Goldspotted oak borer (GSOB): hazard-tree identification for SoCal oaks
GSOB is confirmed by D-shaped emergence holes about 4 millimeters wide in the lower six feet of the trunk, on coast live, California black or canyon live oak. UC IPM's rating calls a tree with crown above 3, exit holes above 25 and bark cracking dead within a few years. A declared Zone of Infestation covers four counties.
The clearance distance owed over a coast live oak in San Diego County is four feet, six feet or ten feet, set by the voltage on the conductor. The goldspotted oak borer changes none of that. It changes the answer to a different question, which is whether the tree that measures at clearance today will still be standing in three years.
A GSOB-infested oak keeps a green crown for a long time after the point at which it should have been written up. The confirming evidence is a hole about four millimeters wide, in the lower six feet of the trunk, and an inspector working from the road will not see it. This page is about where to look, what the state has declared over the ground you are looking at, and what happens to the wood after the tree comes down.
Four counties sit inside a declared zone, and one of them whole
The instrument is a Zone of Infestation, and the authority is Public Resources Code section 4716:
Whenever the director determines that there exists an area that is infested or infected with insect pests or plant diseases injurious to timber or forest growth and that the infestation or infection is of such a character as to be a menace to the timber or timberlands of adjacent owners, the director, with the approval of the board, may declare the existence of a zone of infestation or infection, and describe and fix its boundaries.
The section gives the CAL FIRE director the power to draw the line and the Board of Forestry and Fire Protection the power to approve it. Subdivision (b) then lets the department and its agents "go upon state and private lands within the zone" and requires them to cause the infestation to be eradicated or controlled in a manner the board approves. Subdivision (c)(1) goes one step further and allows the department to remove live vegetation directly adjacent to dead or dying vegetation substantially at risk.
For GSOB that line has been redrawn five times. The Board's declaration of November 4, 2020, signed by Executive Officer Matt Dias, records the current shape:
The State Board of Forestry and Fire Protection approved the proposed expansion of the existing Zone of Infestation (ZOI) for the Goldspotted Oak Borer (GSOB) in the counties of Los Angeles, Orange, Riverside, and San Bernardino… This occurred by unanimous decision during the regularly scheduled Board meeting on November 4, 2020.
County | Year the zone was first established | Acres in the 2020 expansion |
San Diego | Fall 2012, countywide since 2016 | not restated in the 2020 document |
Riverside | December 2014 | 1,563,672 |
Orange | 2016 | 509,740 |
Los Angeles | 2016 | 1,922,992 |
San Bernardino | 2019 | 975,929 |
The four counties in the 2020 expansion total 4,972,333 acres. Orange County is the one to read closely. Its boundary description in the Board's Table 1 runs to four rows, and every row is the county line: lands east of the county border, north of the county border, west of the county border, south of the county border. San Diego County has been in the same posture since 2016. Two Southern California counties are inside the zone edge to edge.
What the declaration does not do is worth stating plainly, because it is the question a foreman asks. PRC 4716 confers authority on the department. It prints no penalty for a contractor who loads oak rounds onto a chip truck and drives them across a county line.
The restriction that actually binds a crew comes from somewhere else: the land manager on federal or state land, the county, or the utility's own specification. California State Parks records that "all Oak firewood has been quarantined in both the Cleveland National Forest and Cuyamaca Rancho State Park."
Los Angeles County is building its layer now. The Chief Sustainability Office's GSOB page records a Board of Supervisors motion adopted May 7, 2024, calling for "a report back within 180 days on recommendations for regulating firewood movement." That report was submitted on November 7, 2024, and its recommendations included a firewood inspection ordinance.
The jumps are 7 to 43 miles, and a beetle does not fly that
The spread record is the reason the zone keeps growing. GSOB was first collected in California in 2004 in San Diego County and was not linked to extensive oak mortality until 2008. CAL FIRE's 2020 expansion proposal, prepared by CAL FIRE forester David Haas and forest pest specialist Kim Corella in August 2020, states the mechanism in one sentence:
Due to the movement of infested firewood throughout the state this insect is moving much faster than its predicted natural spread range of 6 miles per year.
The five infestations that triggered the 2020 expansion show what that means on the ground.
Site | County | Found | Trees confirmed | Distance from nearest known infestation |
Trabuco Canyon | Orange | June 2019 | 12 coast live oaks | 8.5 miles |
Sugarloaf | San Bernardino | July 2019 | 2 black oaks | 15 miles |
Moonridge | San Bernardino | August 2019 | 1 black oak | 13.6 miles |
Hagador Canyon | Riverside | August 2019 | 4 coast live oaks | 7 miles |
Wrightwood | San Bernardino | October 2019 | 7 trees at varying levels | 43 miles |
The Wrightwood find started with one large dead California black oak, in a pine-covered valley at 5,900 feet, and a further infested black oak turned up there in May 2020. Forty-three miles is not a flight. The California Department of Food and Agriculture's pest rating profile for the species reports that flight mill tests produced "an average flight distance of approximately 790 meters in 24 hours." CDFA rated the beetle B as of September 30, 2022, having previously carried it as Q.
The practical consequence for a program is that the map is not a boundary you can plan against. UC ANR's distribution page records satellite infestations in Riverside in 2012, Orange in 2014, Los Angeles in 2015, San Bernardino in 2019 and 2020, and Ventura County in 2025. A circuit that was outside the host range when its inventory was built may not be now.
Three oaks, and a diameter where the tree stops surviving
The host list is short, and it runs at species level. UC IPM's Pest Note 74163, published January 2013 by Flint, Jones, Coleman and Seybold, states it:
GSOB attacks only oaks and prefers those in the red oak group including coast live oak, Quercus agrifolia, and California black oak, Q. kelloggii. GSOB also infests canyon live oak, Q. chrysolepis, and on very rare occasions Engelmann oak, Q. engelmannii.
Engelmann oak is attacked and is not known to be killed. The Board of Forestry's 2020 Annual Report gives the same three-species list and adds the size preference, recording that GSOB "attacks and can kill California black oak, coast live oak, and, to a lesser extent, canyon live oak, preferring larger diameter and older trees."
Diameter at breast height | What the record shows |
Under 5 inches | Rarely recorded |
About 10 inches | Occasionally attacked |
18 inches and greater | Most likely to be killed |
Eighteen inches is the number that matters to a survey order. UC IPM directs that large diameter oaks greater than 18 inches dbh "should be surveyed first if a large area is being assessed." On a distribution circuit running through oak woodland, that sorts the inventory before anybody walks it.
The species at the top of that list is the one that already owed you a basal inspection. Coast live oak fails at the bottom of the tree, with 54 percent of its documented California failures at ground level or below. The GSOB survey sends an inspector to the same six feet of trunk. One walk answers both questions, and a record that notes only crown condition answers neither.
The confirmation is a D-shaped hole in the lower six feet
Crown thinning is what gets a tree noticed. It is not what confirms the pest. UC IPM is explicit about the order:
D-shaped emergence holes are the most definitive indicator of GSOB infestation and are required to confirm an infested tree. Crown thinning and bark staining alone don't always indicate that GSOB has infested a tree.
The hole is about 4 millimeters wide, roughly 0.15 inch, and it extends through the bark surface on the trunk or larger branches. UC IPM records that the holes "are found predominately in the lower 10 feet of the trunk to the root collar." The Field Identification Guide, published January 13, 2011 by Hishinuma, Coleman, Flint and Seybold, tightens that instruction for a surveyor.
It says to "focus on the lower 8 feet of the main stem and directly around the root collar." Within that band the holes are "often found along the lower 6 feet of the main stem, at the very base of the tree, or adjacent to bark-stained areas."
Shape is the discriminator, and several other borers work the same oaks. UC IPM's Table 1 lists what else makes a hole in a Southern California oak.
Insect | Hole shape | Size | Where |
Goldspotted oak borer | D-shaped | 4 mm wide | Primarily the lower trunk |
Lead cable borer | Round | 4 mm diameter | Branches under 5 inches |
Chrysobothris species | Oblong or crescent | 5 to 13 mm wide | Trunk and larger branches |
Oak cordwood borer | Oval | 6 to 10 mm wide | Main trunk, around wounds |
Ambrosia beetles | Round | Under 2 mm diameter | Frequently the main stem |
Western oak bark beetle | Round | Over 1 mm diameter | Most common on small branches |
Bark staining is the symptom most likely to be written up wrong. It appears as dark wet spots or oozing red sap, it is most noticeable in spring and early summer when the soil is moist, and it has many causes. UC IPM names sudden oak death caused by Phytophthora ramorum and western oak bark beetle attack as two of them. It then gives the rule:
If no other GSOB injury symptoms, such as D-shaped holes or larval galleries, are visible, then bark staining is probably due to another agent.
Woodpecker damage carries its own tell. The Field Identification Guide records that woodpeckers foraging for GSOB expose "the brick-red phloem (in coast live oak)," and that the exposed inner bark "darkens to black over time." Red means recent. Black means the birds were there months ago, which dates the infestation without anyone cutting into the tree.
The rating is four numbers, and it prints a prognosis
The Field Identification Guide supplies the scoring system, and it is the closest thing this pest has to a written threshold. The rating is built on four factors.
The severity of crown thinning and dieback.
The density of exit holes.
The degree of bark staining.
The presence or absence of woodpecker foraging.
Score | Crown rating | Exit hole rating | Bark stain rating |
1 | Full, healthy crown, 0% leaf loss | At least one D-shaped exit hole on the main stem | One to five areas of staining on the lower stem, under 8 ft |
2 | Minor twig dieback or light thinning, 10 to 25% leaf loss | 10 to 25 exit holes in clumps on the main stem | Six to ten stained areas |
3 | Moderate thinning and twig dieback, 25 to 50% leaf loss | Exit holes scattered and abundant, more than 25 | More than 10 areas of staining on the lower stem |
4 | Severe dieback to larger branches, over 50% leaf loss | scale stops at 3 | Bark cracking evident on main stem |
5 | Tree is dead | scale stops at 3 | scale stops at 4 |
Woodpecker foraging is scored present or absent. The guide prints the exit hole rating as 0 to 3 and the bark stain rating as 0 to 4, so a zero in either column records a symptom that was looked for and not found. The combination that matters is stated in one sentence:
Trees with a crown rating greater than 3, an exit hole rating of 3, and a bark staining rating of 4 from GSOB injury likely will die in the next few years.
The Pest Note gives the same call in plainer counting terms. Trees with excessive crown thinning and dieback and more than 100 emergence holes on the lower trunk "are considered severely infested and will probably die in the next few years." CDFA's profile puts the interval from larval feeding to death at three to five years, "or less if the tree is weakened, as from drought."
That is a prognosis, on a form, with numbers a second person can check. It is rare enough in vegetation work to be worth using directly.
What the rating does to the knowledge clock
Nothing in the clearance rules names this beetle. What they name is the condition it produces.
A crown rating of 5 is a dead tree, and a dead tree within reach of a conductor on State Responsibility Area land is already answered by the trailing paragraph of PRC 4293, which carries a duty independent of the four-, six- and ten-foot distances.
A crown rating of 4, severe dieback to larger branches, produces dead portions of an otherwise standing tree, which is the fact pattern Rule 35's overhang provision is written around. That provision begins on actual knowledge rather than on distance, and the date knowledge arrived is the date the clock starts.
This is where a confirmed exit hole does real work. A GSOB confirmation is dated, specific and made by a named person standing at the tree. It is knowledge in exactly the sense Rule 35 uses the word, and it converts a tree that nobody had written down into a tree with a record and a date on it.
Which inspection type was supposed to produce it is a separate question with a separate answer, and a 4 millimeter hole at knee height on the far side of a 24-inch trunk is not something a patrol is designed to catch.
The middle of the scale is where the code runs out. Take an oak at crown rating 2 and exit hole rating 1: one confirmed hole, light thinning, no lean, full clearance on every conductor. It is alive, it is infested, and on the published interval it has perhaps three to five years. PRC 4293 reaches trees "weakened by decay or disease," and Rule 35 reaches trees that are "dead, rotten or diseased." Whether a wood-boring beetle is a disease within those words is not settled by either text.
The answer in practice comes from the utility's own vegetation specification and its hazard tree rating procedure, which is the document that decides whether this tree gets a removal prescription or a re-inspection date. What makes a tree a hazard tree in the first place and what level of assessment is owed are answered there, not in the pest literature.
The flight window is the season you are already working
GSOB runs one generation a year, and the calendar is tight enough to plan against.
Period | Stage |
October to as late as June | Prepupae under and in the bark |
Late April through July | Pupae in the outer bark |
Mid-May to September | Adult emergence |
Late June to early July | Peak flight activity in Southern California |
June through September | Females laying eggs on host trees |
Two things follow. Larvae and prepupae are under the bark year-round, so a log cut in January is as capable of producing beetles as a log cut in July. And adult emergence overlaps the months when a Southern California clearance program is most likely to be cutting, moving and staging material, which is also the stretch when seasonal restrictions are tightest for other reasons. UC IPM's grinding recommendation is built around that overlap: grind from October through April, before emergence.
The wood you cut is the vector
UC IPM states the spread mechanism without hedging:
Human-assisted movement of infested cut wood or logs represents the most significant threat for spreading GSOB in California
The same section adds that GSOB "develops in and adults emerge readily from firewood-sized pieces of oak wood." A line clearance crew produces firewood-sized pieces of oak wood as its normal output. Disposal is therefore a technical decision, and four treatments are recognized.
Option | What it requires |
Leave on site | Wood cut and seasoned two years on site should be free of viable GSOB |
Tarp or screen | Clear, UV-resistant plastic of at least 6 mil with edges sealed by soil, or 1 mm wire mesh, in full sun, for at least two years |
Grind | A 3-minus product: strips no more than 3 inches long, 1 inch wide and half an inch thick, ground October through April |
Debark | All bark and phloem removed down to the wood, including pieces thicker than half an inch, with the bark and shavings destroyed or left on site |
Three details in that table cost money and are easy to miss. The tarp has to be clear, because UC IPM specifically warns against opaque tarps, and it works by containment rather than by heat. Grinding to 3-minus is a specification with dimensions attached, and UC IPM notes that "Many chippers used by landscape professionals will not have the capacity to handle the large diameter rounds of oak wood." Debarking leaves the beetles in the bark, so the bark becomes the material that has to be handled.
Two more rules govern where the material sits. Piles of infested wood should not be left "directly under or near uninfested oaks" even inside an infested area. And the stump is a brood source: UC IPM directs that stumps be cut flush to the ground "because GSOB larvae can survive and develop in stumps of cut trees for at least a year."
CAL FIRE's own removal protocol carries the same instruction, specifying that heavily infested trees "will be felled with the stump being cut flush to the ground." A stump left at the usual height in a right-of-way keeps producing beetles into the following summer. Where that sits against the rest of a crew's site clean and BMP obligations is a conversation to have before the unit is priced, not after.
Green Valley is the corridor version of this
The scale is not theoretical. CAL FIRE's 2020 proposal records one Los Angeles County community:
For example, in Green Valley alone, which is located in Los Angeles County, GSOB infested 2,700 coast live oak trees resulting in almost 500 trees already having been removed and over 650 trees still marked for removal.
Twenty-seven hundred infested trees in one community, with more than 1,100 of them either already down or marked. The same document names what makes those trees expensive:
In most cases, access and terrain should not hamper treatments but a tree's proximity to structures, powerlines, roads and other improvements can greatly increase the cost of management.
Powerlines are named in that sentence alongside structures and roads. A removal in a corridor costs more than the same removal in the open, and a program pricing GSOB work off a woodland unit rate will be short.
The stand-level numbers run the same way. A 2015 Forest Service study by Tom W. Coleman comparing fuel loading in severely infested and uninfested oak woodlands found that 77 percent of oaks were infested and 41 percent killed in the infested plots. CDFA reports San Diego County infestation rates exceeding 90 percent in some areas with mortality reaching 45 percent. The Field Identification Guide put San Diego County losses at an estimated 21,500 trees across 1,893 square miles as of 2010.
A program working an oak-woodland circuit inside the zone is managing a mortality wave. It arrives over several seasons, and it leaves standing dead stems in the fall zone of the span.
What a GSOB record has to carry
A general "declining oak" entry loses everything this page is about. Six fields make the record defensible, and each of them comes from something above.
Species at species level, because the host list is three species and the prescription turns on which one.
Diameter at breast height, because 18 inches is the published mortality threshold and it sets the survey order.
Exit hole confirmation: whether D-shaped holes were found, how many, and at what height on the stem.
The four rating scores, recorded as scores: crown 1 to 5, exit hole 0 to 3, bark stain 0 to 4, woodpecker present or absent.
The date and the name of the person who examined the lower 8 feet of trunk and the root collar on foot.
The wood disposition actually chosen, and where the material ended up.
Written out for one tree, that looks like this. Pole and span identified, 12 kV, circuit named. Quercus agrifolia, 24 inches dbh, standing 22 feet from the centerline. Examined on foot at the root collar on March 12 by a named Consulting Utility Forester. Crown 3, moderate thinning with roughly 40 percent leaf loss.
Exit holes confirmed D-shaped, approximately 30 counted between ground level and 6 feet, exit hole rating 3. Bark staining in more than 10 areas on the lower stem, rating 3. Woodpecker foraging present, phloem darkened to black. Prescription: removal, with stump cut flush. Wood ground to 3-minus on site, and the grinding date recorded because it falls inside the October-to-April window.
Nothing in that entry is a clearance measurement. The clearance on this tree was fine, and it will stay fine until the tree comes down.
Where this leaves the reader
The beetle changes three things about a Southern California oak and leaves the fourth alone.
Where to look: the lower 8 feet of the main stem and the root collar, on foot, on the same walk the species already required.
What confirms: a D-shaped hole about 4 millimeters wide, counted and located.
What happens to the wood: one of four named treatments, decided before the saw starts, with grinding to 3-minus priced as the specification it is.
What does not change: the clearance distance, which is still set by voltage in Table 1 and in PRC 4293.
Two calls sit outside the technical literature and both still get made. Whether an infested but living oak is "diseased" within Rule 35 and PRC 4293 is answered by the utility's own specification and hazard tree rating procedure, and whether the answer is a prune or a removal follows from that.
Whether the wood can leave the site is answered by the land manager, the county, or a firewood ordinance that in Los Angeles County was still being drafted as of the Chief Sustainability Office's November 7, 2024 report. Neither answer is in PRC 4716, and a crew that reads the Zone of Infestation as a rulebook will find it is a map.
Further reading
Coast live oak in the utility right-of-way: The same species, the same six feet of trunk, and the failure profile that says why the basal inspection was already owed.
CPUC GO 95 Rule 35: The actual-knowledge trigger that a confirmed exit hole starts, and the date it starts on.
Hazard trees: what CAL FIRE's field guide actually says: Whether a declining tree is a hazard tree is answered there, in words that never mention a beetle.
Frequently asked questions
Which oaks does GSOB attack?
Three native California species are killed by it. UC IPM's Pest Note states that GSOB "attacks only oaks and prefers those in the red oak group including coast live oak, Quercus agrifolia, and California black oak, Q. kelloggii," and that it "also infests canyon live oak, Q. chrysolepis, and on very rare occasions Engelmann oak, Q. engelmannii." Engelmann oak is attacked but is not known to be killed.
What confirms a GSOB infestation in the field?
A D-shaped emergence hole. UC IPM states that "D-shaped emergence holes are the most definitive indicator of GSOB infestation and are required to confirm an infested tree," and that "Crown thinning and bark staining alone don't always indicate that GSOB has infested a tree." The holes are about 4 millimeters wide and are found predominantly in the lower 10 feet of the trunk down to the root collar.
Does the Zone of Infestation make it illegal to move the wood?
Not by its own terms. Public Resources Code section 4716 gives the director, with the Board's approval, the power to declare the zone, and gives the department and its agents the power to enter state and private land within it to eradicate or control the infestation. It does not print a penalty for a landowner or a contractor who hauls oak rounds out. Land managers and counties supply that layer separately, and the Los Angeles County Board of Supervisors asked for recommendations on a firewood inspection ordinance in a motion adopted May 7, 2024.
When do the adults fly?
UC IPM reports that "Adult emergence occurs from mid-May to September with peak flight activity occurring from late June to early July in Southern California." That window sits inside the months a clearance program is most likely to be cutting. UC IPM recommends grinding infested wood from October through April, before emergence.
Does a chipper solve the wood problem?
Often not. UC IPM's grinding standard is a 3-minus product, meaning strips no more than 3 inches long, 1 inch wide and half an inch thick, and it notes that "Many chippers used by landscape professionals will not have the capacity to handle the large diameter rounds of oak wood." The alternatives are leaving the wood on site, tarping or screening it for two years, or debarking it.
