Shot hole borers

AKA the reason I no longer sleep at night

Mar 21, 2024

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If you’ve been reading my newsletters for a while you’ll know that nine times out of ten I dispute the use of the word ‘pest’. After all, a lot of the so-called pests that gardeners wage war on actually play important roles in our overall garden ecosystem, and killing them off ultimately weakens our gardens in the long run.

Not this time.

This time I want to tell you about shot hole borers, and if you are a gardener in WA this is information you truly must read.

You should read it and then you should send it to anyone you know who has a tree in their backyard, or knows someone with a tree in their backyard. So, basically everyone in WA.

Before I tell you about them, I want to give you a very abridged version of what is going on, because a lot of people I speak to either don’t know about the borers at all or don’t realise just how catastrophic their arrival is for our trees.

So here is the story in four dot points:

A beetle has arrived from southeast Asia. It’s been spreading in WA for a few years and we have now totally lost control of it.
It bores small holes into the vast majority of trees we grow (yes, natives too).
Most trees it invades will eventually die (many 100+ year old trees are currently being removed from public parks - Hyde Park, Lake Claremont and many other gorgeous parks around Perth are, frankly, fucked. The Perth Zoo might, in time, lose the majority of its trees).
There is currently no treatment.

I hope that’s got your attention. Now, here’s the info I managed to gather on the borer - how to check your trees for it and what to do if you find it.

Please, please tell everyone you can about it. Our oldest, most beautiful trees, and all the animals who rely on them for food and shelter, are depending on you.

Polyphagous Shot Hole Borers

Heretofore I shall be referring to this bastard of a beetle interchangeably as Shot-hole borers (their actual name) and Shit-hole borers (because they have absolutely ruined my day/week/month/year/life).

What are shit-hole borers? Their full name is Polyphagous Shot-hole Borer (Euwallacea fornicatus) and they are a beetle that hails from Southeast Asia. They’re tiny and black and if you’ve never seen one before I’m not surprised - I hadn’t seen last autumn when I went out poking around our garden.

If you have shot-hole borers in your garden you are far more likely to see the damage they cause before you see the beetle itself.

These beetles reproduce in the bodies of trees and woody shrubs. They burrow into the tree and farm a kind of fungus inside the depths of their burrows. The fungus is called Fusarium fungus and in certain susceptible trees (which at this stage is well over half the trees in our garden), Fusarium fungus will start necrotising the tree from the inside out, causing dieback and, eventually, tree death.

So yeah, basically the worst thing ever.

I first noticed the shot-hole borers in our native mulberry, a tree that I’ve loved growing for the birds in our garden (they like to eat its little white berries) and for the shady canopy it provided. Then I spied them in the base of our elderflower, another tree I bloody loved and was very sad to see go. If you want to look for shot-hole borers in your garden (and you really, really should as keeping on top of them is, at this stage, then only real course of action available to us) there are a few things to look out for.

I found one!! Moments later, I crushed it.

How to look for shot hole borers

Shot hole borers are very small (~2mm) black beetles, who spend most of their time burrowed deep into the trunks of your beloved trees, so it’s pretty unlikely you’ll discover them in your garden if you head out looking for the beetles themselves. What you are more likely to see is the evidence of the beetle - the trail of destruction it leaves in its wake.

Shot hole borers leave tiny holes in the trunks of trees, about the size of the tip of a ballpoint pen. So first, look closely on your trees’ trunks and branches, to see if there are any tiny holes.

You might also see ‘frass’ coming out around the holes. Frass looks like wood shavings that protrude from each tiny hole like a little noodle (it might also be visible at the base of the tree, like small piles of sawdust). Depending on the tree, you might also notice ‘sugar volcanoes’ - a white crystalline substance that forms at the entry holes made by the beetles - or there might be discolouration, staining, sap or resin around the beetles’ entry holes. If your tree is already sick from the fungus, you could also see dieback - where the tree begins to lose its leaves and die (usually from the topmost branches down).

It is a good idea to check your trees now, in autumn, as the cooler weather is the optimal environment for the fungus and borers to proliferate. Both times the borers have been discovered in my garden were in autumn, so keep your eyes peeled.

What to do if you find them

At this stage there are no real treatments for shot-hole borers. Because they burrow deep into the tree itself, you can’t use a spray or any kind of insecticide on the outside of the tree. In fact, doing so will likely kill other beneficial bugs in your garden, and not the beetles.

I’m always a fan of biological control measures (like beneficial insects) but unfortunately as this is a recently introduced species, to my knowledge there are no beneficial insects in Perth that will readily predate it.

So what the hell can you do?! If you’ve found borers in your garden, you have my deepest sympathies. I know I go on about befriending ‘pests’ a lot, but in this instance there is a big difference. When a new species is introduced, we can’t simply step back, avoid interventions and wait for an equilibrium to establish in our gardens because there are no natural predators to move in and help. There is a parasitic wasp from Taiwan that is known to predate borers, but we don’t have that wasp here, so we’re up shot-hole creek without a paddle.

The other reason I take a different view of these beetles compared to other ‘pests’ in my garden is that these beetles kill such a big list of trees. They don’t just affect one or two varieties of shrub. They kill trees that may be many decades old. Trees that give us an urban canopy and provide homes for a wide range of birds and other animals. In this instance, if there was a spray that killed the beetles, I would use it (until another biological control method was found).

Sadly, there are no such sprays, but there are a few things you can do.

Here’s what you CAN do.

Keep your eyes peeled for signs of the borers in your garden.

You can find out which common trees in WA are most susceptible to the borers here or you can peruse the depressingly long and more extensive list of susceptible trees globally here.

You’ll notice on this list that the trees are grouped into two categories; reproductive and non-reproductive.

Reproductive trees are ones in which the beetle (and the fungus it farms) will easily reproduce. These are the trees that are most likely to die. Some common trees that are most badly affected include:

Maple (Acer)
Black Locust (Robinia)
Coral tree (Erythrina)
Plane tree (Platanus)
Fig (Ficus)
Poinciana (Delonix)
Mulberry (Morus)
Willow (Salix)

Non-reproductive trees can still harbour the beetle but the beetles are less likely to set up shop in these trees, and fungus is less likely to kill the tree. If you are looking for new trees to plant in your garden, go for the non-reproductive trees. I will be sharing a list of non-reproductive trees in next week’s newsletter to help get you inspired to fill your garden with trees that will be tougher and harder for the beetles to kill.

When looking for the borer, start with trees that have trunks greater than 2cm in diameter (so pretty much all trees that aren’t saplings) - the borers don’t tend to invade young, thin branches.

Keep your trees healthy.

Healthy, non-reproductive trees are much less likely to be targeted by the beetle. Healthy reproductive trees are still susceptible (WHICH BLOODY WELL SUCKS), but keeping them really healthy still makes them less likely to be attacked.

Make sure your trees are well watered in summer and keep an extra close eye on any trees in your garden that appear to be stressed. Avoid doing things that unnecessarily stress out your trees (like super heavy pruning - the wounds you make when pruning can become an easy point of eentry for the borer)

Report borers

Report borers at the very first sign of them in your garden. Getting in early before they have set up shop will help prevent them spreading further into your garden or into your neighbours’ gardens. You can contact the DPIRD Pest and Disease Information Service on 9368 3080 and arrange for them to come to your house to inspect your trees.

If you are worried that reporting your tree might mean you have to have the tree removed, remember that the borers will eventually kill it anyway. Without a treatment the best thing you can do for the rest of the trees in your garden (and in Perth generally) is to have DPIRD remove the tree, chop it up and treat the wood chips to kill the little shit holes.

Avoid using wood chip mulch

Speaking of wood chips, it’s a good idea to avoid using wood chip mulch in your garden, as it can help the borers spread. Go for compost or grass clippings as mulch instead.

Avoid moving wood around WA unnecessarily.

We currently have a quarantine zone around Perth city, which means that SO FAR the borers don’t seem to have gone too far up north or down south. It is vital that we protect the beautiful trees outside our city by limiting the borers’ spread. This is fortunately relatively easy to do as the borers don’t have more than ~400m range. They can’t spread by themselves. The will spread if we go about moving wood that they are living in. So burn firewood where you buy it and don’t move any wood out of your quarantine area.

Remove affected trees and don’t burn them!

Burning affected wood can actually HELP the beetle to spread. If you have affected trees it’s best to remove the entire tree including the upper roots. DPIRD will organise the removal of affected trees, so the best thing you can do is bring them into your garden ASAP to get the ball rolling.

Finding a silver lining

Well. Are you feeling totally bummed out? Me too. But I’ve decided I need to find a less depressed outlook because, as in all aspects of life, sometimes we encounter things we just can’t change or control, and this is one of those times.

There will, for better or worse, always be new species arriving on our shores. It’s the nature of globalisation - spores will be tracked in on dirty shoes, bugs will arrive hidden in coat pockets, seed packets and wooden trinkets. Some of these will have no impact; others will wreak havoc for a while. Nothing, ultimately, is ever static or certain. And so I guess what I’m saying is that now, more than ever, I have to practise what I preach.

And what I preach is this:

The whole point of gardening is in the act of gardening. Yes, it hurts when we lose plants - some plants much, much more than others. But with every loss, there will be new opportunities that rise up out of the ashes. These will offer us new directions, new ideas, new avenues for hope and excitement. As long as we keep on gardening.

Finding out we have borers in one of our big old trees has been pretty sad. It will fundamentally change our garden when the tree is removed. Suddenly, our neighbours’ big, two storey wall will tower down on us. Suddenly there will be sun on garden beds I had planted for shade. Everything growing in them will need to be shifted.

But this also means I’ll have garden beds to reimagine. I’ll have a whole side of the yard that I can plant with new, tougher trees (more on that in next week’s newsletter). More sun means more opportunities to grow flowers (which I was starting to have to avoid as the trees grew and the garden got shadier and shadier).

In short, this is nothing more or less than a reminder that, in every garden, nothing stays the same for long. And nothing will ever be perfect forever. With an increasingly hot, dry climate, with new bugs knocking on our doorstep and summer heatwaves that blast us for two months of the year, we need to be as tough and resilient as the plants we grow. Most of all, we need to continue growing gardens in the face of adversity. Because we need to keep our planet green and full of life. Because if we don’t, who will? And because gardening is, in the end, always an act of hope.

That said, I’m off to go shopping for trees.

P.S. Speaking of trees, stay tuned for a list of trees that are less susceptible to the shit hole borers - I’ll get working on it ASAP and send it through to subscribers next week!

Until then, stay vigilant amigos, and happy gardening,

Xx Casey

Please share this newsletter!

I always love it when readers share my newsletters, but this time it’s about more than fluffing my own overinflated ego. It’s s, so important that as many Perth gardeners become aware of the signs and symptoms of the shot hole borers as possible, so more of us can act to protect our trees. Please, if you know anyone else who might benefit from this newsletter, pass it along to them. Thanks!

lofi life

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