The hallmark signs of PSHB include tiny ‘shot holes’ the size of a ballpoint pen tip in trunks and branches, as well as intricate networks of tunnels or galleries that are stained black by the fusarium fungus.  

Some trees such as the London planetree (Platanus x hispanica) may display staining from the fungus, while other trees including figs will try to push the beetles out by producing sap, also known as gumming.  

Another symptom that may be seen in avocado trees is the presence of crystallised mounds known as sugar volcanoes, which are caused by the tree’s response to the fungus and disruption to its vascular system.  

Symptoms can vary greatly from tree to tree, depending on the level of infestation, host status and plant species.

East Fremantle resident Joanne Taggart, first noticed symptoms of the invasive beetle on her box elder maple tree (Acer negundo) in August 2021. The tree was displaying signs of dieback and dropping limbs that had unusual markings inside the branches.  

Within hours of submitting photos via the MyPestGuide Reporter app, biosecurity staff and entomologists from the Department of Primary Industries and Regional Development (DPIRD) rushed out to inspect what would soon be the first case of PSHB in Australia.  

An unprecedented biosecurity response is now underway in Western Australia to eradicate the pest.  

DPIRD is leading the $44 million response and has inspected more than two million trees during the past three years. More than 3700 of these trees have been confirmed as positive for PSHB, cut down and chipped, to stem the spread of the beetle.  

While this is the first time that PSHB has been detected in Australia, the beetle has spread from south-east Asia to several other countries, including Israel, South Africa and the United States of America.  

PSHB is considered both an agricultural and environmental pest and has caused substantial economic damage in other countries.  

Dr Trudy Paap, a leading researcher from South Africa, is a research fellow at the University of Pretoria’s Forestry Agriculture and Biodiversity Institute and has been involved in PSHB research since the pest was first detected in South Africa in 2017.  

“There has been up to 100 per cent mortality caused by PSHB to some tree species that are commonly used in landscape design and residential gardens.” Dr Paap said.

“Unfortunately, we haven’t found a chemical treatment that is safe and efficient against PSHB.  

“Tree pruning and removal of heavily infested trees is currently the most effective way to protect surrounding vegetation.”  

Chief Plant Biosecurity Officer at DPIRD Dr Vincent Lanoiselet said the department was engaged in trials to test the efficacy of certain chemicals on early infestations of PSHB in Perth.  

Dr Shannon Lynch, an Assistant Professor from the University of California, shared her unique knowledge about the invasive beetle with DBCA and DPIRD at a public lecture at Murdoch University. Scientists in Southern California have been dealing with the pest for more than a decade, causing devastation in river valleys, where it has killed hundreds of thousands of willow trees.  

Among the lecture attendees were scientists, horticulturalists, arborists, gardeners, program managers and natural area managers across local and state government, academia, community and the private sector.  

Quarantine restrictions are currently in place for the entire Perth metropolitan area to prevent the pest from spreading and to safeguard WA’s native forests and agricultural industries.  

Unseasoned wood cannot be moved outside of the Perth metropolitan area unless it is chipped to less than 2.5 centimetres. Likewise, living plants with stems greater than two centimetres in diameter cannot be moved outside the quarantine area and any machinery used to handle wood or plant material must be cleaned.  

Fortunately, PSHB does not live, breed in or otherwise affect lawn clippings and grasses, so these can be disposed of as normal and with no restrictions.  

The PSHB pest complex has a wide host range, affecting more than 500 plant species globally.  

Polyphagous means ‘feeding on many types of food.’ In WA, the beetle has been found reproducing in 100 different plants and trees, with a clear preference for exotic species.  

In reproductive hosts, PSHB can successfully establish the fungus, complete its lifecycle and reproduce within the plant. Reproductive host species are more susceptible to dieback and can be killed by the pest.  

In WA, the top reproductive host trees for PSHB are all exotic species and include the box elder maple (Acer negundo), black locust or Robinia (Robinia pseudoacacia), coral trees (Erythrina x sykesii), poinciana (Delonix regia), Moreton Bay fig (Ficus macrophylla), Port Jackson fig (Ficus rubiginosa), London planetree (Platanus x acerifolia), white mulberry (Morus alba), mirror bush (Coprosma repens) and castor oil plant (Ricinus communis).

Dr Vincent Lanoiselet said the box elder maple was an amplifier species in which PSHB can breed prolifically, allowing the build-up of large populations of beetles.  

“All box elder maple trees in Perth should be regularly inspected for symptoms of PSHB as they amplify the PSHB population and increase the risk to surrounding trees,” Dr Lanoiselet said.  

Non-reproductive hosts can be attacked by PSHB, but the fungus does not establish, which means the beetle is unable to create breeding galleries and cannot reproduce.

Some Western Australian native tree species have been attacked by PSHB, however they appear to be less susceptible to the beetle and its fungus, with many species listed as non-reproductive hosts.  

Of concern, is the detection of the PSHB pest complex in the swamp paperbark (Melaleuca rhaphiophylla), a species that is endemic to the south-west of WA. The swamp paperbark grows along the banks of rivers and within wetlands, providing food and habitat for a range of native animals and reducing erosion along watercourses.  

Like many of Perth’s inner city historic parklands, Kings Park has not been spared from PSHB infestation. Plantings in the Parklands and Botanic Garden have been impacted, including infestations of iconic Moreton Bay and Port Jackson figs.  

“The most significant area of infestation is centred on the Mount Eliza escarpment with 26 infested trees removed by DPIRD from the scarp this year,” Chelsea Payne, Curator, Arboriculture, Botanic Gardens and Parks Authority (BGPA) said.  

“We had to act quickly to protect the Western Australian Botanic Garden at Kings Park, and the many rare and endangered flora in our collection.”

“The removal of these trees has resulted in a significant landscape change to the Mount Eliza escarpment. However, these works are only a chapter in the long history of changes on the scarp.”  

Restoration of the scarp is underway with almost 18,000 plants from 16 species native to the area included as part of the first planting. These were chosen as species that were known to be endemic to the scarp, perform in tough conditions, with a number being known as species that thrive after disaster.  

BGPA is working closely with Traditional Owners throughout the project to preserve and honour the cultural significance of the area, with consultation emphasising the importance of cultural sensitivity and environmental stewardship.  

The State Government has announced a $7.2 million investment to improve Perth’s canopy cover and replace trees affected by PSHB.  

The funding will allow more than 16,000 trees to be planted at iconic sites including Perth Zoo and Hyde Park. Local governments and residential landholders will also be eligible to apply for and receive funds.  

Up to three trees will be planted to replace every one lost on public lands. The replacement trees will be selected with consideration to their resilience to PSHB and climate change.

The tree replacement program is scheduled to commence in 2025 and is being managed by the Department of Water and Environmental Regulation.