For years, invasive shothole borers and the disease they transmit — Fusarium dieback — were considered primarily a Southern California problem.
But that has begun to change.
As these destructive pests continue expanding into Northern California and parts of the greater Bay Area, it has become important for homeowners and property managers to familiarize themselves with the threat they pose to many common landscape trees.
Below, we’ll explain how invasive shothole borers and Fusarium dieback work, the trees they’re most likely to attack, the warning signs homeowners should watch for, and the treatment options available.
What Are Invasive Shothole Borers?
Invasive shothole borers are tiny ambrosia beetles that are native to Southeast Asia. They were first detected in Southern California more than a decade ago.
In contrast to most other bark beetles, which typically attack weakened or dying trees, shothole borers often infest healthy trees. Adult females bore into trunks and branches, creating small tunnels where they lay eggs and establish new colonies.
However, the tunneling itself isn’t the primary cause of tree decline. Unlike the large galleries created by some other boring insects, shothole borers tunnel primarily through the xylem rather than the phloem and cambium. While this still causes some damage, healthy trees can often tolerate limited tunneling.
Instead, shothole borers are dangerous because they transport pathogenic fungi wherever they go. Every new gallery they create introduces these fungi into the tree’s vascular system, setting the stage for a disease known as Fusarium dieback.
What Is Fusarium Dieback?
Fusarium dieback is a vascular disease caused by several closely related Fusarium fungi that are carried from tree to tree by shothole borers.
Once inside the tree, the fungi colonize the tree’s xylem, which can damage the tree’s ability to transport water. As the infection spreads, water movement becomes increasingly restricted, causing branches to wilt, die back, and eventually fail.
In severe cases, the disease can lead to widespread canopy decline or kill the entire tree.
The relationship between shothole borers and Fusarium dieback is remarkably close. The beetles rely on the fungi as their primary food source, while the fungi depend almost entirely on the beetles for dispersal to new host trees.
This is why arborists typically think of shothole borers and Fusarium dieback as a single pest-and-disease complex rather than two separate problems. Neither the insect nor the fungus would be very destructive in isolation.
Which Bay Area Trees Are at Highest Risk?
One of the reasons shothole borers are so concerning is their exceptionally broad host range.
Researchers have identified hundreds of tree species that may be attacked, although not all are equally susceptible. Some trees support beetle reproduction and often develop severe Fusarium dieback, while others suffer only limited damage.
Some of the Bay Area tree species most commonly affected by shothole-borer-Fusarium-dieback include:
- Coast Live Oak (Quercus agrifolia): One of California’s signature native oaks, coast live oak can suffer branch dieback and significant canopy decline following infestation.
- Valley Oak (Quercus lobata): The valley oak is another iconic native species that is susceptible to both beetle attacks and Fusarium dieback.
- California Sycamore (Platanus racemosa): Among the most heavily impacted native species, California sycamores may develop extensive branch dieback and can decline rapidly.
- Bigleaf Maple (Acer macrophyllum): A common native shade tree, the bigleaf maple can serve as a reproductive host for shothole borers.
- Box Elder (Acer negundo): Box elders (also called ash-leaf maples) are frequently attacked in infested areas and capable of supporting beetle populations.
- Willows (Salix spp.): Several native willow species are susceptible to shothole borers and Fusarium fungi, and they may experience dieback following infestation.
- Cottonwoods (Populus spp.): Common along streams and riparian areas, cottonwoods can also serve as reproductive hosts for these insects and fungi.
- English Oak (Quercus robur): The English oak is a widely planted landscape tree that has shown susceptibility in California.
- Avocado (Persea americana): Although uncommon in most Bay Area landscapes, avocados may suffer from reduced fruit production and failing tree health.
Do note that new host species continue to be identified every year and susceptibility can vary between individual trees. So, you should always take potential signs of shothole borers and Fusarium dieback seriously — even if your tree is a species thought to be safe.
Symptoms of Shothole Borers and Fusarium Dieback
One of the biggest challenges posed by shothole borers is that infestations can go unnoticed until they’re in a relatively advanced stage. A mature beetle is about the size of a sesame seed, and the fungus doesn’t produce any changes until it is already affecting the tree’s vascular system.
That’s why it is so important to understand the signs of a potential attack. We’ll share a few of the most common signs and symptoms of shothole borers and Fusarium dieback below but note that many can also be caused by other pests and diseases.
Accordingly, you’ll want to contact an ISA-certified arborist to have your tree evaluated if you see any of the following clues.
Tiny Entry Holes
The first visible sign is often a series of perfectly round entry holes measuring just 0.03 to 0.09 inches across (roughly the diameter of a ballpoint pen tip). These holes may appear on the trunk, the large branches, or any exposed roots.
Wet Staining, Gumming, or “Sugar Volcanoes”
Trees frequently respond to beetle attacks by producing sap or other exudates around the entry holes. Depending on the tree species, you may notice dark wet stains, amber-colored gum, or small white deposits of crystallized sap, which are sometimes called “sugar volcanoes.”
Frass
As female beetles excavate their galleries, they push a combination of fine boring dust and waste — a substance known as frass — out of the entry holes. This frass may collect on bark crevices, branch unions, or around the base of the tree.
Dark Internal Discoloration
If you peel back the bark on a tree, you may see dark brown or black staining — a sign that Fusarium fungi are colonizing the tree’s vascular tissues. This symptom usually isn’t visible without removing bark (something that should only be done when necessary), but it is a good indicator of Fusarium dieback.
Branch Dieback
As the fungi interfere with water transport, individual branches within the canopy may begin wilting, dying back, or shedding leaves prematurely. Dieback often starts in isolated portions of the canopy before spreading throughout the tree.
Tree Death
Unfortunately, heavily infested trees may eventually succumb to the combined effects of beetle activity and Fusarium dieback. While some trees can survive with prompt treatment, advanced infestations are often difficult to manage successfully.
Can Invasive Shothole Borers Be Treated?
In reality, it can be difficult to treat trees that are infected with shothole borers or Fusarium dieback. However, treatment is sometimes possible, but you’ll have to initiate treatment very quickly, before the tree has suffered extensive damage.
A few of the things our plant health care technicians can help you do include:
- Monitor the Tree: Regular inspections offer the best chance of protecting valuable trees. Identifying an infestation while it’s still confined to a small portion of the tree allows for more management options and can improve treatment outcomes.
- Prune Infested Branches: If an infestation is limited to a few branches, careful pruning may help reduce beetle populations and remove infected wood before the problem spreads further. However, pruning is only effective when infestations are detected early.
- Administer Systemic Treatments: Systemic insecticides may help reduce beetle attacks in some cases, and fungicides may also be used as part of an integrated strategy to help suppress Fusarium dieback. However, these treatments are usually not effective for advanced infections.
- Remove the Tree: Removing severely affected trees may sometimes be the best approach, as it will eliminate a source of beetles, reduce the risk of infestation spreading to nearby susceptible trees, and prevent failures caused by declining structural integrity.
Just remember that every infestation is different, and there is no one-size-fits-all solution. An ISA Certified Arborist can evaluate the severity of the infestation, determine whether treatment is likely to be successful, and recommend the most appropriate management plan for your specific tree and property.
Frequently Asked Questions About Shothole Borers and Fusarium Dieback
Because shothole borers are still relatively new to much of the Bay Area, many homeowners have questions about how these pests spread, which trees they attack, and what can be done if an infestation is discovered. Below are answers to some of the questions we hear most often.
What are invasive shothole borers?
Invasive shothole borers are tiny ambrosia beetles that tunnel into living trees. As they create galleries, they introduce pathogenic Fusarium fungi that cause Fusarium dieback, a disease capable of disrupting water transport and killing susceptible trees.
Are invasive shothole borers found in the Bay Area?
Yes. After becoming established in Southern California, invasive shothole borers have been confirmed in parts of Northern California, including the greater Bay Area. Because these pests continue to expand their range, early detection has become increasingly important for homeowners and property managers.
Which trees are most susceptible?
Many native and ornamental trees can be attacked, including coast live oak, valley oak, California sycamore, bigleaf maple, box elder, willows, cottonwoods, English oak, and avocado. However, not every tree is equally vulnerable, and new host species continue to be identified.
What are the first signs of an infestation?
Early symptoms often include tiny round entry holes, wet staining or sap flow, fine sawdust (frass), or small white “sugar volcanoes” around the holes. As the disease progresses, branches may begin dying back and the canopy may become thin or sparse.
Can an infested tree be saved?
Sometimes. Trees that are diagnosed early may respond well to pruning, systemic treatments, or other management strategies. Once extensive Fusarium dieback has developed, however, treatment becomes much less effective.
Should I remove a tree with invasive shothole borers?
Not necessarily. Many lightly infested trees can be monitored or treated successfully. Removal is generally recommended only when a tree has suffered severe decline, poses a safety hazard, or is unlikely to recover despite treatment.
How can I protect my trees?
Regular inspections, maintaining good tree health, avoiding the movement of potentially infested firewood or wood debris, and scheduling a professional evaluation when symptoms first appear are some of the best ways to reduce the impact of shothole borers and Fusarium dieback.
Protect Your Bay Area Trees Before the Damage Spreads
Shothole borers and Fusarium dieback can progress quickly once a tree becomes infested. The earlier the problem is identified, the better the chances of preserving the tree and preventing the beetles from spreading to nearby hosts.
The ISA-certified arborists at Anderson’s Tree Care Specialists can inspect your trees, identify the cause of the problem, and recommend the most appropriate treatment or management strategy. Contact us today to help protect your Bay Area landscape from shothole borers and Fusarium dieback.
Sources and References
- UC IPM – “Invasive Shothole Borers”
Peer-reviewed Pest Notes on the polyphagous and Kuroshio shothole borers (Euwallacea spp.) and the Fusarium dieback they vector, covering identification, host range, and integrated management.
https://ipm.ucanr.edu/home-and-landscape/invasive-shothole-borers/ - UC Agriculture and Natural Resources – “Invasive Shothole Borers”
Statewide program hub (ishb.org) with pest overview, diagnosis, management guidance, reproductive host lists, distribution maps, and reporting tools for the ISHB–Fusarium dieback complex.
https://ucanr.edu/site/invasive-shothole-borers - Orange County / UC Cooperative Extension – “Polyphagous and Kuroshio Shot Hole Borers + Fusarium Dieback”
Public-outreach flyer summarizing the beetles’ damage, at-risk trees, signs of attack, and firewood and green-waste practices to limit spread.
https://awm.oc.gov/sites/ocpwocerac/files/import/data/files/documents-Flyer.pdf - City of San José – “Invasive Shot Hole Borers”
Municipal urban-forestry page describing San José’s ISHB response, including free trunk-injection treatment of moderately infested street trees, removal of severely infested trees, and resident reporting.
https://www.sanjoseca.gov/your-government/departments-offices/transportation/landscaping/trees/invasive-shot-hole-borers