PSHB – the perfect invader

By Chelsea Payne

Polyphagous Shot Hole Borer (PSHB) has emerged as a significant tree pest in Perth, Western Australia, and presents a threat to Australia’s incredible native flora, and our thriving horticulture and agriculture industries.

Described as the ‘perfect invader,’ the Polyphagous Shot Hole Borer beetle Euwallaceae fornicatus tunnels into living host trees and shrubs creating extensive galleries within structural stems. In addition to causing structural damage from borer galleries, the beetle is a ‘fungi farmer’ transporting and propagating a Fusarium fungus within stems as a food source, which causes stem decline or wilt by disrupting the transport of water and nutrients in the plant’s cells.

At 2mm in length, with a fast life cycle, and an optimum temperature range which matches much of Australia’s climatic zones, PSHB can quickly reproduce, increasing numbers, and disperses via flight whilst going undetected within trees.

Detected in 2021 via WA’s Department of Primary Industries and Regional Development’s (DPIRD) MyPestGuide^® reporter app, a Quarantine Area Notice has been issued over 21 Local Government Areas which limits the spread of untreated green waste to prevent spread of PSHB, and assist control and surveillance programs. An intensive program of trapping and surveillance is ongoing, with extensive engagement with government, industry, and the public.

PSHB has a host range of over 400 species of commonly planted native and exotic hardwood trees including maples, planes, figs, erythrina, avocados and oaks. Attacked trees are considered reproductive hosts if the beetle can establish Fusarium and successfully reproduce within the tree, or non-reproductive where the beetle’s lifecycle is not completed. The Box Elder Maple (Acer negundo) has emerged as a significant reproductive host tree in WA, which amplifies beetle numbers and rapidly declines once infested.

Knowledge shared from international colleagues has provided a sobering insight into the extent of damage if the PSHB disease complex is not controlled, with the potential for extensive economic and biodiversity losses. The global host list assists in predictions on the impact on Australian landscapes, with prominent Australian plants such as Acacias or the WA Red Flowering Gum (Corymbia ficifolia) being known reproductive hosts. As many of Australia’s unique tree species are rarely planted internationally, surveillance and monitoring of our native species is critical to early identification of new host species.

Two significant species which have emerged as a reproductive host species from the infestation in Perth, are the iconic Australian species the Moreton Bay Fig (Ficus macrophylla) and the Port Jackson Fig (Ficus rubiginosa). Widely planted as specimen trees in historic parks and gardens throughout Australia, the infestation of these trees could result in a shocking change to inner-city green spaces, and enormous loss of urban canopy, amenity, and community identity which these trees provide.

Many urban trees are under significant pressure from existing soil pathogens, high intensity horticulture or soil disturbance around their root zones, and climate change. Common street trees plantings such as the London Plane(Platanus x acerifolia), Poinciana (Delonix regia), and Coral Tree (Erythrina sp.) present a challenge to urban canopy management, where monocultures of trees of the same species, age class, and genetic provenance create potential scenarios for large-scale losses throughout urban areas.

Horticulturists and allied professionals are the on-the-ground experts in plant care and maintenance, and play a critical role in surveillance and reporting, prevention and treatment of plant pests and disease.

Prioritising tree health and care is one way the horticulture industry can protect our valuable trees. Tree care principles such as root zone protection, soil health, and always pruning to Australian Standards ensures urban trees stand the best chance of compartmentalising damage, maintaining healthy structural stems and a vigorous immune system to protect them from attack.

Tree custodians throughout Australia must take tree pests seriously. Trees are significant assets to both the private and government sectors; biosecurity is often quantified regarding loss to industry, however the loss of amenity and sense of identity to communities if often understated when communicating the urgency and priority of biosecurity.

Upskilling through the free online resources provided by Plant Health Australia, or State Government Biosecurity Agencies, or industry bodies, empowers horticultural staff to understand threats, seek out diagnostic tools, and report pests of concern, potentially resulting in critical early detection of outbreaks, increasing the likelihood of control and eradication.

Biosecurity is a joint responsibility; together, the horticulture industry can mobilise an interconnected and passionate network of professionals to collectively build a culture of accountability and best practice around biosecurity to protect our incredible plants.

What to look for:

• Multiple small holes, on the trunk and stems, which are round and 1-2mm in diameter

• Unusual decline or death of stems

• Frass (sawdust like powder), gum or sap extruded from entry holes

• Dark inky staining around borer holes or galleries caused by the Fusarium fungus

Monitoring trees is important to identify infested trees. In Western Australia, the free MyPestGuide^® app, is an invaluable resource for pest identification for PSHB and other more common pests. If you suspect that a tree may be infested by PSHB, the location of the tree and photos can be uploaded for diagnosis. Alternatively, people can call DPIRD on 9368 3080 or visit agric.wa.gov.au/borer.

Outside of Western Australia, suspect borer damage (or any unusual pests) can be reported via the MyPestGuide^® app or by calling the Exotic Plant Pest Hotline on 1800 084 881.

More information about polyphagous shot-hole borer, updates and information on the quarantine area, and important updates on the response can be found at the DPIRD website.

Chelsea Payne
Curator, Arboriculture
Botanic Parks and Gardens Authority
chelsea.payne@dbca.wa.gov.au

Main photo: PSHB Beetles are less than 2mm in length, about the size of a sesame seed