and Management Options for Shothole Borer
There is no single treatment for an invasive shothole borer infestation. The right approach depends on the tree species, the extent of the infestation, the tree’s value and condition, and whether damaged limbs pose a safety risk.
Depending on the situation, management may include:
- Monitoring: Trees with light infestations may simply need regular inspections to watch for new entry holes, staining, frass, or dieback.
- Pruning Infested Branches: When beetle activity is confined to individual branches, removing affected limbs may help prevent the infestation from advancing into the trunk.
- Professional Treatments: For some lightly to moderately infested, high-value trees, insecticide and fungicide treatments may be used along with pruning and continued monitoring.
- Proper Wood Disposal: Infested branches and logs can harbor live beetles, so removed wood must be handled carefully to prevent the pests from emerging and spreading to other trees.
- Tree Removal: Severely infested trees may be too compromised to save. Removal may be recommended when the infestation is extensive or the tree has become hazardous.
Importantly, treating healthy, uninfested trees with pesticides as a preventive measure is not currently recommended by UC ANR, because no preventive treatment has been proven effective.
How to Protect Your California Trees from Invasive Shothole Borers
You can’t completely prevent invasive shothole borers from reaching your property, but you can take steps to limit their spread and give your trees a better chance of staying healthy.
- Inspect Trees Regularly: Watch trunks and branches for tiny entry holes, staining, frass, or new areas of dieback. Finding an infestation early provides more management options.
- Keep Trees Healthy: Proper watering and protection from unnecessary injury can help reduce tree stress. Avoid overwatering, underwatering, and excessive pruning.
- Don’t Move Firewood: Invasive shothole borers can travel long distances inside infested wood. Buy firewood near where you plan to burn it rather than transporting it between areas.
- Handle Infested Wood Carefully: Wood from infested trees can continue to harbor live beetles after pruning or removal. It should be chipped, treated, or disposed of according to recommended protocols rather than reused as untreated firewood or mulch.
- Sanitize Pruning Tools: Tools used on infected wood should be cleaned and disinfected before they’re used on healthy trees to reduce the risk of spreading plant pathogens.
If you spot possible signs of an infestation, have the tree evaluated before deciding how to proceed. An ISA-certified arborist can help determine whether shothole borers are responsible, assess the extent of the damage, and recommend appropriate next steps.
Invasive Shothole Borers: FAQ
Homeowners often have questions about how to identify and manage invasive shothole borers. Here are answers to some of the most common.
Can invasive shothole borers kill healthy trees?
Yes. Unlike many native ambrosia beetles that primarily attack stressed, injured, or dying trees, invasive shothole borers can colonize healthy trees. Susceptible reproductive hosts may eventually die from the combined effects of beetle tunneling and Fusarium dieback.
How can I tell if a hole was made by a shothole borer?
Look for a small, round entry hole accompanied by signs such as wet staining, gumming, frass, or sugary buildup. However, several other insects and tree problems can look similar, so the hole alone isn’t enough to confirm an infestation.
Do invasive shothole borers only attack certain types of trees?
No. They can attack a wide variety of native and ornamental trees. However, only some species are reproductive hosts that allow the beetles to complete their life cycle, and susceptibility to serious damage varies considerably among tree species.
Can shothole borers spread from one tree to another?
Yes. Adult female beetles can fly to nearby host trees, while infested logs, branches, and firewood can carry the beetles much farther. This is why proper handling of infested wood is an important part of limiting their spread.
Should I treat a healthy tree before shothole borers attack it?
No. Current University of California guidance does not recommend pesticide treatment for trees that aren’t already infested because no preventive treatment has been proven effective. Regular monitoring and proper tree care are better preventive strategies.
Can I treat a shothole borer infestation myself?
Professional treatment is generally recommended for confirmed infestations. Effective chemical treatments may require products available only to licensed pesticide applicators, and proper identification is important before any treatment begins.
What should I do if I think my tree has invasive shothole borers?
Avoid cutting or moving potentially infested wood until you know what you’re dealing with. A certified arborist can evaluate the tree, determine whether the symptoms are consistent with invasive shothole borers, and recommend appropriate management based on the tree’s condition and the severity of the infestation.
Adney Tree Service: Protecting California Trees from Shothole Borers
Invasive shothole borers can be difficult to spot, and the signs they leave behind can resemble those caused by other pests and diseases.
If you notice tiny entry holes, staining, frass, or unexplained dieback, an ISA-certified arborist can identify the problem, assess the extent of the damage, and determine the best course of action for your tree.
If you’re concerned about shothole borers or the health of your trees, contact Adney Tree Service to schedule an evaluation. We’ll inspect your trees and recommend the appropriate next steps to help protect your landscape.
Sources & References
- Lookout Santa Cruz – “County ag officials warn of invasive beetle in Santa Cruz” – September 2025 reporting on the Santa Cruz County Department of Agriculture and UC Cooperative Extension confirmation of invasive shothole borers in the Tannery area and Paradise Park. https://lookout.co/county-ag-officials-warn-of-invasive-beetle-in-santa-cruz/story
- County of Santa Cruz – “Invasive Beetle Identified as Novel to West Coast” – December 2024 announcement identifying the Felton-area invasive shothole borer as greater shothole borer (Euwallacea interjectus) and describing the local infestation. https://www.santacruzcountyca.gov/portals/0/county/CAO/press%20releases/2024/BorerUpdate.12112024.pdf
- University of California Agriculture and Natural Resources – “Invasive Shothole Borers: Pest Overview” – An overview of invasive shothole borer biology, Fusarium dieback, California distribution, introduction history, and the three shothole borer species currently detected in the state.
https://ucanr.edu/site/invasive-shothole-borers/pest-overview - University of California Agriculture and Natural Resources – “ISHB Reproductive Hosts” — A current list of confirmed reproductive hosts, including California trees susceptible to severe Fusarium dieback and species associated with less severe damage.
https://ucanr.edu/site/invasive-shothole-borers/ishb-reproductive-hosts - University of California Agriculture and Natural Resources – “Invasive Shothole Borers: Management” – Guidance on monitoring, infestation severity, pruning and treatment options, tree removal, firewood movement, sanitation, and proper disposal of infested wood.
https://ucanr.edu/site/invasive-shothole-borers/management - University of California Statewide Integrated Pest Management Program – “Invasive Shot Hole Borers” – Detailed guidance on invasive shothole borer identification, biology, host range and reproductive hosts, signs and symptoms, Fusarium dieback, and management of infested trees.
https://ipm.ucanr.edu/Invasive-and-Exotic-Pests/Invasive-Shot-Hole-Borers/
