Shot hole borers are invasive ambrosia beetles that bore into trees and farm a fungus that blocks the tree's water flow. Polyphagous shot hole borer has spread through Southern California, including Temecula and the city of Riverside. Signs are tiny entry holes, sap staining and dieback — and infested wood must be handled carefully to avoid spreading it.
What Is Shot Hole Borer?
Polyphagous shot hole borer (PSHB) and Kuroshio shot hole borer (KSHB) are tiny beetles from Asia that tunnel into the wood of living trees. They carry a fungus that they farm inside the galleries. The fungus, not the beetle, does the main damage: it spreads through the vascular tissue and can block water movement, causing branch dieback and, in susceptible trees, death. The two species are together called invasive shot hole borers.
PSHB was first found in Los Angeles and Orange counties and has since spread. It has been found in the Temecula Valley, including as far south as Temecula and inland in the city of Riverside, and it continues to spread through the region.
Which Trees Are at Risk?
These beetles attack dozens of species. Some are true reproductive hosts where the beetle and fungus thrive; others are attacked but the beetle does not reproduce well. Common Southern California hosts include:
- California sycamore and other sycamores and London plane
- Coast live oak and other oaks
- Avocado
- Box elder and maples
- Willows and cottonwoods
- Liquidambar (sweetgum)
- Castor bean and Persian silk tree
Signs to Look For
- Tiny, round entry holes about the size of a pencil lead in the bark
- Wet-looking or dark staining, sometimes with white powdery residue
- “Sugar volcano” crystalline sap on oaks
- Sawdust-like frass, sometimes as toothpick-like tubes
- Branch dieback, thinning leaves, and browning canopy
- Cracking or splitting of infested limbs
Infested wood should not be moved, and cutting an infested tree the wrong way can spread beetles. An arborist assessment confirms whether it is shot hole borer or something else.
Managing Infested Trees
- Confirm the diagnosis. Similar damage is caused by other borers and diseases.
- Sanitation. Infested wood should be chipped to small pieces, solarized or disposed of correctly — never stored as firewood or moved to other areas.
- Prune only what is necessary. Removing branches with active infestation can help, but heavy pruning stresses the tree.
- Protect high-value trees. Trunk injections of registered products can help protect or treat valued trees, applied by licensed applicators; results vary.
- Remove trees that are beyond saving. Severely infested trees can fail suddenly and become hazards.
- Watch irrigation. Keep trees hydrated without waterlogging.
Don't Spread It
The most important rule: do not move infested wood, firewood, green waste or chips out of the area. Beetles emerge from cut wood. If you have a tree that may be infested, tell your tree company and ask how they dispose of green waste.
Where Local Advice Matters
Sycamores along creeks and drainages, coast live oaks and landscape trees in Temecula, Murrieta, Riverside and nearby communities are among the most exposed. We check for shot hole borer during assessments and prune infested trees with care. See also oak tree care and drought and tree health.
How the Beetle and Fungus Work Together
Female shot hole borers bore tunnels (galleries) into the wood of living trees and introduce a fungus they carry in special pouches. The fungus grows on the gallery walls and feeds the beetle's larvae. In susceptible trees the fungus also spreads through the water-conducting tissue, causing branches to wilt and die back. Because the beetles are tiny — about the size of a sesame seed — and the entry holes are only a millimeter or so wide, infestations often go unnoticed until dieback appears.
Host Trees by Risk Level
| Category | Examples | What it means |
|---|---|---|
| Reproductive hosts | California sycamore, box elder, castor bean, willow, cottonwood, Liquidambar, coast live oak (attacked) | Beetle and fungus thrive; infestations can build and spread |
| Susceptible but less productive | Avocado, maple, Persian silk tree | Trees suffer damage; beetles may not reproduce as well |
| Rarely affected | Many conifers and palms | Typically not a host |
Steps If You Suspect an Infestation
- Photograph the entry holes, staining and dieback.
- Do not prune or remove branches until the tree has been assessed.
- Do not move wood, firewood or chips off the property.
- Have an arborist confirm the diagnosis.
- Decide together on management: sanitation, protective treatment or removal.
- Have all infested wood chipped and handled by a company that understands disposal rules.
- Re-inspect nearby host trees.
Prevention and Tree Vigor
- Keep host trees hydrated with deep, infrequent watering
- Avoid wounding trees with unnecessary pruning or mower damage
- Mulch properly without piling against the trunk
- Do not transport firewood
- Inspect valued trees every season, especially in spring and summer
What This Means for Property Owners
A confirmed infestation is not automatically a death sentence, but it is a reason to be proactive. Trees near homes, walkways and play areas that show significant dieback can become hazards. A written assessment documents the situation for insurers, HOAs and neighbors, and a clear management plan helps avoid surprises.