Invasive shot hole borer complex: SoCal tree mortality and ROW hazards

Sep 21

15 min read

Invasive shot hole borers bore 0.85 millimeter holes in trunks and branches and introduce Fusarium fungi. The Board of Forestry declared a Zone of Infestation over eight Southern California counties totaling 15,775,396 acres. On 51 of 66 California hosts the result is branch dieback, so the corridor hazard is limb failure from a tree still at clearance.

A sycamore carrying an invasive shot hole borer infestation can hold full clearance on every conductor in the span and still put a primary limb through it. The beetle is about two millimeters long. The hole it leaves is smaller than that, and it is as likely to be on a branch as on the trunk.

That is the part of this pest a line clearance program has to solve. The state has drawn a zone over eight Southern California counties, six of them edge to edge. Inside that zone the published management thresholds are counts of holes you can barely see, and the prescription they produce turns on how hazardous the tree is, which in a corridor is decided by the conductor over it. The clearance distances do not change. What changes is which trees keep generating work, and for how long.

Eight counties, and six of them entire

The instrument is a Zone of Infestation, declared under Public Resources Code section 4716. What that section does, what it authorizes and what it conspicuously does not prohibit is set out with the goldspotted oak borer, which sits under the same authority and went to the same Board meeting.

The invasive shot hole borer version is larger. CAL FIRE's Zone of Infestation proposal is dated August 2020 and went to the Board as binder item FULL 15 for the meeting of November 4, 2020. Eight people prepared it.

Kim Corella, CAL FIRE Southern Region forest pest specialist.
Eric Just, Gregg Bratcher, David Haas and Alan Peters, foresters in the San Diego, Riverside, San Bernardino and San Luis Obispo units.
David Erickson, Orange County Fire Authority wildland resource planner.
Rob Durbin, Los Angeles County Fire Department forestry division chief.
Dr. Stacy Hishinuma, USDA Forest Service forest entomologist.

County	Acres in the zone	Boundary
Los Angeles	2,529,514	Entire county
Orange	509,466	Entire county
Riverside	1,989,419	By section boundary
San Bernardino	3,102,923 across four sections	By section boundary
San Diego	2,712,195	Entire county
San Luis Obispo	2,124,278	Entire county
Santa Barbara	1,633,621	Entire county
Ventura	1,173,980	Entire county

The eight total 15,775,396 acres. Table 3 of the proposal describes six of them with the same three words, "Entire County," and only Riverside and San Bernardino get a written boundary description. The Board of Forestry's 2020 Annual Report records the outcome:

An ISHB zone of infestation (ZOI) was approved by the Board of Forestry and Fire Protection in counties impacted by the beetles in areas with known reproductive host trees, which support beetle reproduction. The ZOI includes all of San Diego, Orange, Los Angeles, Ventura, Santa Barbara, and San Luis Obispo Counties as well as portions of Riverside and San Bernardino Counties.

The same report separates the two beetles by county. Polyphagous shot hole borer is established in Los Angeles, Orange, Riverside, San Bernardino, San Diego and Ventura. Kuroshio shot hole borer is established in Los Angeles, Orange, Santa Barbara, San Diego and Riverside. For field purposes that distinction does not survive contact with a tree, because UC IPM states that the two are "visually identical and require DNA analysis to distinguish them."

Scale, for the two ends of the range. The proposal records more than 375,000 trees infested across 597 acres of primary riparian forest in the Tijuana River Valley. At the other end, UC ANR reports a 2017 study by USDA Forest Service scientist Gregory McPherson and collaborators estimating that 23.2 million of Southern California's 70.8 million urban trees, 32.8 percent, are susceptible to the complex.

The hole is 0.85 millimeters, and it is on the branches too

UC IPM published Pest Note 74179 in August 2023, authored by Nobua-Behrmann, Eskalen, Hishinuma, Lynch, Eatough-Jones and Kabashima. It gives the confirming sign in one sentence:

The holes are small (1/32 inch or 0.85mm in diameter) and perfectly round, roughly the size of the tip of a medium ball point pen.

Perfectly round is the discriminator. The goldspotted oak borer leaves a D-shaped hole about four millimeters wide in the lower six feet of an oak trunk, which is a different shape, roughly five times the width, and confined to a band a surveyor can walk.

Invasive shot hole borer holes carry no such band. CAL FIRE's proposal states that they are "most commonly seen on the trunk of the tree but they can attack branches as small as 2cm in diameter." Two centimeters is about three quarters of an inch, and the proposal gives that figure again in inches: branches "as small as 0.78 inches."

The hole is the only sign that is always there. UC IPM is explicit that the rest varies by host:

Since every host plant species responds differently to ISHB-FD, the combination of signs and symptoms observed might vary; however, entry holes are always present in infested plants.

Accompanying sign	What it looks like
Staining	Wet dark discoloration around the hole
Frass and boring dust	Fine sawdust-like material at or below the hole
Gumming	Sap or resin extruded at the entry point
Sugary exudate	White crusted buildup, common on avocado

One line in the Pest Note breaks the habit most vegetation programs run on. The usual heuristic is that borers follow stress, so the declining tree gets the attention and the vigorous one does not. UC IPM says the opposite applies here, and says it while distinguishing the native look-alikes:

Unlike ISHB, which colonizes mostly healthy trees, most of these look-alike species attack only stressed, injured, or dying trees.

A well-irrigated street sycamore in full leaf is a candidate. So is a vigorous willow in a wash the circuit crosses. CAL FIRE's proposal says which species to walk first: box elder and sycamore are the hosts ISHB prefers, and "these hosts should be the prioritized in ground surveys to determine if an area is infested."

Fifty-one species that keep standing and drop limbs

UC IPM counts over 78 confirmed reproductive hosts worldwide, plus "another 247 species in 64 plant families" that are attacked without the beetles reproducing in them. The California list is shorter and it is published with the split that matters. UC ANR's reproductive host page sorts 66 confirmed California hosts under two headings: 15 species "Susceptible to ISHB-FD (may cause tree death)," and 51 species "Less susceptible to ISHB/FD (causes branch dieback)."

Read those two headings against a corridor inventory.

Susceptible, may cause tree death	Less susceptible, causes branch dieback
Box elder, Acer negundo	Coast live oak, Quercus agrifolia
California sycamore, Platanus racemosa	Canyon live oak, Quercus chrysolepis
London plane, Platanus x hispanica	Engelmann oak, Quercus engelmannii
Valley oak, Quercus lobata	Jacaranda, Jacaranda mimosifolia
Fremont cottonwood, Populus fremontii	Carrotwood, Cupaniopsis anacardioides
Arroyo, red and black willow, Salix spp.	Chinese flame tree, Koelreuteria bipinnata
Castorbean, Ricinus communis	Southern magnolia, Magnolia grandiflora

The left column is a riparian list and a street-tree list at the same time. CAL FIRE's proposal cites a 2016 UC Cooperative Extension survey of Orange County Parks that found infestation rates above 50 percent in California sycamore, willow, cottonwood and London plane. The CDFA pest rating profile put the exposure more bluntly in 2015: the known reproductive hosts then "account for approximately 25% of street trees in southern California."

Valley oak is in the left column, which is the oak whose documented California failure profile is already a large live lateral limb in August. Coast live oak is in the right column, which is the oak whose failures are mostly at the base. The pest does not respect either profile. It adds branch dieback to the second and mortality to the first.

The right column is the one that changes a maintenance plan. Fifty-one species that are colonized, hold their structure, and produce dead branches is not a removal problem. It is a recurring supply of exactly the condition Rule 35's overhang provision reaches, which is dead, rotten or diseased portions of otherwise healthy trees that overhang or lean toward a span.

A tree in the left column produces that condition once and then comes down. A tree in the right column produces it every cycle, on a stem that will still be there in ten years.

Both lists move. American sweet gum and Japanese maple appear in the hosts-killed column of Table 1 in the August 2023 Pest Note and appear in the branch-dieback column of UC ANR's current page. Any inventory field that stores host status has to store the date it was written and the document it came from.

The corridor is what sets the hazard level

UC IPM's management matrix is the closest thing this pest has to a written prescription, and it is built on two axes a utility reader will recognize. One is infestation level, counted in entry holes. The other is hazard level, and the Pest Note defines it by consequence rather than by biology:

High Hazard Level: Trees that might pose a high risk to people or property (e.g., trees adjacent to walkways, playgrounds, high-use lawns, parking lots)

The examples are a park inventory. The definition is not. A reproductive host whose primary branches sit over an energized conductor is a tree that might pose a high risk to property, on the plain words, and the matrix's own footnote says the classification belongs to the site:

Definitions for tree value and hazard level vary. Classification must be determined by site and site use (e.g., economic or cultural value and risk to people or property).

The infestation levels are counts of entry holes.

Low: fewer than 50.
Moderate: 50 or more and fewer than 150.
Heavy: 150 or more.
Severe: 150 or more with ISHB-related dieback.

Now the cell. For a low-value reproductive host with a low infestation, the matrix prescribes Monitor if the hazard level is low. The same species, the same count of holes, with the hazard level set to high, gets "Monitor & remove hazard branches." One row down the table the divergence widens: at a heavy infestation the low-hazard cell still says monitor and remove infested branches, while the high-hazard cell offers "Remove infested/hazard branches*, or remove tree & stump."

That is the whole corridor argument in one table. Putting a conductor over the tree advances the prescription by a full infestation level, and it does so under the matrix as published, without anyone having to argue an exception. The asterisk attached to those cells carries the timing:

If ISHB attack is confined to the branches of host tree, prune affected branches immediately to prevent advancement to the trunk.

The matrix is University of California extension guidance. No CPUC general order and no Public Resources Code section adopts it. Whether a utility's specification imports it, and whether the answer on a given stem is a prune or a removal, is a decision the utility makes in its own hazard tree rating procedure. What level of assessment that procedure owes is answered there as well.

Counting to fifty on a sycamore

The threshold is 50 holes, and the qualifier is what makes it hard. UC IPM states that "Only active entry holes with live beetles inside the associated gallery should be considered to determine infestation levels," and that "the entry holes may remain visible on the trunk and branches even after the beetles left or died." A tree that was heavily infested three years ago and recovered still carries the holes.

The published test for whether a hole is active involves going back:

One other way to confirm if there are live beetles within the galleries is painting over selected entry holes with water-based latex paint. If an ISHB female is alive within the gallery, it will re-open the entry hole during the next 24 hours, proving that the entry hole is active.

Twenty-four hours is a second visit. Wet staining and frass are the faster indicators, and UC IPM names them as good ones, but neither is a count. The honest position is that the number that decides the prescription cannot be produced from a vehicle, and is not reliably produced by a single walk-by either.

Which inspection type is supposed to produce what is a separate question with a separate answer, and a 0.85 millimeter hole on a primary limb 30 feet up is not something any of them are designed to catch.

Two further facts close off the idea of a quiet season. CAL FIRE's proposal gives the flight season as "January through September or October," and says adults may emerge any time inside it if the wood retains enough moisture. UC IPM reports two dispersion peaks, one around February and one from September to November, and adds that patterns "vary by year and location." Temperature models in the Pest Note put the beetle at 5 to 11 generations per year in Southern California.

Where the code is genuinely ambiguous is worth saying plainly. Rule 35 reaches trees that are "dead, rotten or diseased," and PRC 4293 reaches trees weakened by decay or disease. Fusarium dieback is a disease of the tree's vascular system, and on that reading an infested limb is within both.

Neither text says so, and neither says a beetle gallery is decay. The answer that a crew acts on comes from the utility's vegetation specification, not from the pest literature, and what makes a tree a hazard tree in the first place is answered in a document that never mentions a beetle.

The saw and the chipper are named in the sanitation rule

The beetle carries the fungus into the tree. Steel can carry it too, and the published guidance says so directly. UC IPM's mechanical control section gives the practice and the agents:

Pruning tools should be disinfected between trees to avoid spreading the fungus and other opportunistic pathogens. Effective disinfecting agents include a 70% solution of ethanol, a 5% solution of bleach, or commercial disinfectant solutions like Lysol.

UC ANR's management page extends the list past hand tools and then gives a sequencing recommendation:

Any pruning, sampling, or other tools that come into contact with infected wood should be sanitized before being used on uninfected trees. This includes chainsaws and wood chipping equipment. If several infested and uninfested trees need to be maintained on a property, we recommend managing the trees in batches. First take care of all of the uninfested trees, then move to the infested trees so that your tools won't need to be cleaned until the end of the day.

That recommendation assumes a property. A line clearance crew works a circuit, pole to pole, in the order the spans run, and the infested sycamore is wherever it happens to be planted. Batching by infestation status means either resequencing the day's work or sanitizing between trees, and the second of those is a per-tree step nobody priced when the unit rate was set. Whoever builds the work plan owns that call, not the crew standing at the tree.

A different vector question is already answered elsewhere in this set. Seed and soil carried on equipment between North Bay work sites sits under a specific permit measure with a specific scope, and that measure is about washing mud off vehicles. This one is about fungal spores on a cutting edge, at a scale of seconds rather than sites, and there is no permit measure behind it at all.

Sanitizing the saw also solves only half the problem. It stops the crew moving Fusarium from stem to stem. It does nothing about a female flying in on her own, which is the ordinary way the next tree gets infested.

One inch, and then six weeks or six months

Cut wood is live inventory. UC IPM states that invasive shot hole borers "can survive in cut wood for weeks or even months," and directs that infested branches and logs not be moved offsite until they have been sanitized. The published process has two steps, and a line clearance operation already performs the first one.

Chipping is step one, and the specification is a dimension with a kill rate attached:

The preferred chip size is 1 inch or smaller, which will kill 99.9% of the beetles. Chips that are 3 inches or smaller will kill 98% of the beetles.

Those figures come from Eatough Jones and Paine's 2015 study in the Journal of Economic Entomology, volume 108, issue 4. The difference between one inch and three inches is 1.9 percentage points of kill, which sounds small until it is applied to the beetle load in a mature sycamore.

Chipping is not the end of the process either. UC IPM says it "should be followed by other sanitation methods like composting or solarizing," and that chips which have not been composted or solarized "should not be used as mulch in a different location."

Step two, where it is solarization, is a written specification a chip yard has to be built for.

Requirement	Specification
Sheeting	Clear, sturdy, UV-resistant plastic, at least 6 mil
Opaque tarps	Not to be used
Containment	Plastic both underneath and over the material, edges sealed with soil, holes sealed
Pile depth	2 logs deep maximum, chips as thin as possible
July to August	Covered at least 6 weeks
September to June	Covered at least 6 months

Six weeks against six months is the line that decides whether a yard can turn material over. A January removal ties up covered ground until July. A July removal is clear by September. That is a scheduling input, and it belongs next to the site clean and BMP obligations a crew already carries, not in a separate conversation after the wood is on the ground.

The stump is the last piece, and it is the one most easily left at the usual height. UC IPM directs that stumps be ground or cut to ground level after removal "since ISHB can survive and reproduce on stumps for years."

One thing the zone does not supply is a prohibition. CDFA rates polyphagous shot hole borer B, and the March 24, 2015 rating justification explains why it is not an A: the pest "has been present in the state since 2003 and is not under official control." There is no state quarantine on the material a crew generates. What restricts it is the land manager, the county, and the utility's own specification.

What an ISHB record has to carry

A "declining sycamore" entry loses everything above. Six fields make the record usable, and each comes from something published.

Species at species level, and which column of the host list it sits in on the date of the entry.
Whether round entry holes about 0.85 millimeters across were found, and where: trunk, primary branches, or both.
An approximate count, and whether the holes were assessed as active, with the basis given as staining, frass, beetles seen, or a paint test.
The infestation level that count produces: low, moderate, heavy or severe.
The hazard level assigned, with the reason, which in a corridor is the conductor and the fall geometry.
Wood disposition: chip size, what happened after chipping, and where the material went.

Written out for one tree, it looks like this. Pole and span identified, 12 kV, circuit named. Platanus racemosa, 26 inches diameter at breast height, canopy 8 feet above the primary. Examined on foot on April 14 by a named Consulting Utility Forester. Round entry holes approximately 0.85 millimeters, confirmed on the trunk and on two primary branches over the span.

Approximately 70 counted, wet staining and frass present at the majority, assessed active. Infestation level moderate. Hazard level high, primary branches directly over an energized conductor. Prescription per the utility specification: removal of the two infested branches over the span, tools disinfected before the next tree. Wood chipped to 1 inch on site, hauled covered, solarized at the yard from April 14, uncovering not before October 14.

None of that is a clearance measurement. The clearance on this tree was compliant on the day it was written, and the entry is the reason somebody will look at it again before it is not.

Where this leaves the reader

The pest changes three things about a Southern California corridor tree and leaves the fourth alone.

Where to look: trunk and branches, on foot, on species the ground survey should already be prioritizing, with box elder and sycamore first.
What confirms: a perfectly round hole about 0.85 millimeters across, located and counted, with activity assessed rather than assumed.
What happens to the wood: chip to one inch, then compost or solarize, with the yard calendar built around six weeks in high summer and six months the rest of the year.
What does not change: the clearance distance, which is still set by voltage.

Two calls sit outside the technical literature and both still get made. Whether an infested but structurally sound limb is "diseased" within Rule 35 is answered by the utility's specification and its hazard tree rating procedure.

Whether the corridor sets the hazard level to high on UC IPM's matrix is a decision somebody has to write down, because the matrix hands that classification to the site and then says nothing more about it. A program that leaves both unanswered will still be doing the work. It will just be doing it one cycle later than the tree required.

Frequently asked questions

How many beetle species are in the invasive shot hole borer complex in California?

Two in Southern California, and a third in the north. UC IPM states that "The two ISHB species currently found in California are the polyphagous shothole borer (Euwallacea fornicatus) and the Kuroshio shothole borer (Euwallacea kuroshio)," and that "Visually, these two species are identical and require DNA analysis to distinguish them." UC ANR records that a third species, greater shothole borer (Euwallacea interjectus), was found and identified in Santa Cruz County in October 2024.

What confirms an ISHB infestation in the field?

The entry hole. UC IPM states that the holes are "small (1/32 inch or 0.85mm in diameter) and perfectly round, roughly the size of the tip of a medium ball point pen," and that "entry holes are always present in infested plants." Staining, gumming, frass and sugary exudate accompany the holes but vary by host species.

Does the Zone of Infestation cover a whole county?

Six of the eight, yes. CAL FIRE's 2020 proposal lists Los Angeles, Orange, Santa Barbara, San Diego, San Luis Obispo and Ventura Counties as "Entire County," with Riverside and San Bernardino described by section boundaries. The eight counties total 15,775,396 acres.

How small a branch will the beetle attack?

About three quarters of an inch. CAL FIRE's Zone of Infestation proposal states that ISHB "also infests young, small branches as small as 0.78 inches," citing Mendel et al. 2012, and that entry and exit holes "are most commonly seen on the trunk of the tree but they can attack branches as small as 2cm in diameter."

What chip size kills the beetles?

One inch. UC IPM states that "The preferred chip size is 1 inch or smaller, which will kill 99.9% of the beetles," and that "Chips that are 3 inches or smaller will kill 98% of the beetles." Chipping alone is not the end of it; UC IPM directs that chipping "should be followed by other sanitation methods like composting or solarizing."

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