A beetle no bigger than a few millimetres could wipe out nearly half the tree canopy in Sydney, Melbourne and Brisbane if it reaches Australia’s east coast. New research says the polyphagous shot-hole borer could kill 47 per cent of Sydney’s urban trees in an invasion scenario, with the greatest damage likely in cities where the climate and tree mix suit the pest.
The warning lands as Perth is already living with the insect after it was discovered in Western Australia in 2021. Scientists published their findings last month in Frontiers in Forests and Global Change, after a stocktake of Sydney’s street trees and public parks found the beetle could threaten Moreton Bay figs, Port Jackson figs, Illawarra flame trees, paperbarks, some eucalyptus species, oak trees and plane trees.
Angus Carnegie, one of the researchers, said the 47 per cent figure was the extreme end of the risk. “That’s the worst-case scenario,” he said, adding that a more realistic outcome would be for 10 to 15 per cent of Sydney’s trees to die within five years if the beetle were not managed. If nothing at all were done, he said, losses in Sydney might rise to about 30 per cent, though dead branches would likely be removed as part of any control program.
The beetle, which originated in South-East Asia, does not eat wood directly. Instead it farms Fusarium fungus as a food source, and that fungus causes branch dieback and often kills the host. A single female can reproduce without a mate by laying unfertilised eggs that hatch only males, then mating with her own offspring. The insect can fly only short distances, but it can survive for months on small pieces of wood, a combination that has helped it spread across four new continents over about a quarter of a century: North America, South America, Africa and Australia.
That spread has already forced hard choices in Western Australia. After removing 4,000 infected trees in Perth, the West Australian government gave up eradication efforts last year and shifted to containment. Kingsley Dixon said Perth’s eucalyptus and banksia trees had proved reasonably resilient, but that the beetle had devastated east coast imports such as figs, flame trees and European deciduous trees. “In those environments with particularly moist summer conditions and higher humidity and a range of important and larger trees that will be vulnerable, it is imperative that the borer is kept out,” he said, adding, “I would strongly urge it’s an experiment that shouldn’t be tried in Sydney and Melbourne [and Brisbane].”
The risk is not limited to city parks and streets. Carnegie said keystone species such as she-oaks, broad-leaved paperbarks and tuckeroo were at risk outside urban areas, and said treatments might save some trees or agricultural crops such as avocados, but not at the scale needed in native forests. That is the difficult answer at the heart of the new research: if the polyphagous shot-hole borer crosses to the east coast, the places least able to recover may be the ones that define the landscape.
