Managing California’s Shot Hole Borer Infestation

Fall 2016 Ecesis, Volume 26, Issue 3

Adult beetles come in a range of shades between black (females) and brown (males) and are very small, ranging from 0.05 to 0.1 inches in length. Photo: Stacy Hishinuma, UC Davis, Department of Entomology. 

Adult beetles come in a range of shades between black (females) and brown (males) and are very small, ranging from 0.05 to 0.1 inches in length. Photo: Stacy Hishinuma, UC Davis, Department of Entomology. 

The polyphagous shot hole borer (PSHB), an exotic forest pest from Southwest Asia, is causing millions of dollars of damage and devastating native woodlands in California. 

Proving to be one of the most dangerous of both native and non-native tree pests in California, PSHB has the ability to attack and kill hundreds of tree species, including native coast live oak, western sycamore, and red willow. 

Unlike most bark beetles, PSHB does not feed on the tree itself, but on a fungus, which it introduces and transports from tree to tree. The fungus spreads throughout the tree, damaging the tree layer that carries water and minerals up the tree from the roots—effectively starving the tree. The resulting damage varies from tree to tree but can include: 

  • Discolored wood 
  • Leaf discoloration and wilting 
  • Weakened structural integrity 
  • Dieback of limbs from the tree’s tips and roots inward 
  • Increased susceptibility to other pests and disease
  • Tree death 

PSHB infestation has contributed to the decline of native woodlands, reduced canopy cover near river and stream banks, and reduced habitat for several threatened and endangered species. Additionally, the devastation has resulted in millions of dollars of damage to native restoration efforts and agricultural production. 

Gold spotted oak borer (GSOB) and Kuroshio shote hole borer (KSHB) are also presenting serious threats to California native trees, including black oak, canyon live oak, and coast live oak 

Managing the Threat

The University of California, Riverside, is researching treatment methods, but they are not yet available for public use. Currently, management programs for these pests are focused on 1) limiting their spread into new areas, and 2) protecting healthy trees where possible. 

Measures to limit the spread of infected/infested trees include minimizing firewood transportation (“Buy Where You Burn”) and disposing infested material by chipping, grinding, and tarping (using heat generated by the sun to kill beetle larvae)(USDA 2013). 

The protection of healthy trees includes: 

  • Practicing appropriate cultural practices (irrigation, mulching, pruning, and fertilization at optimal levels) 
  • Using plants and soil that are known to be free of pests and pathogens
  • Cleaning/sterilizing tools that come into contact with infected plant material 

Dudek arborists certified by the International Society of Arborists are providing ongoing pest inspection monitoring throughout Orange County and portions of Los Angeles County. Monitoring efforts include identifying pests through rapid woodland assessment monitoring, ongoing pest trapping, and visual woodland and urban forest assessments. Once pests are found, Dudek arborists record the location and extent of the outbreak, establish an action threshold, and provide management options and treatment recommendations. — by Chris Kallstrand, DUDEK

For more information on Dudek’s effort to treat the infestation, contact Chris Kallstrand at ckallstrand@dudek.com or Ryan Gilmore at rgilmore@dudek.com.