Unprecedented biosecurity response hopes to tackle wood-boring beetle outbreak
Melbourne would be very different without its avenues of plane trees, while Brisbane and Sydney streetscapes would not be the same without their towering figs.
However, if the polyphagous shot hole borer (PSHB) spreads, that scenario could become reality. The wood-boring beetle from Southeast Asia has made its home in parts of Perth in Western Australia. It not only has the potential to deplete that city’s urban tree canopy, but also to degrade native ecosystems and impact agriculture.
First detected in Australia three years ago in a suburban backyard, the PSHB (Euwallacea fornicates) has a symbiotic relationship with a fungus, which it farms inside trees as it tunnels its way through trunks and branches. The fungus causes dieback that can kill the tree the beetle has made its home. A chemical treatment to kill infestations of the beetle has not yet been found.
While the outbreak is understood not to have gone beyond WA, there are concerns about its impact if it is not eradicated.
Extreme measures
Efforts to tackle PSHB have seen a quarantine area established across Perth and thousands of trees removed from streets, parks and private properties.
Such is the threat to biodiversity on a national level that the Australian Government and all state and territory governments are cost-sharing a more than $40 million eradication plan.
The surveillance program is the biggest of its kind for WA, with the state’s Agriculture Minister Jackie Jarvis describing the project, funded for three years, as an “unprecedented biosecurity response”.
It is not known how long the beetle had been in WA when a resident came across tree damage in their backyard and reported it. The insect likely arrived undetected in untreated wood.
A quarantine area encompassing 17 local government areas was implemented in November 2021, which has since been expanded to 25 LGAs. Wood and plants are not allowed to be moved out of the zone.
However, it has recently been made public that multiple infestations of the borer have been found outside of the quarantine area boundary, in the Perth Hills.
More than 3000 trees have been removed and mulched, including a number of specimens estimated to be more than 100 years old, and almost 1000 others have been cut back to try to stop the beetle’s spread.
A national plan
While the federal Department of Agriculture, Fisheries and Forestry (DAFF) co-ordinates what it calls the “national technical response”, on the ground WA’s Department of Primary Industry and Regional Development (DPIRD) is responsible for developing and leading eradication measures.
CSIRO Principal Research Scientist, Health and Biosecurity Bruce Webber says, “The already low tree canopy cover in Perth will be significantly impacted by this beetle, and we could see the further decimation of tree canopy across Australian cities if it were to spread to other parts of the country.”
He and University of Western Australia Associate Professor in Applied Entomology Theo Evans are members of a recently formed scientific advisory group created by DPIRD in response to the outbreak.
“If we see the same impact on the figs we’re seeing in Perth over east, then Sydney and Brisbane will be very different,” Webber says.
“The beetle infestations also target plane trees and with the many plane tree avenues throughout Melbourne, take them away and the city will be a very different place.
“The time it will take to replace those trees at the size they are now, will have an enduring impact for decades,” he says.
Effect on the ecosystem
Biodiversity Council Australia Director James Trezise says the loss of urban canopy would also contribute to the heat island effect in cities.
He says the beetle’s spread would become “exceptionally difficult to manage” if it reached native forests.
If the beetle was to spread beyond the WA capital, Trezise says it would “transform” natural environments and would see “a gradual decline in the health of those ecosystems”.
“[We would] likely start to see the level of die back and old trees that are suddenly looking brown and dying off, because if it did become uncontrolled you wouldn’t see the removal of infested trees.”
He says the response would shift to an asset protection mode in that case where environmentally sensitive areas would be prioritised.
“That’s a really scary scenario we’re dealing with.”
The loss of trees could also affect native animals through the destruction of habitat. Trezise says it would have a compounding impact on threatened species already experiencing habitat decline.
He adds that scenario could already be playing out in Perth, where large trees in Kings Park, Hyde Park and Rottnest Island have been infested.
“WA’s iconic black cockatoos like to feed on a number of different species, and so the biggest impact on them would be the loss of trees for foraging, roosting and nesting,” he says.
“Losing even that urban canopy for threatened species is going to be really significant, that is a huge number of urban trees to have lost.”
What’s in a name?
The very name of the polyphagous shot hole borer gives insight into why it could pose such a threat.
Evans says while “Most insects that eat plants are monophagous, meaning they only eat one species,” polyphagous means these beetles eat many different species.
It has so far been found to make its home and reproduce in about 100 different species in Western Australia.
If the beetle were to find its way into commercial orchards, it could critically cost growers.
In Israel, where an outbreak has also occurred, the avocado industry has been severely impacted and Webber says given the beetle has been shown to make avocado, mango and macadamia trees their home in Australia, those industries could “potentially be significantly impacted”.
It is estimated PSHB will cost the South African economy more than $23 billion in the 10 years to 2032, and Trezise says, “If it were to spread throughout Australia, the cost would easily be in billions.”
Control challenges
While outbreaks in other countries may provide insight, Evans says there was a risk in simply using “control methods developed elsewhere” as “information true in one location may not be true for another location”.
“[If] you look at the list of trees that have been attacked over there, figs are not listed, but these beetles are hammering two native species of fig in Perth,” he says.
Evans says in California, where there are many Eucalyptus species, “Essentially no attack on eucalypts” was reported, however, “We’re finding that they can attack eucalypts in WA.”
DPIRD says it has conducted its own trials of insecticides and fungicides as treatments or preventative measures against the beetle but the entirety of the joint funding – due to be exhausted sometime next year – has been allocated to eradication measures.
It took more than a year for the joint funding arrangement behind the eradication plan to come into place.
“We probably missed a trick with the PSHB, it got in through the net, it got a bit of stamina, and spread to the point where eradication is now a very large and expensive exercise,” he says.
Evans says the fact the beetle does not naturally disperse far works in favour of the possibility of eradication. While the PSHB is capable of flight, it has not been found to fly often or far.
DPIRD has a network of more than 3000 sticky traps within the quarantine area and surrounds, in an effort to determine the spread of the pest.
Evans says the biggest factor that could increase the beetle’s reach was humans.
A DPIRD spokesperson says they were unable to provide a current map of the outbreak, however Evans says there were a number of isolated trees within the quarantine zone where the PSHB had been found.
“It’s literally one tree, and there are no other infested trees nearby,” he says. “You would expect humans must have transported some infested material to those locations.”
Trezise says, “Enforcement of the quarantine zone is where the game will really be won. Making sure that people aren’t moving green waste, live plants or products outside that quarantine zone is going to be really critical.”
