Two large maple trees once dominated Joanne Taggart’s backyard in Fremantle, casting dappled shadows on her back wall and shading the lawn from the searing heat of a West Australian summer. Now the only evidence the trees existed are faint rings in the yellowing grass.

Nearly four years ago, she noticed one of her box elder maple trees was dying, and had it cut down. A few months later, she noticed tiny holes in the second tree and insect tunnels in the wood of a dropped limb.

This time, within hours of Taggart alerting authorities, a phalanx of tree and insect specialists descended on her house. “It was like something out of a Ghostbusters movie,” she says, adding her lawn soon resembled a forensic crime scene. “They spread tarpaulins everywhere and the remaining tree was carefully removed, segment by segment. It was very thorough.”

The pest that Taggart and the “Ghostbusters” team had discovered was new to Australia, and her box elder maple trees were the nation’s first official victims. That pest’s unwieldy name, the Polyphagous shot-hole borer, is now striking fear into the hearts of tree-lovers nationwide.

The name sounds like a joke without a punchline. It’s a preposterously small beetle, no bigger than a sesame seed or rice grain. Yet it poses a massive threat to many tree species across the nation if it ever escapes across the Nullarbor Plain.

The borer is a form of ambrosia beetle (Euwallacea fornicatus) that burrows into a healthy tree trunk, leaving tiny “shot-holes” like a blast from a small-bore shotgun. Once inside, the borer creates a maze of tunnels into which it deposits fungal spores that grow to form the beetle’s food source. Left unchecked, the fungus clogs up the tree’s vascular system – water and sap can no longer move around – and that can cause branches on the host tree, or ultimately the entire tree, to progressively die.

The borer has comically tiny wings that allow it to fly short distances, but it can hitch a ride on garden waste, firewood or chipped mulch. And the female beetle has a deadly advantage – she can produce offspring without a mate and start a whole new colony on her own. “The word polyphagous means ‘eats many foods’,” says senior botanist Kingsley Dixon, a professor in the School of Molecular and Life Sciences at Curtin University. “That means it eats anything. It’s not like Dutch elm disease that’s specific to elm trees. What we’ve got here is the Dutch elm disease of everything, the COVID-19 of tree infestation, or a terrorist cell waiting for an opportunity to break out.”

It’s hardly surprising that the tiny borer has triggered the largest and most complex biosecurity response ever undertaken in Western Australia. Taggart’s backyard discovery has so far led to the inspection of 2.6 million trees, the removal of 4500 large trees – mostly mature exotics – and the lopping of thousands of branches. The tree toll goes on.

An internationally renowned expert in plant disease and ecological restoration, Dixon is emotional when he describes the havoc wrought so far in Perth, a city that already suffers from having the smallest tree canopy cover of any Australian capital city. The most visible loss has been at picturesque Kings Park, the city’s globally renowned botanical park where, for years, Dixon was chief scientist.

“The large Moreton Bay and other figs from the 1880s were very much a Victorian garden design element that featured in Kings Park,” he says. “These were gigantic, century-old individuals that grew around the heritage precinct, along the Swan River banks and natural springs and the great grotto gardens that form part of Kings Park.

“And then, a few months ago, many were gone. All you see today is their stumps. It’s an extraordinary scene of devastation, and a significant loss for all cultures because they were such iconic and fantastic trees.”

Mass tree removal has been mandated by a $42 million federal government program delivered under the Emergency Plant Pest Response Deed, a national framework funded via a state cost-sharing arrangement to fund programs for pests of national significance. That directive is being executed by WA’s lead agency, the Department of Primary Industries and Regional Development, which has swung into action. Any badly affected tree is chopped down and double-chipped to ensure the beetle hasn’t a hope of survival.

State Agriculture Minister Jackie Jarvis is unrepentant about the loss. “The only known treatment around the world is to actually remove the trees,” she told reporters in January last year, as complaints about mass tree removal hit the headlines. “We are fighting a battle to try to stop the spread of this pest.”

Yet the tiny beetle has now spread to more than 30 local government areas, representing more than 80 Perth suburbs; last September, the quarantine zone was extended to the entire Perth metropolitan area. The most affected – “Zone A” – centres on Perth’s older, leafier suburbs and riverside parks, and the less-affected “Zone B” stretches north and south, and eastward to include Perth’s forested hills.

The news is hardly reassuring, although Dr Mia Carbon, deputy director-general of WA’s Primary Industries department, sought to reassure a local government conference in February that the level of positive detections amounts to 0.17 per cent of 2 million inspected trees: “It’s not rampant or in every second tree.” And while the two quarantine zones now cover the entire Perth area, she says, “the level of infestation is not uniform. In Zone B, the incidence is very low or nil.”

It was quite by chance that Taggart, a keen gardener, had spotted the borer in the wood, and sheer luck that she’d recently put a pest watch app on her phone. “It was the best thing I could have done,” she says. “The app has a geolocator, so you take a photo, put your email address in and press send. Literally 10 minutes after I sent it, I got a phone call saying, ‘We’re interested to come and look at your tree. Are you home now?’ ”

So how has the polyphagous shot-hole borer, a native beetle of South-East Asia, hitchhiked its way to Australia’s west coast? A clue is that Taggart’s house is only two kilometres from Fremantle Harbour. Authorities believe a likely scenario is that the borer entered the country in untreated wood from an Asian country. More certain is that the outbreak has raised a dilemma – is the brutal remedy as devastating as the threat itself? In the case of other invasive biohazards – think myrtle rust, varroa mite, fire ants – all attempts at destruction are applauded. But the prospect of losing thousands of trees in an attempt to eradicate a beetle has raised public alarm.

The borer chose some of Perth’s best postcodes to continue its assault, invading its leafiest, oldest and most affluent suburbs, such as Nedlands, Peppermint Grove, Cottesloe, Wembley and Kings Park. Businessman Jock Clough has a background in engineering and marine science, but not in the insect that attacked his trees in Peppermint Grove. “I had some trees removed from my house,” he says. “That didn’t cause me too much alarm, but it focused my attention.”

It was the disappearance of century-old Moreton Bay figs in Kings Park that shocked Clough. “I’m not a tree-hugger, but it doesn’t make sense. I wanted an expert to give me a view, to see if there are better ways to mitigate this problem rather than chopping trees down.”

Clough and a couple of mates decided to fund a private report by a local entomologist. They’d been told the polyphagous shot-hole borer had already been inadvertently introduced to Israel, the US and South Africa, so how had they dealt with it? The review found that the sole strategy of beetle eradication had not been adopted in other countries, often because it was discovered too late. It noted a nuanced approach to suppress and contain, but not eradicate. Only the most susceptible and infected trees were removed; lightly infested trees were treated with insecticides and branch removal, and strong quarantining practices observed.

“The report also says what I thought it might – that most affected trees don’t die,” says Clough. In an example involving nearly 2500 affected trees in California, the rate of tree mortality was 5.6 per cent. “I’ve heard the minister say that trees infected with this beetle will die within a couple of years. But the overseas evidence is that tree mortality is between five and 10 per cent. The most affected die, and the more robust don’t.”

A visiting expert seemed to confirm aspects of the review when she gave a seminar in Perth in September. University of California Assistant Professor Shannon Lynch, who identified the first borer invasions in Californian avocados, said the insect posed a complex problem. She said the combined US and South African experience shows the borer will attack more than 300 tree species, but key to its spread are 77 host species that support reproduction of both the beetle and its fungus. In the worst instance, 200,000 native willow trees growing in river valleys around San Diego were killed in a single year. But Lynch emphasised that the beetle’s impact can vary greatly, from mass tree deaths to dying branches in species that survive if limbs are removed. “There’s no one-size-fits-all approach to management of this problem.”

In California, only severely infested or hazardous trees are removed, and branch removal is far more common. “We have seen the recovery of trees when we have done this … that has been the most effective approach in reducing the beetle populations in the urban forest,” Lynch said. Lynch cited the case of Disneyland, California’s popular playground campus, where 16,000 trees and more than 600 tree species are dotted across the landscape. Intense monitoring, limited pesticide use and branch removal have seen a drastic drop in outbreaks. “They now live with the beetle,” she says.

Jock Clough suspects it’s already got away in Perth, and “we should be moving into managing this problem, not eradicating it.” He sent his review to minister Jackie Jarvis with suggested alternative strategies.

“I would have liked to have met her, but she wasn’t interested in engaging,” he says. “I fear what might happen is we’ll cut them down and then, in six or 12 months, the minister will come out and say, ‘We did our best, but it wasn’t containable.’ I would like to hear that sooner rather than later, so we can save the magnificent old trees – most of which won’t die – from being cut down [by asking the minister to take a more moderate approach, such as by removing limbs on only partially infected trees].”

Dixon concurs. “There’s no evidence from any overseas country that you can control the borer by removing every infected tree. Doing that is contrary to standard epidemiological practice. The federal money is for removing trees and mulching them – it’s eradication of trees, not the borer.”

Western Australia’s intrusive little borer is now on the radar of every urban forester. The list of potentially affected trees is enough to make them blanch – more than 100 species including box elder maple (Acer negundo), coral trees (Erythrina), poinciana (Delonix), Black locust (Robinia), fig species (Ficus), plane trees (Platanus), mulberry (Morus) and hibiscus (Hibiscus rosa-sinensis). A long list of other species are susceptible, including jacaranda and eucalypts, although the borer may not always reproduce in them.

A big fear is that the tree life of cities including Melbourne, Brisbane and Sydney could be decimated if the borer reaches them. In Melbourne, 80,000 public trees in parks and streets are maintained and monitored by the city, under the care of senior urban forester Freya Thomas. Thomas recently flew to Perth to address a local government conference about increasing the tree canopy in cities. She produced detailed maps of Melbourne’s public tree species – many of them mature exotics like elms and plane trees – showing the potential damage that a variety of insect pests could inflict on each type. “I have not put up the shot-hole borer map because it’s terrifying,” she told the crowd.

Sitting next to Thomas was CSIRO principal research scientist Bruce Webber, a Perth-based expert in invasive species and climate change. He described another worrying scenario. “We know that avocados, mangoes and pears are impacted by the borer – and these are very important industries in Australia. If we see a 20 per cent or greater decline in yield, will some of those industries survive? I’m surprised there hasn’t been more interest in this aspect. And there’s been less interest than I would expect over in the east.”

Webber says he recently contacted national grower groups, including those representing Queensland, the state that produces 43 per cent of the nation’s avocados and 46 per cent of its mangoes. Were they aware that the borer, if it escaped WA, could threaten industries that last year alone earned Queensland $589 million [avocados] and $221 million [mangoes]? In Israel, the beetle has led to the bulldozing of entire avocado orchards, but the industry has been less affected in California.

“We can’t just take knowledge of an infestation from another country and say, ‘That’s going to happen, or not happen, here,’ ” says Webber. “South Africa found the shot-hole borer too late – eradication was never an opportunity. Their conservative cost-benefit analysis found this beetle will cost South Africa $28 billion over 10 years. But overseas environmental conditions are often different, and are known to impact tree vulnerability to the shot-hole borer. We don’t have a complete cost-benefit analysis for WA, and nothing at all to guide discussion for the national impact. Without it, it’s hard to frame a discussion around whether $42 million [for the mandated mass tree removal] is money well spent. We need to frame the discussion around the potential impact on the nation if we don’t get this right.”

‘Take it seriously now. Don’t think this is a problem that will necessarily stay in WA. There is absolutely no room for complacency.’

Alison Xamon, City of Vincent mayor

Webber later meets me at Hyde Park, in the heart of Perth’s quarantine Zone A, a place where he and his family have spent a lot of time. “We’re dwarfed by these fig trees – you could hide people in the buttress roots,” he says, as we settle under the shade of one that soars 25 metres above us. Rows of mature elm and plane trees surround the lake in front.

Until a few months ago, the trees of Hyde Park seemed doomed. About 20 per cent of the 900 park trees were destined to be removed due to infestation by the shot-hole borer. In desperation, City of Vincent [a local government area of Perth] mayor, Alison Xamon, reached out to arborists and scientists, including Webber, in an attempt to change the trees’ fate. Xamon says the government’s eradication strategy has made people reluctant to report borer infestations. “I’ve heard them say, ‘If I report these little holes, they’ll come and take the tree altogether.’ Our residents have been devastated by this. We are determined to keep as much of the tree canopy as we can.” The advisers Xamon contacted said the city should conduct a valuation of Hyde Park’s trees.

“One of the best ways to help people understand the value of doing something is understanding what they have to lose,” says Webber. “An arborist was able to show that infested trees targeted for removal are worth tens of thousands of dollars. Some of the big fig trees are worth more than $500,000 each, like the one we’re sitting under. In a city, no amount of infinite resources will instantly create huge trees. And on a hot day like today, we’re sitting in shade that is generally 10 to 15 degrees cooler. It’s a huge temperature difference, so valuing trees is a no-brainer.”

Webber and his colleagues argue that while badly infested trees must still be felled, many trees could be saved by lopping off limbs. “That’s without derailing the eradication campaign, because every tree must be assessed on its merits and there will still be trees needing removal,” he says. “But the earlier we are aware of infestations, the more likely we can save it by taking off a limb.”

Xamon says local governments have been forced to sign non-disclosure agreements with the Primary Industries department in order to find out where outbreaks lie in their own communities. “It’s ridiculous. We need information about an infested area, so we can keep an eye out and inform our residents,” she says. “The department needs to partner with us, and we will work collaboratively with them.”

Another frustration aired by Xamon and others is that years after the first detection, not one dollar of the $42 million has been spent on research into controlling the borer. In November, $2 million was made available by the state for research to make the eradication program more effective. Yet no announcement has yet been made on who will receive it or when it will start.

Meanwhile, Webber and his academic colleague Theo Evans, a University of Western Australia expert in wood-eating insects and pesticides, believe the problem cannot wait. They have begun their own testing of chemicals and biocontrol agents in a UWA-funded project they hope will produce useful results soon. “Control methods have been attempted overseas and the view is that chemical pesticides don’t work well enough because the beetle is protected from insecticide contact inside the wood,” Evans says. “But in the whole scientific literature, only 13 chemicals have been tested against the borer; in Australia, we have 2000 chemicals registered as insecticides. There are a few more things we could try. If we can solve the problem here, then we’ve solved it for the country.”

Beetle detection traps now dangle from roadside stakes across Perth’s metropolitan area and further south. So far, the traps have not returned any positive detections in Western Australia’s valuable horticultural industry in the south-west. But is it just a matter of time? Will Australia go the same way as other countries – and California’s Disneyland – in moving to contain the beetle instead of eradicating it?

That option is under regular review by the national consultative committee overseeing the Emergency Plant Pest Response Deed. “Four years down the track, it’s too late,” says Kingsley Dixon. “Contain it in Perth and have strict quarantine. Do not allow any WA plant material into eastern Australia or their trees will be king-hit. We are dealing with something that is difficult to spot – you only need one little hole tucked under a leaf that no one saw, and you’ve just introduced [the borer] into a new location.”

Xamon has a warning for her counterparts in the rest of Australia. “Take it seriously now. Don’t think this is a problem that will necessarily stay in WA. There is absolutely no room for complacency.”

Back in Fremantle, Joanne Taggart is planning her new garden. A tree-replacement program has been rolled out where “you choose your preferred tree and the government will reimburse you for the cost”.

Now that there’s a chance that they may only be lopped, and not chopped down, Taggart hopes people will report those trees with the telltale tiny shot-holes on their trunks. “We all need to understand that this threat is serious.”

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