Racing to Stop the Destructive Shot Hole Borer

Internal symptoms on coast live oak Photo: Akif Eskalen

Editor’s Note:  This shot hole borer article is featured in the current issue of CNPS Flora magazine. In the magazine, we mention new findings coming out of current research. CNPS is still awaiting an update from researchers and will post the news as soon as it becomes available.


Tiny Terror

By Kathy Morrison

If someone wanted to design the perfect pest, it would have all the qualities of a shot hole borer.

A type of beetle, the shot hole borer tunnels into trees, far from the reach of chemicals and pesticides. The fungi it farms for food can damage or kill its host tree. The pest is too tiny to notice casually, and the symptoms of its invasion vary in different hosts. What’s more, it reproduces prolifically.

No bigger than a sesame seed, the shot hole borer invaded Southern California early in the millennium and has been a growing problem since 2010, damaging or destroying trees of many species, natives as well as non-natives. The pest already is a nightmare for arborists, foresters, botanists and orchard farmers. How to stop it is a huge challenge for researchers and resource managers, says Sabrina Drill, PhD, a Ventura County-based expert for the University of California Division of Agriculture and Natural Resources (UCANR) who presented at the recent CNPS Conservation Conference in Los Angeles. But the future of California’s vast green canopy depends on finding a solution.

Identifying the invader

The first known appearance of a shot hole borer in the state was in Los Angeles County in 2003, Drill explains, but the tiny beetle was misidentified as a tea shot hole borer, which it resembles. Then, in 2010, an entire Long Beach street of box elders failed. The beetle found in those trees showed up later in a backyard avocado tree in South Gate. The pest was determined to be a previously unknown species and was dubbed the polyphagous shot hole borer (PSHB). (“Polyphagous” means “eats lots of things.”)

Drill says the PSHB is a type of ambrosia beetle, apparently from Vietnam. Unlike bark beetles, these pests tunnel into a host tree, bringing along fusarium fungi that they farm as food. The shot hole borers reproduce inside the galleries in the host, mating with siblings. The fusarium infection interrupts the flow of nutrients in the host tree, causing branch dieback and potentially killing the tree.

Female PSHB Photo: Akif Eskalen

The pests’ spread across Southern California was aided by a huge December 2011 windstorm in the San Gabriel Valley, Drill says. Thousands of trees and branches were brought down by the strong gusts. The felled wood was removed to the parking lot of the Rose Bowl, then dispersed to several sites.

Eventually the pest invaded several species of trees at the Huntington Botanical Gardens and the Los Angeles County Arboretum & Botanic Garden. Researchers there saw the varying effects of its invasion, now identified as PSHB/fusarium dieback complex. Some species showed minor damage, while others became reproductive hosts to fungal colonies.

The PSHB also is well established now in Orange County and western Riverside and San Bernardino counties. It has spread into eastern Ventura County as well.

Meanwhile, a very similar beetle made an appearance in the San Diego area. It apparently came from Taiwan, Drill says. Named the Kuroshio shot hole borer (KSHB), after an ocean current, it subsequently has affected trees in Orange County and has been found in San Luis Obispo and Santa Barbara counties, having jumped over L.A. and Ventura, she says. It also has killed thousands of willows in the Tijuana River Valley.

“These things really like our riparian trees,” Drill says. The pests seem to have no preference when it comes to attacks: They’ve been found in palms, cottonwoods, maples, oaks, sycamores and liquidambars. “Big trees, little trees, it doesn’t seem to matter.” Healthy trees, as opposed to drought-weakened ones, are the beetles’ preferred hosts.

Seeking a solution

The symptoms of a tree under attack vary in type and intensity, with some species showing a natural resistance to the pests. That could hold the key to stopping the shot hole borer, Drill explains. “In the long term, we’re really looking into bio control.”

Beetle entry hole on an avocado branch Photo: Akif Eskalen

Much of the research on shot hole borers has been funded by the California Avocado Commission. The KSHB is a particular concern in the state’s avocado-growing regions. Drill also credits the U.S. Forest Service, the Huntington Botanical Gardens and Orange County Parks for their contributions to the research.

Yet something good may emerge from this disaster, Drill said. “We are not used to working on things that are both wildland pests and urban pests and agricultural pests.”

“I think we’ve learned a lot about working together, because it’s everybody’s issue,” she says. “Hopefully we’ll be better prepared for the next thing that shows up along these lines. And there will be a next thing.”

Symptoms of shot hole borer attack

A shot hole borer entry hole is round, about the size of a ballpoint pen tip, UCANR guidelines say. Attack symptoms can include staining, gumming, sugary exude or frass. Advance infestations lead to branch dieback and overall decline.

Fusarium infection causes dark discoloration of the wood beneath the bark and around the beetle gallery. Lightly scraping away the bark around the entry hole will reveal the staining.

What can a tree owner do?

The UCANR advises these steps:

  • Use only local firewood, and know the source of any mulch you use.
  • Keep trees healthy.
  • Watch trees for signs of distress or damage. Symptoms can mean problems other than a shot hole borer invasion.
  • If a tree is seriously damaged, have it removed and chopped into mulch pieces no larger than 1 inch.
  • Suspect trees in regions of the state where the beetles are not well established should be reported. Local or county officials should be notified about suspect public or street trees as soon as possible.

Native Trees to Monitor

These 19 California natives are among 64 tree species confirmed by Dr. Akif Eskalen, UC Cooperative Extension plant pathologist, as reproductive hosts of shot hole borer/fusarium dieback complex. Reproductive hosts are tree species that are capable of 1) supporting beetle reproduction and 2) growing the fungi that cause fusarium dieback.

Sabrina Drill emphasizes that this should not be considered a “do not plant” list but rather a guide for observation. Residents with these species should watch for changes in their trees and report them. (See below for how to do that.)

  • Box elder (Acer negundo)
  • Big leaf maple (Acer macrophyllum)
  • California buckeye (Aesculus californica)
  • White alder  (Alnus rhombifolia)
  • Mulefat (Baccharis salicina)
  • Blue palo verde (Cercidium floridum)
  • California sycamore (Platanus racemosa)
  • Fremont cottonwood  (Populus fremontii)
  • Black cottonwood (Populus trichocarpa)
  • Mesquite (Prosopis glandulosa)
  • Coast live oak (Quercus agrifolia)
  • Canyon live oak (Quercus chrysolepis)
  • Engelmann oak (Quercus engelmannii)
  • Valley oak (Quercus lobata)
  • Goodding’s black willow (Salix gooddingii)
  • Red willow  (Salix laevigata)
  • Arroyo willow (Salix lasolepis)

The full list, including non-native trees and links to pictures of symptoms and damage, is at

How to Report a Suspect Tree

Submit the following information to UC Riverside (home of the Center for Invasive Species) at

  • Your contact information, including name, city, phone number and email
  • Suspect tree species
  • Description of suspect tree’s location (and/or GPS coordinates)
  • Description of tree’s symptoms
  • Photos. Take photos of suspect trees from several distances. Include 1) the trunk or symptomatic branches; 2) up-close view of the symptoms; 3) the entry/exit hole, if visible, with a ballpoint pen tip for scale (remove gumming or exudate if necessary). If dieback is observed, include a picture of the entire tree.

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  1. Is this beetle prevalent in South Africa? Has an article about the beetle and its research been featured in our local Flora Magazine, or any Botanical Society publication?

  2. Sadly, our two decade collection was removed in East San Diego, CA just yesterday, Blue Spruce, Box Elder, Red Bud, Orn Plum, Oak, Maple. All collected from different nurseries- years apart. After reading this article wondering if the bagged bark/mulch purchased at big box stores is to blame. Off to replant.

  3. Is there any recommended treatment to rid a healthy tree of the hole bore beetle? I have a canyon oak that seems healthy but one limb appears damaged by HBB

  4. The Beetle has just been spotted in Australia, Sept 30 2021… In Fremantle Perth and a couple of suburbs have been quarantined, no movement of trees, shrubs or pot plants is allowed out of the designated areas.

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