06-07-2025
Warning over common garden product as destructive threat spreads across Aussie city
Keen gardeners in one Aussie state are being warned to think twice before buying and using wood mulch on their yards. The word of caution comes amid the outbreak of a destructive pest that is infesting thousands of trees throughout Perth.
The invasive beetle, known as the polyphagous shot-hole borer, digs deep into tree trunks, spreading fungal disease and eventually killing its host. It has triggered a major biosecurity response in Western Australia, with a quarantine zone set up around Perth's metropolitan area.
Bob Hunter, chairman of the Western Australian Horticultural Council, urged residents to take care before spreading wood mulch in their gardens.
'Only use it if you know, A - where it's come from and B - if it's been made safe,' Mr Hunter told Yahoo News.
He said mulch from unreliable or unknown sources posed the greatest risk.
'The ones that are not safe are the ones that you get from some source that you don't know — and that's either a communal pile or from a tree lopper who's dumping off a load,' he said.
'If you don't know it's safe, I would recommend it wasn't used.'
Mr Hunter added that there's no risk for those with a home mulcher. If it is necessary to purchase wood mulch, opt for one that has been heat-treated and cut to the recommended 2.5cm diameter or smaller.
While his property hasn't been impacted by the borer yet, Mr Hunter said "just about every tree' in his yard is high on the host list.
The state has identified the box elder maple, the black locust and the coral tree as its highest risk host trees.
Find the full list of host trees here.
The shot-hole borer has taken a serious toll on Perth's already struggling urban canopy. Thousands of trees have been removed across the city, including at some of the state's most iconic locations — Kings Park, Perth Zoo, and even Rottnest Island.
There's concern that the insect may have made it to Rottnest by hitching a ride in contaminated mulch, although this theory is yet to be confirmed.
Once a tree is infected, there is little chance of recovery. The insect creates networks of tunnels inside trunks and branches while cultivating a fungal symbiont as a food source for themselves and their larvae.
The fungus disrupts the tree's ability to transport water and nutrients, eventually leading to its death.
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Three weeks ago, WA biosecurity officials announced it was no longer possible to wipe out the shot-hole borer from the city. Instead, the government determined it will transition to managing the pest rather than eradication strategies.
The Invasive Species Council slammed the move, saying it could put the rest of the nation at risk.
"This is tragic news not just for Western Australia, but potentially for native trees, towns and cities, and the horticultural industry across the entire country," Dr Carol Booth, the Invasive Species Council's policy director, said.
A quarantine zone remains in place across much of the Perth metropolitan area in an attempt to stop the borer from spreading.
Under the restrictions, movement of any host material within the restricted area is discouraged, while the movement of any host material from the quarantine area to outside areas is prohibited.
The quarantine zone covers 30 LGAs across 6400km², but biosecurity experts believe it's only a matter of time until the pest breaks through quarantine lines and spreads throughout the state.
Some councils have begun trialling ways to contain or treat infestations. In the City of Canning, a pilot treatment was initially hailed as a possible success, however Local Government Minister Hannah Beazley urged caution.
'I'm aware of the media reports that have been generated from the City of Canning, but despite significant global research, no chemical treatments have been found to be effective in eradicating shot hole borer and its symbiotic fungus anywhere in the world,' Beazley told WAtoday.
Yahoo News contacted the City of Canning regarding the success of its treatment trial but did not receive a response before the time of publication.
If you suspect a shot-hole borer infestation on your property, contact the DPIRD Pest and Disease Information Service on (08) 9368 3080, or by email at padis@
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