How to Recognize and Manage an Invasive Shot Hole Borer (ISHB) Infestation

Invasive shothole borer (ISHB) is a group of two species of ambrosia beetles that are native to Southwest Asia. Suspected to have first arrived in Southern California in 2003, the beetles were previously misidentified as a less aggressive pest until approximately 2012. Since then, thousands of trees in forests, cities, and residential areas have become infested with and/or died from ISHB, including California native tree species such as coast live oak, western sycamore, and red willow.

Recognizing an ISHB Infestation

While most ambrosia beetles feed on the tree itself, ISHB feeds on a fungus (Fusarium spp.) that it introduces, “farms,” and transports from tree to tree. The fungus spreads throughout the tree, damaging the tree layer that carries water and minerals up from the tree’s roots, effectively starving the tree. Signs and symptoms of an infestation can include:

  • Oily, black bark staining
  • Perfectly round, 1/8-inch entry holes, similar to the size of a pen tip
  • Insect frass (sawdust and excrement)
  • Gumming
  • White powder on trunks
  • Limb dieback and general tree decline
  • Tree death

ISHB infestation has contributed to the decline of native woodlands, reduced canopy cover near riverbanks and urban areas, and reduced habitat for several threatened and endangered species. A Dudek urban forester warns that given ISHB’s wide host range, there is high potential for the pest to spread further north and continue to jeopardize more of California’s natural areas and ecosystems. Understanding the zones of infestation, and movement of ISHB will be a key component of its management.

Managing the Threat of Infestation

Because the threat of ISHB is serious, researchers at the University of California, Riverside, and University of California Agriculture and Natural Resources Extension have developed methods of ISHB treatment and management that vary based on where the infested tree is found. The associated risk management matrix helps determine the appropriate management strategy by taking into account the location of the tree, severity of the infestation, and potential for economic impact, as well as other factors. Based on these factors, the tree is recommended for treatment, removal, or other management programs that focus on limiting the spread of ISHB into new areas and protecting healthy trees.

Spread of ISHB from infected/infested trees can be limited by:

  • Minimizing firewood transportation (“Buy Where You Burn”) and
  • Disposing of infested material by chipping, grinding, and tarping (which uses heat generated by the sun to kill beetle larvae).

Methods to promote tree health include:

  • Adequately watering, mulching, pruning, and fertilizing trees to avoid stressing them;
  • Using plants and soil known to be free of pests and pathogens, and
  • Cleaning/sterilizing tools that come into contact with infested plant material.

Dudek’s team of certified arborists and plant pathologists have provided ongoing ISHB monitoring for projects throughout Southern California. Dudek’s efforts have involved ongoing ISHB trapping, pest identification through rapid woodland assessment monitoring, and broad-based forest health 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.

In 2021, Dudek began a 3-year effort with the Inland Empire Resource Conservation District to develop a Regional Priority Plan (RPP) to set management actions and standards for ISHB in San Bernardino and Riverside Counties. The RPP will take into account years of field trapping data, a large-scale literature review, changes in aerial imagery, and more, to identify the leading edge of the zone of infestation (i.e. where the beetles are moving), and prioritize demonstration projects to help slow the spread of ISHB.

For more information, contact Senior Urban Forestry Specialist Chris Kallstrand.