What are Invasive Shothole Borers?

Invasive shothole borers (ISHB) are a group of non-native ambrosia beetles within the genus Euwallacae. There are two ISHB species currently found in California, the polyphagous shothole borer (Euwallacea fornicatus) which was first detected in 2003, and the Kuroshio shothole borer (Euwallacea kuroshio) which was first reported in 2015. As of 2024, ISHB have established populations in seven Southern California counties: Los Angeles, Orange, Riverside, San Bernardino, San Diego, Santa Barbara, and Ventura; and in two Northern California counties, Santa Clara and Santa Cruz (current distribution map). 

Like other ambrosia beetles, ISHB do not feed on wood; instead, they feed on symbiotic fungi that they “farm” within the tunnels they bore into the tree. Each beetle species is associated with specific symbiotic fungi, Fusarium euwallaceae is associated with PSHB and F. kuroshium is associated with KSHB. These fungi penetrate the plant's vascular system, causing the disease Fusarium dieback. This disease has led to the widespread death of tens of thousands of trees in Southern California, and hundreds of trees in recently introduced areas of Santa Clara County.

Identifying ISHB and Their Damage

ISHB adult females are small, ~2 mm (0.08 in), and range from brown to black in color. Male beetles are even smaller (~1.6 mm/0.06 in), flightless, and light brown to black. Because ISHB are very small and spend most of their lives inside the tree, visual identification can be challenging, but the signs of infestation are often clear. 

The most reliable visual indicator of ISHB infestation is the presence of entry holes with the characteristic size and shape. These entry holes are typically perfectly round and about 0.85 mm (less than 0.04 in) in diameter, about the size of the tip of a medium ballpoint pen. Entry holes are often accompanied by symptoms like wet staining, frass, gumming, or sugary buildup, with each tree species reacting differently to ISHB attacks.

Over 78 species of trees and shrubs have been identified worldwide as ISHB-FD reproductive hosts (trees in which the fungus can grow, and the beetles can reproduce). In California, ISHB has been associated with 66 different reproductive hosts, including many California native and common landscape tree species (ISHB Reproductive Host List). Another ~ 250 species in 64 plant families are attacked but the beetles cannot successfully reproduce in them. 

Since many other pests may create similar symptoms, accurate identification of ISHB is essential. Unlike native beetles, ISHB typically attacks healthy trees rather than stressed or dying ones. Female ISHB typically disperse when temperatures exceed 68°F, attacking new trees in close proximity.

Management Strategies for ISHB

While ISHB eradication is improbable in established areas, there are effective strategies for containment and management. While management decisions will defer between production nurseries and urban forest trees, all effective management programs require an integrated approach that includes consistent tree health monitoring, infestation identification, removal of severely infested trees, and proper disposal of infested wood. The ISHB-FD management matrix (UC IPM Pest Notes, Publication 74149) developed for urban forest management decisions is a great place to help determine first steps. 

Monitoring

• Routine inspections are crucial to detect early infestations. Look for signs such as small entry holes and associated symptoms. Active infestations are confirmed by live beetles within galleries or fresh indicators like frass and staining.
• Trapping can help detect ISHB presence. Using traps with quercivorol lures and placing them strategically can help monitor beetle activity, though trapping alone is not an effective control measure.

Mechanical Control

• Heavily infested trees should be removed promptly to prevent ISHB from spreading. Branch removal is effective for cases with localized infestations.
• After removal, infested wood should be chipped as small as possible, with chips ideally no larger than 1 inch, which effectively kills 99.9% of beetles. If a smaller size is not feasible, chips should be kept under 3 inches to reduce likelihood of beetle survival.

Cultural Control

• Prevention starts by avoiding the movement of infested wood, which can spread ISHB to uninfested areas. Sourcing firewood locally or using kiln-dried wood is advisable.
• Tree health is paramount in pest resistance; ensure proper irrigation, soil care, and protection from mechanical damage. In the urban landscape, a diverse tree community is more resilient to pests, so avoid high concentrations of susceptible tree species.

Biological Control

• Although there are no biological control options available yet, researchers are exploring parasitic wasps, nematodes, and fungi that may naturally suppress ISHB populations. These biological tools hold promise but require thorough testing before they can be implemented.

Chemical Control

• Chemical treatments are available but should only be applied to trees with active infestations by State-licensed pesticide applicators. 
• Trunk sprays using the contact insecticide bifenthrin in combination with the fungicide Bacillus subtilis have been demonstrated to offer some control for dispersing beetles.
• Systemic pesticides applied as soil drenches (imidacloprid), or trunk injections (emamectin benzoate with propiconazole) also reduce beetle attacks. Systemic pesticides should be applied either in the spring or in the fall to moist soil to ensure effective uptake and movement of the pesticides within the tree.

RESOURCES

If you are interested in learning more about this pest, please visit ISHB.org or check out the UC IPM Pest Notes, Publication 74149. 

Christopher Shogren is an Environmental Horticulture Advisor with UC Cooperative Extension in Los Angeles County.  He can be reached at cjshogren@ucanr.edu. 

Beatriz Nobua-Behrmann is an Urban Forestry and Natural Resources Advisor at the South Coast Research and Extension Center, Irvine. She can be reached at benobua@ucanr.edu.