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If you have a tree in your yard, you probably aren’t spending much time looking for a beetle that’s smaller than a grain of rice. But when it comes to invasive shothole borers, it’s worth knowing what to look for.
These tiny, non-native beetles have become a serious threat to trees in California. Invasive shothole borers tunnel into the trunks and branches of trees and introduce a fungus that causes a disease called Fusarium dieback. The fungus interferes with the tree’s ability to move water and nutrients, eventually causing branch dieback and, in severe cases, tree death. The beetles can attack both healthy and stressed trees.
The shothole borer was confirmed for the first time in Santa Clara and San Mateo counties in August and November of 2024. The eradication of these beetles is virtually impossible once they are established. A coalition of city, county, state and federal agencies is working together to identify the extent of the infestation and to build awareness. Along the Peninsula, city and county agencies have been monitoring these beetles and focusing on long-term management and containment. Surveying and trapping efforts are ongoing in the region.
What should homeowners look for?
The beetles themselves are difficult to spot because they spend most of their lives inside the tree. Instead, look for very small, perfectly round holes in the bark, roughly the size of a medium ballpoint pen tip. Those holes may be accompanied by dark staining, gumming, a white powdery or sugary substance, or fine boring dust around the entry point. Dead or wilting branches can also be a warning sign, especially in combination with these other symptoms.
The important thing is not to assume that every small hole in a tree means shothole borer. There are other insects and tree problems that can produce similar symptoms, so proper identification is an important first step.
Which trees could be at risk?
One reason this pest is concerning is its broad host range. In California, invasive shothole borers have been documented in more than 65 tree species, including several trees commonly found in residential landscapes. Sycamores, oaks, cottonwoods and box elders are among the more susceptible hosts, along with other ornamental and native species.
See signs? Here’s what to report
If you suspect an invasive shothole borer infestation, contact the appropriate county agriculture department. When reporting, have the following information ready.
- Location: The address or location of the affected tree.
- Tree type: Identify the species if you can. Common hosts include box elder, sycamore, oak and willow.
- Signs: Note any tiny, round entry holes, staining, gumming, boring dust or sugary buildup around the holes.
Santa Clara County Division of Agriculture: 408-918-4610.
San Mateo County Department of Agriculture/Weights and Measures: 650-363-4700.
That doesn’t mean homeowners should remove these trees simply because they are potential hosts. In fact, a healthy tree is still worth protecting. Proper watering and maintenance can help trees maintain resilience, while regular monitoring gives homeowners a better chance of catching problems early.
What should you do if you find the signs?
Don’t immediately reach for the chainsaw. Early detection matters because lightly or moderately infested trees may be candidates for management, while severely infested trees can become a safety concern and a source of beetles that spread to nearby trees. Have suspected infestations properly assessed so the appropriate management approach can be determined.
Homeowners should also be careful about moving potentially infested branches, firewood or green waste. The beetles can be transported in cut wood, which can help spread the pest to new areas. Always use locally sourced or properly treated firewood and properly dispose of infested green waste.
When should you call an expert?
If you notice the combination of small round holes, staining or gumming, boring dust and branch dieback, that’s a good reason to have the tree evaluated rather than waiting to see what happens.
The goal isn’t simply to determine whether a tree has a pest. An arborist can first take a look at the tree as a whole: its health, structure, level of damage and potential risk. Then, steps can be taken to determine whether monitoring, treatment, pruning or removal is appropriate.
For Peninsula homeowners, the best defense against a tiny pest is knowing what to look for before a small problem becomes a big one.
George Reno is a local arborist at The Davey Tree Expert Company in Menlo Park, which serves communities on the Peninsula, including Menlo Park, Atherton, Palo Alto and Redwood City. Reno can be reached at 866-923-5658.