Yes, in many cases a homeowner can treat tree borer damage using insecticide injection or other targeted methods — but the results vary significantly depending on the borer species involved, how advanced the infestation is, and whether you follow safe handling procedures. For heavily infested or high-value trees, calling a qualified arborist is often the most reliable option.
Understanding Tree Borers in New Zealand
New Zealand has several borer species that attack trees and timber. The most common ones homeowners encounter include:
- Huhu beetle (Prionoplus reticularis) — NZ's largest native beetle; larvae tunnel through dead or dying native timber and trees.
- Kanuka longhorn borer (Ochrocydus huttoni) — attacks kanuka, manuka, and related species.
- Willow borer (Agrilus suvorovi) — a serious pest of willows and poplars.
- Two-toothed longhorn borer (Ambeodontus tristis) — found in a range of hardwoods.
- Common house borer (Anobium punctatum) — more often associated with structural timber than living trees.
Identifying the species correctly matters because different borers respond to different treatments, and some (like the huhu) are native and play an ecological role in breaking down dead wood — they may not need treatment at all if the tree is already dying.
Signs of Active Borer Infestation
Before reaching for an insecticide, confirm you have an active infestation rather than old damage:
- Fresh frass (sawdust-like material) around entry holes or at the base of the tree
- Sap oozing or staining near holes
- Visible exit holes (round or oval, depending on species)
- Wilting or dieback in branches above the affected area
- Audible chewing sounds in quiet conditions (for larger larvae)
Old, empty tunnels with no fresh frass usually mean the borer has already completed its life cycle and moved on.
Can You Inject Insecticide Yourself?
Yes, DIY insecticide injection is possible for accessible, smaller trees. Here's how the process generally works:
1. Choose the Right Product
Look for systemic insecticides labelled for tree borer use in NZ. Products containing imidacloprid or **** are commonly used, though always check the current label — registered uses change over time. Products are available at garden centres and hardware stores such as Bunnings or Mitre 10. Always read the label in full before purchasing.
2. Injection Method
- Drill small holes (typically 6–10 mm diameter) at a downward angle into the trunk, spaced evenly around the circumference and at a height appropriate to the infestation.
- Inject the diluted insecticide solution using a syringe applicator or purpose-made tree injection kit.
- Seal the holes with grafting wax or a suitable tree wound sealant after treatment.
- Some products use a soil drench method around the root zone instead of direct injection — follow the manufacturer's instructions precisely.
3. Timing
Treat when larvae are actively feeding, typically late spring through summer. Systemic products need time to move through the tree's vascular system, so early treatment before peak larval activity gives the best results.
Safety First
Insecticides are regulated under the HSNO Act in New Zealand. Always:
- Wear gloves, eye protection, and a dust mask when drilling and handling chemicals
- Keep children and pets away from treated areas until the product has dried or as directed on the label
- Never pour excess chemical down drains — dispose of according to label instructions
- Check WorkSafe NZ guidance on safe handling of agrichemicals if you are unsure
Limitations of DIY Treatment
| Situation | DIY Suitable? |
|---|---|
| Small to medium tree, early infestation | Often yes |
| Large or mature specimen tree | Seek professional advice |
| Tree near power lines or structures | Call an arborist |
| Infestation in main trunk/root crown | Professional recommended |
| Unknown borer species | Get an ID first |
| Protected or heritage tree | Check with your council |
Note that removing or significantly pruning trees may require a resource consent or be subject to rules in your district plan — always check with your local council before doing major work, especially on large or protected trees.
Biosecurity Considerations
If you suspect an exotic or unusual borer species not commonly seen in your area, do not disturb the tree further. Report it to the Ministry for Primary Industries (MPI) via the Biosecurity NZ pest and disease hotline (0800 80 99 66). Early reporting of new pests is critical for NZ's biosecurity.
When to Call a Professional
Contact a qualified arborist (ideally one with NZ Arboricultural Association membership or an NZQA arboriculture qualification) if:
- The tree is large, structurally compromised, or near buildings, fences, or power lines
- You cannot identify the borer species with confidence
- The infestation is severe or widespread through the canopy
- Previous DIY treatment hasn't worked
- The tree has significant sentimental, ecological, or monetary value
- You need a tree health assessment or written report for insurance or council purposes
A professional arborist has access to commercial-grade systemic treatments, specialised injection equipment, and the expertise to assess whether a tree is worth saving or needs to be safely removed. You can find qualified local arborists on tradedirectory.co.nz.
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