Invasive Shot-Hole Borers And Fusarium Dieback

By Beatriz Nobua-Behrmann and John Kabashima

Parks in Southern California are losing many of their trees to the Invasive Shot-Hole Borers (ISHB). These tiny ambrosia beetles are carrying the fungus that is causing increasingly extensive damage to urban trees, riparian natural forests, and avocado groves. Thousands of severely infested trees have been killed or removed in natural and landscaped areas.

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The Invasive Shot-Hole Borers (Euwallacea spp.) are small beetles (roughly the size of a sesame seed) that belong to two closely related and physically identical species: Polyphagous Shot-Hole Borer and Kuroshio Shot-Hole Borer. Both species are believed to have been accidentally introduced into California via wood products and/or shipping material from Southeast Asia. Since ISHB was first identified in Los Angeles County in 2012, the infestation has spread to six other counties, including Orange, San Diego, Ventura, Santa Barbara, San Bernardino and Riverside. Once the beetles arrive at a new location, they steadily spread to neighboring areas, infesting more and more trees. Movement of infested firewood and green waste can spread the infestation over even longer distances.

These beetles bore into trees, creating a series of small galleries. Inside these galleries, they “farm” a fungus (Fusarium spp.) they use as their main food source. The fungus colonizes the trees’ vascular system and causes a disease called Fusarium Dieback. Infested trees have trouble transporting water and nutrients up and down their trunks, which manifests itself as branch dieback, general tree decline, and, in many cases, tree death.

Currently, there are 64 confirmed species of tree in which the beetles can successfully grow the fungus and complete their life cycle. Susceptible trees include many of the species commonly used for landscaping, like sycamores, willows, oaks, cottonwoods, and box elder. To see the the complete list of ISHB hosts species, visit www.pshb.org.

How To Determine Tree Infestation
Each tree species reacts differently to ISHB attack. When inspecting trees, look for the following signs:

  • Beetle entry holes: When the beetles excavate their galleries in the trees, they make a perfectly round, small hole, 0.8 mm wide (roughly the size of the tip of a medium ballpoint pen) (a).

  • Tree symptoms: Entry holes are usually accompanied by one of these symptoms: wet staining (b), gumming (c), white powdery exudate (d), and/or frass (thin sawdust) (e). Each species of tree shows different symptoms.

  • 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 branch collar.

 ISHB symptoms.  Photo credit: Monica Dimson, UCCE Orange (a, c), Beatriz Nobua-Behrmann, UCCE Orange (b), Akif Eskalen, UC Riverside (d, e).

ISHB symptoms.

Photo credit: Monica Dimson, UCCE Orange (a, c), Beatriz Nobua-Behrmann, UCCE Orange (b), Akif Eskalen, UC Riverside (d, e).

Management On Landscape Trees
ISHB-infested trees can quickly become a public safety hazard, especially those with heavily infested branches, where the combination of tissue decline caused by the fungal pathogen and the mechanical damage from the beetle’s galleries weakens the wood, causing the limb to break and fall.

Early detection is the key for controlling this pest. So far, no effective preventative treatment has been reported, so best practices include regular monitoring and only treating trees when they become infested. Chemical and biocontrol management strategies are currently being investigated for this pest-disease complex. If you suspect ISHB is affecting trees in your area, please contact a local agricultural commissioner’s office or consult a licensed pest-control advisor familiar with ISHB control.

Trees that are severely infested (with more than 150 beetle attacks and ISHB-related branch dieback) are not likely to survive, even after being treated. These declining trees pose a physical hazard and become a source of beetles that can infest the neighboring areas. Therefore, they should be removed and their wood properly disposed. Even after a tree is removed, ISHB can continue to live and reproduce in the stump, so following a tree removal with stump grinding is always recommended.

Disposing Of Infested Wood
Live ISHB can survive in cut wood for weeks or even months. It is vital to take care of green waste appropriately in order to avoid spreading this pest to new areas.

  • Chip infested wood to 1 inch or smaller, which will kill 95 percent of the beetles.

  • To ensure the elimination of all beetles and fungal spores, have wood chips solarized, composted, or kiln-dried. Untreated chips can be used as mulch only in areas that are already heavily infested with ISHB.

  • Chipped material can be taken to a composting facility that has earned Seal of Testing Assurance (STA) from the U.S. Composting Council. Facilities in the STA program are tested for proper decomposition and pathogen control. To find a local STA facility, visit www.compostingcouncil.org/participants.

  • Chips can also be sent to the local landfill to be used as Alternative Daily Coverage.

  • If chipping is not possible, logs should be kiln-dried or solarized under a clear tarp to ensure total beetle elimination. Visit www.pshb.org for more information on solarization and composting guidelines.

  • When moving infested material to a different location (e.g., to be treated), make sure to cover it in-transit to prevent beetles from escaping.

Beatriz Nobua-Behrmann, Ph.D., is a Staff Research Associate for the University of California Cooperative Extension, Orange County. Reach her at benobua@ucanr.edu.

John Kabashima, Ph.D., is an Environmental Horticulture Advisor, Emeritus for the University of California Cooperative Extension, Orange County. Reach him at jnkabashima@ucanr.edu.

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What Else Can I Do?
Get educated: Learn how to identify this pest. Visit www.pshb.org to learn more about ISHB and obtain the most updated information.

Catch it early: Inspect trees regularly before they become infested. If you suspect trees are infested with ISHB, consult a licensed pest-control advisor familiar with ISHB control.

Take proper care of green waste: Dispose of infested wood properly to avoid spreading the beetles to other areas.

Don’t move firewood: Use locally sourced firewood to help stop the spread of this and other non-native pests.

Spread awareness in your community: Tell friends, family, HOAs, etc.