Shot Hole Borer in the Bay Area

In late 2023, the invasive Shot Hole Borer complex was detected in the San Francisco Bay Area for the first time — confirmed in San Jose by Santa Clara County agriculture officials. By November 2024, the Polyphagous Shot Hole Borer ( Euwallacea fornicatus ) had been confirmed in San Mateo County as well. The beetle had been devastating urban forests in Southern California since 2012 — killing tens of thousands of trees in Los Angeles, Orange County, and San Diego — and its arrival on the Peninsula had long been predicted by UC researchers.

That has changed. And for Peninsula homeowners with mature native trees, this pest represents a fundamentally different threat than Sudden Oak Death or drought stress. Unlike SOD, which can be prevented with phosphonate treatments, there is currently no effective systemic treatment once a tree is infested with Shot Hole Borer. The beetle farms a fungus inside the tree that blocks water transport and kills branches from the inside out. By the time external symptoms are visible, the infestation is often well advanced.

This article covers what you need to know right now.

What Is the Shot Hole Borer?

The Shot Hole Borer isn’t a single species but a complex of closely related invasive ambrosia beetles native to Southeast Asia. The two species of concern in California are the Polyphagous Shot Hole Borer (PSHB, Euwallacea fornicatus ) and the Kuroshio Shot Hole Borer (KSHB, Euwallacea kuroshio ). Both behave identically from a homeowner’s perspective, and the detection on the Peninsula involves PSHB.

What makes this beetle unusually destructive is its relationship with Fusarium fungi. The female beetle bores into a tree, excavates a gallery system, and cultivates a specific Fusarium fungus on the gallery walls. The fungus serves as food for the beetle and its larvae. But the fungus also colonizes the tree’s vascular tissue — the xylem — blocking water and nutrient transport. The disease caused by this fungal infection is called Fusarium Dieback.

The beetle is tiny — about 2mm, roughly the size of a sesame seed — and easily transported in firewood, green waste, and untreated wood products. This is likely how it moved from Southern California to the Peninsula.

Which Trees Are at Risk?

Researchers at UC Riverside have documented over 100 tree species as hosts for PSHB. Not all hosts are affected equally. The most important distinction is between “reproductive hosts” (where the beetle successfully breeds and the Fusarium causes significant damage) and “non-reproductive hosts” (where the beetle may attack but cannot establish a colony).

For Peninsula homeowners, the reproductive hosts that matter most are the trees you’re most likely to have — and the ones most heavily protected by local ordinances. Check your city's permit complexity on the map.

How to Identify a Shot Hole Borer Infestation

Early detection is the single most valuable thing a homeowner can do, because the management options narrow dramatically once an infestation is established. Here are the signs to look for:

Entry Holes

The beetle creates perfectly round entry holes approximately 0.85mm in diameter — slightly smaller than the tip of a ballpoint pen. These are easy to overlook. On smooth-barked trees like sycamore, the holes are more visible; on rough-barked oaks, you may need to look closely at branch unions and the lower trunk.

Sugar Volcanos

On some host species, the tree responds to boring activity by exuding a white, crystalline sugar residue around the entry hole. These small mounds of white powder at the hole entrance are one of the most distinctive signs of Shot Hole Borer activity and are easier to spot than the holes themselves.

Staining and Wet Spots

Dark, wet-looking staining on the bark around entry holes indicates the Fusarium fungus is actively colonizing the wood. On coast live oaks, this can look similar to SOD bleeding cankers, but the pattern is different — Shot Hole Borer staining radiates from small, discrete entry points rather than producing the broad, diffuse bleeding typical of Phytophthora ramorum .

Branch Dieback

As the fungus colonizes vascular tissue, you’ll see individual branches wilt and die while the rest of the crown remains green. This patchy dieback pattern — sometimes called “flagging” — is a later-stage symptom and indicates the infestation has been present for months or longer.

Frass Noodles

Thin, thread-like strands of compressed sawdust (frass) may protrude from or accumulate below entry holes. These are produced as the beetle excavates gallery tunnels inside the wood. They’re fragile and often washed away by rain, so inspect trees during dry weather.

Why There Is No Effective Treatment

This is the difficult reality. Unlike bark beetles that can be managed with preventive insecticide applications, or SOD which can be suppressed with phosphonate injections, there is currently no registered systemic treatment that reliably kills Shot Hole Borer inside a living tree.

The beetle lives deep inside the wood in its gallery system, physically shielded from contact insecticides. Systemic insecticides injected into the tree have shown limited efficacy in research trials — the beetle’s protected position inside the xylem makes it difficult to deliver a lethal dose to the galleries without damaging the tree. And even if you could kill every beetle, the Fusarium fungus remains in the wood, continuing to block vascular tissue.

Research is ongoing. UC Riverside’s Eskalen Lab and the UC Cooperative Extension are testing biological control agents and new treatment approaches. But as of early 2026, the honest answer is that management options for infested trees are limited to monitoring, pruning infested branches when the attack is localized, and removal when the infestation is advanced.

What Homeowners Should Do Now

1. Learn the Signs

The identification guide above covers the main symptoms. Spend 10 minutes walking your property and looking at the trunks and lower branches of your oaks, sycamores, and maples. You’re looking for tiny round holes, white sugar residue, dark staining, or unexplained branch dieback.

2. Don’t Move Firewood

The single biggest factor in Shot Hole Borer spread is human transport of infested wood. Do not move firewood, green waste, or untreated wood products from one property to another. If you have a tree removed, confirm that the wood is chipped to pieces smaller than one inch — the research standard for destroying beetle galleries. Larger chips or rounds can harbor live beetles that emerge and infest new trees.

3. Schedule a Baseline Assessment

If you have coast live oaks, valley oaks, sycamores, or box elders, a professional arborist assessment establishes a baseline condition record for each tree. This matters both for early detection and for documentation if you later need to pursue a removal permit for an infested protected tree. Ask specifically whether the arborist is familiar with Shot Hole Borer identification — the symptoms are not yet well known among all tree care professionals in the Bay Area.

4. Report Suspected Infestations

If you find signs consistent with Shot Hole Borer, report it to the Santa Clara County or San Mateo County Agricultural Commissioner’s office. You can also submit samples and location data through UC Riverside’s ISHB distribution tracking project. Early detection reports help researchers and county officials map the spread and target monitoring resources.

5. Maintain Overall Tree Health

Healthy trees are more resistant to all pests. Deep root watering during drought, proper pruning practices, avoiding soil compaction in the root zone, and treating existing conditions like Sudden Oak Death all contribute to a tree’s ability to compartmentalize and resist beetle colonization. This won’t prevent a determined infestation, but it improves the odds.

What This Means for Peninsula Tree Ordinances

Every city we cover protects coast live oaks. Most protect valley oaks. Several protect sycamores. If Shot Hole Borer establishes itself in Peninsula urban forests the way it has in Southern California, cities are going to face a surge of removal permit applications for infested protected trees.

This hasn’t happened yet. The current detection is limited, and it’s possible that the cooler, foggier Bay Area climate may slow the beetle’s spread compared to the warmer conditions that drove explosive infestations in SoCal. But the experience in Los Angeles suggests that the question is not whether infested tree removals will become an issue, but when.

From a permitting standpoint, the key principle is the same as with SOD: document everything. An arborist report that documents the presence of Shot Hole Borer, identifies the Fusarium fungus, and concludes that the tree cannot be saved is the foundation of a removal permit application for a protected tree. Cities that have dealt with SOD removals for the past decade have established precedent for disease-based removal permits — Shot Hole Borer will likely follow the same administrative path.

What This Costs

Most Shot Hole Borer-related costs are for monitoring and assessment rather than treatment, because effective treatment options are so limited.

Shot Hole Borer vs. Sudden Oak Death

Peninsula homeowners with coast live oaks now face two major threats. They overlap in some ways but differ in critical ones:

The practical takeaway: if you’re already doing phosphonate treatments for SOD prevention on your coast live oaks, keep doing them. SOD remains the more immediate and widespread threat. But add Shot Hole Borer awareness to your monitoring routine, and make sure your arborist is looking for both.

For Peninsula Homeowners With Native Trees

Shot Hole Borer is not yet a crisis on the Peninsula. The detection is recent, and the extent of local establishment is still being assessed. But the pattern from Southern California is instructive: early infestations went unnoticed for years before populations exploded and started killing trees at scale. The Bay Area’s climate may slow that trajectory, but it’s unlikely to prevent it entirely.

The best thing you can do right now is pay attention. Walk your property, look at your trees, and learn the signs. If you have high-value native oaks, sycamores, or maples, a professional baseline assessment is a small investment relative to the replacement value of those trees. And if you see something that looks like Shot Hole Borer activity, report it — the earlier the detection, the more options everyone has.