Borer damage in garden trees is more common in New Zealand than many homeowners realise, and catching it early can mean the difference between saving a tree and losing it entirely. The good news is that with a careful eye, you can identify the warning signs yourself — and then decide whether to act or call in a professional arborist.
What Are Borers?
Borers is a broad term used in NZ for insects — usually beetles or moths in their larval stage — that tunnel into the wood of living or recently felled trees. Common culprits in New Zealand gardens include the Huhu beetle (Prionoplus reticularis), the Kanuka longhorn beetle, and introduced species such as the Eucalyptus longhorn borer. Some species target stressed or dying trees, while others will attack healthy specimens. Identifying the type of borer can help determine the best course of action.
Key Signs of Borer Damage
Here's what to look for when inspecting your trees:
1. Exit Holes in the Bark
The most obvious sign is small, round or oval exit holes in the trunk, branches, or exposed roots. These are where adult beetles or moths have emerged after completing their larval stage. Holes can range from a few millimetres to over a centimetre in diameter depending on the species. Fresh holes often have clean, sharp edges.
2. Frass and Sawdust
Frass is the powdery or pellet-like waste that borer larvae push out of their tunnels as they feed. You may notice small piles of fine sawdust or coarse, fibrous material collecting at the base of the tree, in bark crevices, or around exit holes. Fresh frass is a strong indicator of active infestation.
3. Sap Bleeding or Resin Oozing
Some trees respond to borer attack by producing excess sap or resin around entry points. Look for sticky, wet patches or dried resin blobs on the bark — these can indicate where a larva has bored in.
4. Bark Discolouration and Cracking
As larvae tunnel beneath the bark, they disrupt the flow of water and nutrients. This can cause patches of bark to discolour, crack, or lift away from the wood. In advanced cases, you may be able to peel back loose bark and see the winding galleries (tunnels) the larvae have carved.
5. Wilting or Dying Branches (Dieback)
If borers have girdled a branch — meaning they've tunnelled all the way around it, cutting off the vascular system — you'll notice sudden wilting or dieback of leaves and smaller branches. This can look similar to drought stress or disease, so check for other borer signs before drawing conclusions.
6. Visible Larvae or Adult Insects
Occasionally you may spot the larvae themselves (creamy-white, grub-like) under loose bark, or adult beetles on or near the tree, particularly in warmer months (late spring through summer is peak activity for many NZ species).
How to Inspect Your Trees
Use this simple process to check your garden trees:
| Step | What to Do |
|---|---|
| 1 | Walk slowly around the full circumference of the trunk |
| 2 | Check from ground level up to the main branch unions |
| 3 | Look for exit holes, frass, sap, and bark changes |
| 4 | Gently probe any soft or discoloured bark with a screwdriver |
| 5 | Check major branches, especially where they join the trunk |
| 6 | Note whether damage appears fresh (active) or old (historic) |
Old exit holes with no frass and hardened edges may indicate a past infestation that has already run its course. Fresh frass and clean holes suggest active borers.
Trees Most at Risk in NZ Gardens
While borers can attack many species, some are particularly vulnerable:
- Pines and other conifers — susceptible to several bark beetle species
- Eucalyptus — targeted by the eucalyptus longhorn borer
- Native trees (pōhutukawa, kāhikatea, rimu) — Huhu beetles favour native timbers, especially stressed or dying trees
- Fruit trees (apple, plum, cherry) — can be attacked by clearwing moths and other borers
- Recently transplanted or drought-stressed trees — stress makes trees far more attractive to borers
What to Do If You Find Borer Damage
If you suspect active borer damage:
- Don't panic — a single infested branch doesn't necessarily mean the whole tree is lost.
- Remove and dispose of heavily infested branches by bagging and binning (not composting) to reduce spread.
- Improve tree health through appropriate watering and mulching — healthy trees are more resistant.
- Avoid wounding trees unnecessarily, as fresh wounds can attract borers.
- For significant infestations or large/valuable trees, consult a qualified arborist who can assess the extent of damage and recommend treatment or removal.
When to Call a Professional
You should contact a qualified arborist if:
- The infestation appears widespread across the trunk or multiple major branches
- The tree is large and poses a safety risk if it were to fail
- You're unsure whether the tree can be saved
- The tree is a protected species or near a boundary, structure, or power lines
- You need to consider tree removal — felling large trees safely is specialist work
A certified arborist can accurately identify the borer species, assess structural integrity, and advise on treatment options. You can find qualified local arborists on tradedirectory.co.nz. For guidance on tree work near power lines or in public spaces, always check with your local council, as some work may require consent or notification.
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