19-06-2025
Lab Notes: The tiny beetle ravaging Perth's trees
Belinda Smith: Dramatic scenes have been unfolding in Perth's majestic parks.
News Grab: Trees in Perth's iconic Kings Park ground into dust. A bare scar on the hill where Moreton Bay fig trees once stood.
Belinda Smith: And just last month, a couple of kilometres away at Hyde Park, dozens of trees were chopped and chipped.
News Grab: Seeing the trees on the islands go, it's so, so sad.
Belinda Smith: And it's all because of a beetle about the size of a sesame seed called the Polyphagus shot-hole borer. So how has this tiny pest caused such massive problems? Hi, I'm Belinda Smith and you're listening to Lab Notes, the show that dissects the science behind new discoveries and current events. To explain how the beetle spreads and what can be done about it is Theodore Evans, an entomologist at the University of Western Australia.
Now, more than 50 huge old Moreton Bay fig trees in Kings Park have been chopped down. What impact has this had on the area?
Theodore Evans: It looks like a war zone, as people have used those words. And they're horrified. And people would go for walks through that part of Kings Park and now they aren't because there's no shade on a 40 degree day. It's deeply unpleasant.
Belinda Smith: Yeah, I imagine, you know, in a hot and dry city like Perth, losing all that tree canopy must be pretty extreme.
Theodore Evans: Perth already has the lowest tree canopy of any capital city in Australia. So it's a not good situation that is going to get even worse.
Belinda Smith: The reason Perth is chopping and chipping hundreds of trees is because of a wood-boring beetle. And their favourite food, you guessed it, is...
Theodore Evans: Wood. And wood is made up of cellulose, lignin and hemicellulose. And cellulose in particular is the most abundant biological molecule on the planet. And it's one of the hardest things to digest. So it's a huge resource if you can crack the secret to digest it.
Belinda Smith: And the polyphagous shot hole borer has figured it out.
Theodore Evans: So polyphagous comes from the Greek, which means many eating, because they eat a very wide variety of plants. And
Belinda Smith: this varied vego menu is pretty rare for insects.
Theodore Evans: Think about a caterpillar, for example. They only eat one species of plant or a very small number of close relatives. And we call those monophagous, meaning one eating. It's much harder to be polyphagous because you have to be able to digest plant matter from all these different plant families that aren't related. And they probably have a whole range of defensive chemicals. And you have to be able to overcome that much wider range of defensive chemicals.
Belinda Smith: Why did this polyphagousness evolve in this particular beetle species?
Theodore Evans: This beast evolved in South and Southeast Asia, along with a whole bunch of its relatives. And that's probably got something to do with its polyphagous nature, because those diverse rainforests, they have huge numbers of diverse plant species. It's kind of hard to specialise on one species when you've got such diversity around you.
Belinda Smith: What do the beetles look like?
Theodore Evans: Their head is kind of pushed underneath their thorax. If you remember, insects have three body parts, head, thorax and abdomen. And so they kind of look from above, they almost look like they have no head. And the whole body is almost like a short cylinder. And they sort of chew and they chew and they turn as they chew. Like a drill, almost. Exactly, just like a drill. Not surprising, given what they're living in.
Belinda Smith: And these holes are usually the first sign of the pest's presence. But it's not the holes or even the beetles themselves that kill the tree. So
Theodore Evans: part of their trick of being polyphagous is to have a symbiotic friend. In this case, it's a fungus. As I said, it's very hard to digest cellulose and those other compounds in wood. And no animal has evolved the capacity to do this on its own. They always do it with a microbial friend. So with termites, for example, they have a range of bacteria and protozoa. With the case of these wood-boring beetles, they use a fungus. They're normally not an organism that can invade the tree on their own. They usually piggyback on somebody else.
Belinda Smith: So when a polyphagous shot hole borer drills into a tree, it's the piggybacking fungus that digests the wood.
Theodore Evans: And then these beetles just eat the fungus.
Belinda Smith: But the real problems arise when the fungus gets thirsty and starts growing into the water vessels inside the tree.
Theodore Evans: And eventually they clog up those vessels and they basically starve the tree of water and nutrients. And that's how the tree ends up dying.
Belinda Smith: Right. Okay. So it's actually the fungus that causes that clogged circulation, which then spills the end of the tree. Eventually. That's correct. Right. Okay. So when was the shot hole borer first discovered in WA? It
Theodore Evans: was first found by the West Australian State Government Department of Primary Industries on the 6th of August in 2021. And it was found in two box elder maple trees in East Fremantle. These were trees planted by the owner of the property and they were her pride and joy. She's a very keen gardener and these trees were looking very unhealthy. The leaves were dying, turning yellow, falling off. And she looked carefully at the bark and realised that there were these shot holes. And within a short time, the experts had identified it as polyphagus shot hole borer, Eulacia fornicatus. And that has been spreading in different parts of the world over the last roughly 20 years. And so it's well known as an invasive species. Yeah,
Belinda Smith: it's been found in California, Israel and South Africa before. And while the East Fremantle infestation was the first confirmed report of polyphagus shot hole borer in Western Australia and Australia more broadly.
Theodore Evans: This can't be the original site of the very first infestation for a couple of reasons. But one is those trees were dying. So the beetle must have been in those trees for a minimum of two years and possibly three or four because it takes that long for the beetle to breed up, for the fungus to spread through the tree and clog its vascular system.
Belinda Smith: How did they get to Western Australia?
Theodore Evans: So what we think has happened is the beetle came in on green wood, so wood that hasn't been properly dried, used as dunnage. And dunnage is essentially the bracing, the filler for large heavy items. And so it's likely that something got brought into Perth. It had green wood dunnage to hold it in place and it hadn't been heat treated to dry it out and kill any organisms, which they're supposed to do. But obviously that takes time and money to do and it's often skipped.
Belinda Smith: How well has the shot hole borer been contained since its discovery a few years ago?
Theodore Evans: I would say the average person in Perth who follows the shot hole borer news would say not very well because every six months or so we hear a new report of an infestation that was outside the previously defined quarantine zone. The
Belinda Smith: shot hole borer may be awesome at drilling into wood,
Theodore Evans: but... This beetle is a terrible flyer, like all very small beetles. Oh really? Terrible flyer. When you're only one and a half millimetres long and you spend most of your life living inside a tree... And your
Belinda Smith: head's tucked away underneath your thorax.
Theodore Evans: Exactly. You know, you're not going to be an acrobat. And so in field experiments done in the United States, the distances that they flew were around 30 to 35 metres and they all stopped flying at high wind speeds.
Belinda Smith: So if they're barely flying and they're not being blown around, how do they spread between suburbs?
Theodore Evans: That's all humans.
Belinda Smith: Yeah, we are shuttling the shot hole borer around, particularly when trees get pruned. The
Theodore Evans: arborist takes those cut branches away and they might transport them dozens of kilometres. This beetle can survive in these cut wood for up to seven months.
Belinda Smith: So shot hole borer love wood, but there are particular trees that they really enjoy.
Theodore Evans: So number one on the list is Acer negundo, the box elder maple. Number two on the list are Erythrina x sykesii, which are coral trees. And Erythrinas are found in Africa through to India. So these trees get attacked very heavily and so they succumb rapidly.
Belinda Smith: Others are robinia, hibiscus, plane trees and figs, like the giant morton bay figs recently chopped down in Perth. And people are worried about trees outside of the metro area too. We
Theodore Evans: know that the borer does attack WA native forest trees, including a range of paperbark Melaleuca, including Corymbia eucalyptus species such as the Marri. And they also attack Callitris. So these are important trees in not just WA's bush, but there are close relatives throughout the bush across Australia.
Belinda Smith: Agriculture could be affected too, because in other parts of the world, avocado trees, for instance, are highly susceptible to the pest.
Theodore Evans: They also affect pears and apples. They attack and can kill macadamias and mangoes and mulberries.
Belinda Smith: How concerning is the threats that the borer poses beyond WA's borders?
Theodore Evans: That's a very hard question to answer. There's been some modelling to look at where it might thrive in Australia and essentially wherever there are trees, it will do well. Not so well in Tasmania, because it's a bit cold, but it should absolutely thrive on the east coast and particularly sort of north of Sydney from Brisbane up into the tropics, because that most closely matches the temperatures of its native range. How badly the plants are going to suffer is a harder question to answer.
Belinda Smith: Right now, the only approved way to stop the beetle spread is cutting down trees and putting them through a wood chipper. Some insecticide sprays have been tested on the borer, but they didn't work.
Theodore Evans: And the reason they didn't work is because if you're spraying an insecticide through the air and it lands on the bark of the tree, it doesn't get to the beetle. The beetle is inside the tunnels and nobody has yet tried to test some of the chemicals that can get into the wood of the tree, or they haven't done very much of it. And so there is hope in trying to look at these other approaches.
Belinda Smith: And Theodore is among the researchers who have been testing these other approaches. My
Theodore Evans: very first tree that I started experimenting on with my most favoured chemical, because this particular chemical is both fungicidal and insecticidal, so it kills the borer and the fusarium fungus. And we have managed to protect one tree, which is a stone throw from Hyde Park. So it's constantly being attacked by Polyphagous Shot Hole Borer. And we've managed to protect that tree now for a year.
Belinda Smith: Oh, congratulations. Thank you. I feel like that is well, that is worthy of a celebration.
Theodore Evans: We'll never get rid of 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 have only, that are early in the infestation, I think some of these methods are going to work and they're going to save the tree.
Belinda Smith: That was Theodore Evans, an entomologist at the University of Western Australia. Thanks for listening to Lab Notes on ABC Radio National, where every week we dissect the science behind new discoveries and current events. I'm Belinda Smith. This episode was produced on the lands of the Wurundjeri and Menang Noongar people. Fiona Pepper's the producer and it was mixed by Riley Mellis. We'll catch you next week.
