Protect Urban Trees from Invasive Shothole Borers

Although invasive shothole borers could spread throughout much of California and other states if left unchecked, many trees can be saved, and even heavily infested areas can recover over time.
woman with ponytail and ballcap surveys a tree for signs of infestation of invasive shothole borers

Urban trees provide us with many benefits to our health and our wallets. The trees around us reduce our stress levels, provide shade and allow for energy conservation, improve air quality, reduce stormwater runoff, and provide habitat for wildlife. It is important to protect them from invasive pests, like the invasive shothole borers (ISHB), that could potentially kill up to one in three urban trees in California. Experts believe ISHB were introduced to Southern California, presumably via infested wood products or shipping material, between approximately 2003 to 2012.

A pest-disease complex: Invasive Shothole Borers and Fusarium dieback

The invasive shothole borers (ISHB) are beetles that tunnel into trees and introduce a fungus that they use as their food source. The fungus causes a disease called Fusarium dieback that disrupts the flow of water and nutrients in trees. In severely infested trees, it can cause tree decline, branch dieback, and tree death. Trees with heavily infested branches can be especially hazardous, since the combined damage of the fungal disease and the beetle’s tunneling activity weakens the wood, causing limbs to break and fall.

Trees at Risk

The most highly susceptible trees include many of the species com­monly used for landscaping, such as sycamores, oaks, cottonwoods, and box elder trees. ISHB beetles attack healthy trees as well as stressed or diseased trees in a variety of urban, suburban, and riparian settings. Visit www.ishb.org to find the full list of reproductive hosts in California.

Female beetles can fly short distances, allowing the pest-disease complex to spread into new areas near already infested trees. Beetles can also be transported in infested firewood and green waste, leading to spread over much greater distances. ISHB are currently present in Southern California and the Central Coast, and they could spread throughout much of California and other states if left unchecked.

Identifying an ISHB Infestation

Correct identification is the first step to successful ISHB management. The following are typical signs and symp­toms of ISHB infestation:

  • Entry holes to the beetles’ tunnels. Perfectly round and about the size of the tip of a medium ball-point pen (0.8 mm).
  • Additional signs and symptoms accompanying entry holes (vary by tree species): wet staining, gumming, sugarlike buildup, and/or boring dust (resembles fine sawdust).
  • Dieback: Dead or wilting branch­es can be a sign of a severe infesta­tion. If you see dieback on trees, check for entry holes on the trunk or the branch collars.

The University of California Cooperative Extension has created an ISHB Management and Detection Tool that can be used to help identify ISHB infestations, obtain general management recommendations, and report infestations in new areas or new reproductive hosts.

Managing ISHB

Several steps can be taken to prevent pest problems and manage infestations

  1. Keep trees healthy. Proper irriga­tion and maintenance will keep trees strong and help protect them from ISHB and other pests.
  2. Prune out infested branches. Re­moving branches that have clusters of 50 or more ISHB holes would help control this pest.
  3. Reduce beetle populations by removing severely infested trees (with more than 150 entry holes and ISHB-related dieback) which, unfortunately, are not likely to survive and will become a source of infestation for the surrounding trees.
  4. Properly dispose of infested plant material. The easiest way to kill the beetles in down wood is chipping it (ideally, to one inch or less). If the chips will be used as mulch somewhere else, they should be solarized or com­posted to ensure they are completely pest-free. If chipping is not an option, logs can also be solarized or kiln dried to exterminate the beetles.
  5. Prevent the spread. Avoid spread­ing this pest by not moving firewood or mulch that hasn’t been prop­erly solarized or composted. If you must move infested green waste (for example, to bring it to a composting facility) make sure the load is tightly covered while in transit.
  6. Consider treating infested high-value trees. Make sure you contract a licensed professional. Trees that aren’t already infested should be monitored but not treated.

Biocontrol options (including natural enemies, entomopathogenic fungi, endophytes, and nematodes) are currently under research by University of California Statewide Integrated Pest Management Program. But these biological management options might take time before they are tested and available. Until then, prevention, early detection, and rapid response are our best weap­ons to keep trees healthy and alive.

For more information on invasive shothole borers and their manage­ment, visit www.ishb.org.

Photo at top by Jennifer Shedden.

Dr. Beatriz Nobua-Behrmann

Dr. Beatriz Nobua-Behrmann

Dr. Nobua-Behrmann is the Urban Forestry Advisor for the University of California Cooperative Extension in Orange and LA Cos. She obtained her Ph.D. from the University of Buenos Aires, in Argentina, studying the ecology of insect-plant interactions. Her current research and extension program at UCCE is focused on management of invasive tree pests.

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