Protecting Trees from the Polyphagous Shot-Hole Borer: Strategies and Solutions

The polyphagous shot-hole borer (PSHB, Euwallacea fornicatus), a tiny beetle native to Southeast Asia, has emerged as a significant threat to trees worldwide, including in urban, agricultural, and natural ecosystems. First detected in Australia in 2021 in Perth, Western Australia, and with established infestations in regions like California, South Africa, and Israel, PSHB has wreaked havoc on over 600 tree species, causing economic losses, environmental degradation, and public safety concerns. Its symbiotic relationship with the Fusarium fungus, which disrupts a tree’s vascular system, makes it particularly destructive, often leading to tree death within two years. Protecting trees from PSHB requires a multifaceted approach, combining early detection, quarantine measures, innovative treatments, and community engagement. This 2,000-word article explores the biology of PSHB, its impacts, and the latest strategies for safeguarding trees, drawing on recent advancements as of 2025.

Understanding the Polyphagous Shot-Hole Borer

Biology and Behavior

PSHB is a small ambrosia beetle, with females measuring about 2 mm and males slightly smaller at 1.6 mm. Unlike bark beetles, PSHB bores deep into the trunks, stems, and branches of trees, creating galleries where it cultivates Fusarium fungi (Fusarium euwallaceae or related species) as a food source for itself and its larvae. The fungus, carried in specialized mouthpart sacs called mycangia, colonizes the tree’s vascular system, blocking water and nutrient transport. This leads to Fusarium dieback, characterized by wilting, branch dieback, and, in susceptible species, rapid tree death.

PSHB attacks a wide range of hosts, including over 100 reproductive hosts where it completes its lifecycle and more than 500 non-reproductive hosts where it causes damage without reproducing. Key reproductive hosts include box elder maple (Acer negundo), avocado (Persea americana), oak (Quercus spp.), and willow (Salix spp.). The beetle’s ability to fly up to 400 meters and spread through human-mediated movement of infested wood (e.g., firewood, green waste) makes it highly invasive. Its lifecycle, from egg to adult, takes 22–40 days, allowing rapid population growth, especially in warm climates.

Global and Local Impacts

Since its introduction to California in 2003 (initially misidentified until 2012), PSHB has caused the loss of over 4,000 trees in Perth alone by 2025, devastating iconic urban spaces like Hyde Park and Kings Park. In South Africa, it threatens over 100 tree species, with potential economic damages exceeding R275 billion over a decade. The beetle’s impact spans:

Urban Environments: Loss of canopy cover increases cooling costs, reduces property values, and creates safety hazards from falling branches.
Agriculture: Damage to crops like avocado and pecan threatens food security and export markets.
Natural Ecosystems: Infestation of native species disrupts biodiversity and ecosystem services like water regulation.

In Western Australia, a $41.29 million eradication campaign, funded by federal and state governments, underscores the urgency of controlling PSHB. However, its cryptic nature—small size, deep tunneling, and variable symptoms—complicates management.

Challenges in PSHB Management

Traditional pest control methods are largely ineffective against PSHB due to its unique biology:

Chemical Limitations: Systemic insecticides and fungicides struggle to reach the beetle and fungus within tree galleries, and surface sprays are ineffective as PSHB spends most of its life inside the host.
Rapid Spread: Human movement of infested materials, such as firewood or wood chips, facilitates long-distance dispersal, bypassing natural barriers.
Host Range: The beetle’s ability to attack diverse species, from ornamental to native trees, makes comprehensive protection challenging.
Lack of Resistant Species: While some trees show natural resistance, insufficient research exists to identify universally resistant species for widespread planting.

Historically, the primary control method has been the removal and destruction of infested trees, which is costly, socially unacceptable in urban settings, and insufficient to eradicate the pest. Recent advancements, however, offer hope for more sustainable solutions.

Strategies for Protecting Trees

Protecting trees from PSHB requires an integrated pest management (IPM) approach, combining prevention, early detection, innovative treatments, and community action. Below are the latest strategies as of 2025.

1. Prevention Through Quarantine and Biosecurity

Preventing PSHB spread is critical, especially in regions like Western Australia, where a Quarantine Area (QA) covers 30 local government areas around Perth. Key measures include:

Movement Restrictions: Prohibit the transport of wood, bark, potted plants, firewood, mulch, or green waste out of the QA. Permits are required to move mature trees with stems over 2 cm in diameter, ensuring inspection for PSHB symptoms. For example, permits can be obtained by emailing PSHB@dpird.wa.gov.au or calling 9368 3080.
Firewood Management: Avoid moving firewood long distances. The “buy it where you burn it” campaign encourages sourcing firewood locally to prevent PSHB dispersal, as burning infested wood releases beetles.
Mulch Alternatives: Use grass clippings or compost instead of wood chips, which can harbor PSHB if sourced from infested areas.
Biosecurity at Borders: Strengthen import regulations to inspect wooden packaging and plant materials, as PSHB likely entered Australia via untreated wood products.

These measures aim to contain the pest within known areas, preventing its spread to regions like Queensland or rural Western Australia.

2. Early Detection and Surveillance

Early detection is crucial to limit PSHB damage. The Western Australia Department of Primary Industries and Regional Development (DPIRD) has implemented extensive surveillance programs, including:

Trapping: Yellow sticky traps with Querciverol lures (an aggregation pheromone) are deployed across the QA for at least six months to monitor PSHB distribution. These traps, designed to avoid capturing non-target species, provide data on pest spread.
Community Reporting: Residents are urged to inspect trees, especially box elder maples, for signs of infestation, such as:
- Tiny entry/exit holes (<1 mm) resembling shotgun patterns.
- Frass (wood dust and insect waste) around holes or frass tubes.
- Sugar volcanoes (white, crystalline exudate) on avocado trees.
- Wilting, dieback, or dark staining under bark.
Reports can be submitted via the MyPestGuide Reporter app or by calling the Exotic Plant Pest Hotline (1800 084 881). Including a pen or ruler in photos of bore holes aids identification.
Targeted Inspections: DPIRD prioritizes high-risk species like box elder maples and conducts regular checks in public spaces, parks, and backyards.

Community awareness campaigns, such as digital displays at Yagan Square, encourage vigilance and reporting, amplifying detection efforts.

3. Innovative Treatment Methods

Recent trials have introduced promising alternatives to tree removal, offering hope for preserving infested trees:

Chemical Injection Trials: In 2025, the City of Canning in Perth reported success in a four-month trial, saving 131 infested trees using a vitamin-like capsule containing insecticide and fungicide. The capsule, injected deep into the tree and sealed with a plug to prevent leaching, targets both the beetle and Fusarium fungus. The treatment, developed by BioHerbicides Australia, costs approximately $2,000 for 131 trees and may require reapplication every 3–4 years. While still in the trial phase, this method is being considered for broader adoption across Perth, pending further validation.
Plastic Wrapping: Source Certain, a Perth-based research firm, proposed a novel plastic matrix wrap infused with copper to kill PSHB and its fungus by excluding oxygen and disrupting fungal growth. Trials at Kings Park and Perth Zoo have shown promise for high-value trees, though the method is not suitable for widespread use and does not support eradication goals.
Systemic Insecticides with Pruning: A 2023 study in Western Australia found that biennial systemic insecticide treatments combined with pruning necrotic limbs were more cost-effective than tree removal and replanting. This strategy, costing less than removal, preserves tree canopy and ecosystem services, though long-term efficacy requires further research.

These treatments mark a shift from destructive methods, offering hope for saving significant trees like Moreton Bay figs or urban canopy staples.

4. Tree Removal and Sanitation

When infestation is severe, removal remains necessary to prevent PSHB spread. DPIRD issues Pest Control Notices (PCNs) to authorize tree removal, ensuring compliance with eradication protocols. Key practices include:

Proper Disposal: Infested wood must be chipped, covered, and disposed of at designated facilities to prevent beetle dispersal. Burning infested wood is discouraged, as it releases beetles.
Tool Sanitation: Pruning tools, chainsaws, and woodchippers must be disinfected with bleach or alcohol before use on uninfested trees to avoid cross-contamination.
Strategic Pruning: For lightly infested trees, removing affected branches can slow spread, though this is less effective than chemical treatments.

5. Planting Resistant Species

While no tree is entirely immune, planting resistant or less-preferred species reduces vulnerability. DPIRD recommends native species over exotics, though specific resistant species are still under study. In South Africa, researchers are identifying resistant trees to replace susceptible ones, a strategy B.C. could adopt. Avoid planting high-risk species like box elder maples in infested areas.

6. Community and Industry Engagement

Community involvement is critical to PSHB management. Initiatives include:

Education Campaigns: DPIRD’s outreach, including videos and workshops, educates residents on identifying PSHB and adhering to quarantine rules.
Collaboration: Local councils, universities (e.g., University of Queensland), and private firms like ArbWest partner with DPIRD to test treatments and share findings.
Indigenous Involvement: Engaging First Nations communities in surveillance and management respects their connection to the land and leverages traditional ecological knowledge.

7. Research and Biocontrol Prospects

Ongoing research aims to develop long-term solutions. In California, Richard Stouthamer is exploring biocontrol agents like Phymastichus sp., a parasitic wasp, though host specificity tests are incomplete. Other research focuses on resistant tree genetics and fungal inhibitors. Funding from organizations like the California Avocado Commission and U.S. Forest Service supports these efforts, which could benefit B.C. if adapted to local ecosystems.

Economic and Environmental Stakes

The economic cost of PSHB is staggering. In South Africa, potential losses could reach R275 billion over a decade, affecting forestry, agriculture, and urban infrastructure. In Perth, the loss of urban canopy increases energy costs (due to reduced shade), lowers property values, and requires millions in tree replacement. Environmentally, PSHB threatens biodiversity by attacking native species, disrupting ecosystems, and increasing fire and flood risks due to dead trees.

Protecting trees preserves ecosystem services like carbon sequestration, air purification, and habitat provision, which are worth billions annually. A 2023 study estimated that preserving urban trees through pruning and insecticide treatments saves significant ecosystem service costs compared to removal.

Practical Steps for Tree Owners

For individuals and communities in B.C. or other at-risk areas, the following steps can protect trees:

Monitor Regularly: Check trees for bore holes, frass, or dieback, especially on high-risk species. Report suspicions to local authorities or biosecurity agencies.
Follow Quarantine Rules: Avoid moving wood or plants from infested areas. Source mulch and firewood locally.
Maintain Tree Health: Healthy trees resist PSHB better. Ensure proper watering, avoid over-pruning, and provide suitable soils.
Support Trials: Advocate for funding and participation in chemical or wrapping trials to save valuable trees.
Plant Wisely: Choose native or resistant species for new plantings, consulting local guidelines.
Sanitize Equipment: Clean tools after pruning to prevent accidental spread.

Conclusion

The polyphagous shot-hole borer poses a formidable challenge to trees in British Columbia and beyond, threatening urban canopies, agricultural productivity, and natural ecosystems. However, recent advancements in 2025, such as chemical injection trials and plastic wrapping, offer hope for saving infested trees, while traditional methods like quarantine, surveillance, and tree removal remain essential. Community engagement, biosecurity measures, and ongoing research into biocontrol and resistant species are critical to long-term success. By adopting an integrated approach, B.C. can protect its trees, preserving their economic, environmental, and cultural value for future generations.

To stay proactive, residents and policymakers must collaborate, leveraging lessons from regions like Western Australia and California. Whether through reporting suspicious symptoms, adhering to quarantine protocols, or supporting innovative treatments, every action counts in the fight against PSHB. Protect your trees today—report, monitor, and act to safeguard B.C.’s green heritage.