The polyphagous shot-hole borer kills trees. Why is it so lethal, how is it treated, and can it spread across Australia?

The Western Australian government yesterday announced that it would stop trying to eradicate an invasive pest that's killing trees in and around Perth and start managing it instead.
The pest, the polyphagous shot-hole borer (Euwallacea fornicatus), is a tiny beetle native to South-East Asia that has been silently spreading in the Perth metropolitan area for at least four years.
The only approved treatment for the sesame-seed-sized pest is to chop down plants it has infested and chip them into tiny pieces.
It's something that local councils have been forced to do to hundreds of trees, including dozens of huge, old Moreton Bay figs, to try to stop the beetle's spread.
Last year, a $41 million plan was approved to wipe the pest from Australia's shores, but the WA state government this week said eradication was now no longer feasible.
So why is this particular pest so problematic, and if it can't be eradicated, are there other treatments on the horizon?
The beetle drills tunnels into tree trunks and branches, leaving distinctive "shot holes" in the bark.
But despite being adept at chewing through wood, the polyphagous shot-hole borer doesn't eat it. Wood is mostly made of cellulose which, while plentiful, is incredibly hard to digest.
"No animal has evolved the capacity to do this on its own — they always do it with a microbial friend," Theo Evans, an entomologist at the University of Western Australia, said.
"With termites, for example, they have a range of bacteria and protozoa. In the case of these wood-boring beetles, they use a fungus," Dr Evans said.
The fungus eats the wood, and the beetle eats the fungus.
And it's the fungus — not the beetle, nor its tunnels — that ends up killing the tree.
Once inside a branch or trunk, the growing fungus needs water, so it sends out filaments to tap into the tree's circulatory system.
These fungal filaments can block those vessels, and essentially starve the tree of water and nutrients.
The pest was first detected in WA in East Fremantle in August 2021 when a resident noticed two box elder maples in her garden looked unwell, and had shot holes in their bark.
It was soon confirmed that the infestation was the polyphagous shot-hole borer, a pest that had been wreaking havoc in places like California, Israel and South Africa for more than a decade.
But while this was the first confirmed report of the beetle in the state (and Australia more broadly), it would not have been the site of the first infestation, Dr Evans said.
"Those trees were dying, so the beetles must have been in those trees for a minimum of two years, and possibly three or four because it takes that long … for the fungus to spread through the tree and clog its vascular system."
It's also unclear how the beetle reached WA in the first place.
What seems likeliest is the pest hitched a ride on wood used as packaging or filler around large, heavy items such as farm machinery, Dr Evans said.
This wood filler, called dunnage, is supposed to be treated to kill any pests inside, but sometimes that doesn't happen.
The beetle can survive in cut wood for up to seven months.
Being a beetle, the polyphagous shot-hole borer has wings, but "it's a terrible flyer", Dr Evans said, capable of only flying around 30 metres at a time.
"When you're only 1.5 millimetres long and you spend most of your life living inside a tree, you're not going to be an acrobat."
Nor does it get transported on the wind. The minute a breeze picks up, the beetle retreats into a tunnel until it dies down again.
It's thought the pest spread across the Perth metropolitan area in plant prunings. For instance, affected branches lopped off by arborists to protect power lines could've been unwittingly transported kilometres away.
Chipping infested wood to under 2.5 centimetres results in a shot-hole borer death rate of more than 99 per cent.
But "if it's a mature tree and it's got literally tens of thousands of beetles in it, that's still hundreds of beetles that survive", Dr Evans said.
The shot-hole borer attacks WA native forest trees, including the marri (Corymbia calophylla) and karri (Eucalyptus diversicolor).
"These are important trees, not just for the WA bush, but there are close relatives throughout the bush across Australia," Dr Evans said.
"Now we don't know how badly affected the trees are going to become, because normally when the beetles are found infesting the trees, they get cut down and chipped.
"So we don't see if the tree can survive a long time with the beetle … but it could end up being quite bad."
Nor do we know exactly how the shot-hole borer might affect orchard trees in Australia. There is some information from fruit trees grown in Perth backyards, but nothing from commercial operations outside of the city.
One lesson learnt from growers overseas is in avocados. The fruit seems to be highly susceptible to the polyphagous shot-hole borer, but the beetle tends to attack branches rather than the trunk.
"With some careful pruning, which obviously costs more money and it does lower productivity of the tree a little, people in Israel and parts of the US have found that it's not a particularly bad problem," Dr Evans said.
That said, when avocado trees were introduced into Australia, we managed to avoid bringing any pest species with them. So if the polyphagous shot-hole borer gets into local avocado orchards, "it will actually be quite a big problem, because growers don't have to think about those sorts of insect pests [at the moment]", he added.
And just because the pest has behaved a certain way abroad does not mean it will act the same way here.
The beetle has attacked Moreton Bay fig trees in Perth, but has had less of an impact on figs overseas.
"So there's obviously local variation, probably a combination of the climate, the soil type, and water availability that changes the susceptibility of the plants," Dr Evans said.
It's hard to say, Dr Evans said.
Unsurprisingly, the beetle seems to thrive in climates similar to its native home.
So while it might not do too well in relatively chilly Tasmania, it may well get a foothold along the east coast, especially from Brisbane up.
Annual long, hot summers and wet winters may make plants more susceptible to the pest too.
During punishingly dry summers, water-stressed plants may simply have very little capacity to fend off the beetle and fungus combination, and simply succumb to infestation.
Trials mostly in the US have tested a handful of insecticides against the shot-hole borer with very little success — mostly because they were sprayed chemicals that might've landed on the bark of the tree, but couldn't get to the beetle inside its tunnel.
So researchers, including Dr Evans, are trialling a combination of insecticide (to kill the beetle) and fungicide (to kill the fungus), which can be administered inside the tree.
While he can't comment on his findings yet, "the results are very promising", he said.
"We'll never get rid of [the need to] chop and chip. I think there are going to be some trees that are just too far gone and chop and chip is the only option.
"But for trees that are early in the infestation, I think some of these methods are going to work and they're going to save the tree."
For more on the polyphagous shot-hole borer, check out the full episode of Lab Notes.

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