Invasive shot hole borers (ISHB) are small ambrosia beetles that bore into trees. Not all beetles that bore into trees are the same and each must be treated differently. Unlike other boring beetles, such as Asian long-horned beetles or emerald ash borers, invasive shot hole borers do not eat trees. Instead, they bring a fungus called Fusarium into the trees which affects tree health.

Fusarium dieback is a disease caused by a fungus. ISHB and Fusarium work together to survive. Female invasive shot hole borers carry spores of the fungus and introduce it to trees when they bore into the tree trunk or branches. Once established, the fungi spread through the tree and put the tree at risk of decline or death.

The symptoms of Fusarium include pale and wilted leaves and branch dieback.

The beetles make perfectly round entry holes in bark the size of the tip of a ball point pen. There is sometimes staining near the holes. Other signs may be present depending on the tree species.

It can be. Effectiveness of treatment depends on the level of infestation. Trees with 150 or fewer entry holes are considered low to moderately infested. Trees with 150 or more entry holes are severely infested. Low to moderately infested trees are treatable. For severely infested trees, removal is the only effective treatment option to protect surrounding trees.

Trunk injection is the appropriate treatment method for moderately infested street trees (with 50 to 150 entry holes). The City will have contractors who possess a Qualified Applicators License from the California Department of Pesticide Regulation treat moderately infested street trees with two controlled chemicals: a fungicide and an insecticide. The insecticide kills the beetles, and the fungicide controls the fungus that the beetles feed on. For street trees, this treatment will be free to residents and will begin in spring 2025. 

Trunk sprays and soil drenches are alternative treatment options. These treatments are applicable for trees in open areas such as parks or private property.

To apply these chemicals and perform these treatments for hire, a business must be licensed with the Department of Pesticide Regulation and registered with the Santa Clara County Division of Agriculture.

Treatment will be most effective during the spring.  When chemicals are injected into the trunk of a tree, they mix with the sap being distributed throughout the tree. This process of treatment uptake works best in the spring when trees are most active. Injection treatment is most appropriate for moderately infested trees, with 50 to 150 entry holes). Injection treatment is not recommended for lightly infested trees as the procedure can do more harm than good to the tree.

The beetles stop moving when the daily high temperature is 68 F or lower. The beetles will stay inside the trees to keep warm and will not further infest trees. When daily temperatures are 68 F or higher and trees are experiencing their spring growth, they are able to efficiently uptake and distribute the chemical treatments throughout the trunk and branches to eradicate the beetles and control the fungus.

Trees should be treated with the insecticide and fungicide trunk injection treatment once every two years.

Contact a tree care company with an International Society of Arboriculture, Inc. (ISA) certified arborist and qualified applicator on staff to have private trees assessed. ISA-certified arborists can be found at treesaregood.org. You may also contact arborist@sanjoseca.gov for a list of City-approved contractors who are familiar with ISHB and associated processes.

Company must be licensed and registered with the  County of Santa Clara.

Don’t panic! Improper timing of chemical treatment leads to less effective results. Monitor trees and contact the Arborist office at arborist@sanjoseca.gov or 408-794-1901 if you suspect signs of ISHB.

We have been monitoring the presence of ISHB in San José since November 2023.

The City has already:  

• Created outreach materials for the community 
• Begun launching a street tree treatment program to treat infested street trees for property owners
• Developed a plan for removal and disposal of infested trees
• Notified the City’s approved tree contractors of the infestation and discussed proper treatment and disposal 
• Notified city leadership of the infestation and the response plan

The City is currently:  

• Checking traps to monitor the spread of the infestation
• Testing trees with signs of infestation to confirm positive identification
• Identifying and removing severely infested trees to slow the spread and reduce the beetle population
• Preparing for spring treatments 
• Meeting with community leaders and groups to inform them of the pest, what to look for, and the City’s next steps

The City will soon be: 

• Launching a door-knocking campaign to notify residents of positive infestations and upcoming treatment in affected neighborhoods
• Mailing postcards to notify residents and property owners that their street trees will be treated in spring 2025

How can I get involved?

• Contact the Arborist office at arborist@sanjoseca.gov if you would like to be a community leader and/or assist with outreach efforts