Why is something attacking my tree?
Pests and diseases tend to follow stress. Gardening Australia says borers go for trees struggling with root competition, a lack of water or nutrients, or poor pruning, and that keeping a tree healthy is the first defence.
Bad cuts open the door. Whitehorse City Council says a lopping cut (one that leaves a stub) makes it very hard for a tree to seal the wound, which can leave it exposed to fungal infection. Our lopping vs pruning guide explains the difference.
Growing conditions play a part too. The City of Sydney notes that eucalypts growing fast in rich conditions with no fire can become structurally weak, with foliage and bark more open to insect attack.
What you might see, and what it could be
| What you might see | What's going on | |
|---|---|---|
| Borers | Tiny round or slightly oblong holes in the trunk or branches, with fine sawdust-like frass | Larvae of beetles or clearwing moths feeding inside living wood, weakening it over time |
| Termites | Often nothing on the outside, because they live in dead stumps, the root crown or the bowl of the tree | Some species nest in living eucalypts and damp wood, and some travel a long way underground |
| Phytophthora dieback | Plants declining and dying, sometimes several in one area | A soil-borne disease that infects and kills the roots |
| Myrtle rust | Badly infected new growth that may be stunted | A fungal disease of the myrtle family: eucalypts, paperbarks, lilly pillies and bottlebrush |
| Lilly pilly psyllid | Pimples that disfigure the leaves of some lilly pillies | A small insect, Trioza eugeniae, the most serious lilly pilly pest |
| Fungi on wood | Mushrooms or brackets growing from a stump, the trunk or the base | Wood being broken down: normal on a stump, worth a closer look on a living tree |
Borers
Borer damage builds slowly. The larvae feed on living wood until a branch can break or the tree is ringbarked (damaged all the way around the trunk, so it can't move water and food).
Because borers pick stressed trees, the fix usually starts with the tree rather than the insect: water through dry spells, mulch, and keep the root zone from being dug up or driven over. If holes keep appearing in a big limb over the house, the real question is how much sound wood is left, and that's an arborist's call.
Termites in trees
Termites in a tree are common and aren't always the end of it. A South Australian Museum zoologist told the ABC that Coptotermes acinaciformis usually turns up in dead stumps, root crowns or the bowls of trees, often eucalypts and oaks. ACT City Services says another species, Porotermes adamsoni, nests in living eucalypts and damp wood.
The house is the bigger worry. That same zoologist said Coptotermes acinaciformis can travel up to 50 metres underground looking for new food. A termite nest in a tree or stump near the house is worth a timber pest inspection of the house, as well as an arborist's look at the tree.
Phytophthora dieback
Phytophthora is a disease that lives in soil and infects and kills roots. If the roots are badly infected, or it reaches the base of the trunk, the whole plant dies.
In Western Australia's south-west, the state's biodiversity department says over 40 per cent of native plant species are considered susceptible, including banksias, eucalypts and grass-trees, along with many garden plants.
It moves in soil, infected roots and run-off, and on animals' feet and fur, but people moving infested soil have spread it further and faster than anything else. If you suspect it, clean soil off boots, tools and wheelbarrows before moving around the garden, and be wary of soil from places you don't know.
Rust and leaf pests
Myrtle rust is a fungal disease first found in Australia on the NSW Central Coast in April 2010. The City of Sydney lists eucalypts, paperbarks, lilly pillies and bottlebrush among the plants it can affect.
On lilly pillies, the psyllid leaves pimply, disfigured leaves on susceptible species. The Australian Native Plants Society says Syzygium smithii is rarely affected, which is worth knowing if you're choosing a hedge. Our lilly pilly guide has more.
Is a weakened tree safe to stand under?
Not always. Borers and rot can weaken a limb long before it looks bad, and decay can hide inside: City of Sydney arborists found rot at the base of a Moreton Bay fig only with a drill test, spreading into its supporting roots, and the tree had to come out. Keep people and cars out from under any limb with holes, cracks or dead wood, and don't climb the tree or use a ladder to get a closer look.
What to do if you spot something
Photograph it
Close-ups of the holes, frass, fungus or leaves, plus the whole tree, with the date.
Note what's changed
When you first saw it, which parts are affected, and whether nearby plants have it too.
Take the stress off
Water deeply in dry spells, mulch over the roots but not against the trunk, and keep digging and parking off the root zone.
Don't make it worse
Skip the lopping and the wound paint. The pruning standard, AS 4373-2007, lists both lopping and wound painting as unacceptable practices.
Get an assessment
An arborist can tell you what it is, how much sound wood is left and whether treatment, pruning or removal makes sense. Our tree health page explains what that involves.
Check the rules before anything comes down
A sick tree can still be a protected one. Ask the council before removal or heavy pruning, and see our tree permit pages.
What you can do, and what's arborist work
You can
- Water and mulch a stressed tree
- Prune small dead twigs from the ground with clean secateurs or loppers
- Clean soil off boots and tools where dieback may be about
- Photograph the tree and watch it through a season
- Book a timber pest inspection of the house if termites turn up nearby
Leave to an arborist
- Working out what's really causing the decline
- Testing for hidden decay in the trunk or roots
- Taking weakened or dead limbs out at height
- Removal, if the tree can't be saved
- A report for the council if approval is needed
Tree pests and diseases: what to do next
With the tree's stress, not the bug. If you've found borer holes or termites in a big tree near the house, we'd get an arborist's assessment first, because how much sound wood is left is hard to judge from the ground. Where termites are involved, we'd book a timber pest inspection of the house in the same week.
Want an arborist to look at it?
Tell us the job and your postcode, and a local arborist calls you to talk it through.