Scale insects are a common pest on NZ garden trees, and the good news is that most light to moderate infestations can be managed by a diligent homeowner. The key is identifying the type of scale, acting at the right time of year, and choosing the most effective treatment for your situation.
What Are Scale Insects?
Scale insects are small, sap-sucking pests that attach themselves to the bark, stems, and leaves of trees. They get their name from the waxy or shell-like covering that protects their bodies. In New Zealand gardens you're most likely to encounter:
- Soft scales (e.g. black scale, brown soft scale) – produce sticky honeydew that encourages sooty mould
- Armoured scales (e.g. greedy scale, oleander scale) – protected by a hard, separate shell and do not produce honeydew
- Cottony cushion scale – identifiable by a white, fluffy egg sac
Heavy infestations cause yellowing leaves, dieback, and a general decline in tree vigour. The sticky honeydew produced by soft scales also attracts ants and promotes black sooty mould on foliage.
How to Identify a Scale Infestation
Before treating, confirm you're dealing with scale:
- Look for small, flat or domed bumps on stems and branches that don't move when disturbed
- Check for sticky residue or black sooty mould on leaves beneath affected branches
- Notice unusual ant activity climbing the trunk (ants farm scale for honeydew)
- Scrape a bump gently with your fingernail — if it's an insect, you'll see a soft body underneath
When Is the Best Time to Treat?
Timing matters. The most vulnerable stage is the crawler stage, when juvenile scale insects have just hatched and are moving across the tree before settling and forming their protective covering. In NZ, this typically occurs in late spring to early summer (October–December), though it varies by species and region. Treating during this window gives the best results.
For dormant-season treatments (such as winter oils), late autumn to early winter — before new growth flushes — is ideal.
DIY Treatment Options
1. Horticultural Oil (Spraying Oil)
Horticultural or spraying oil is one of the most effective and widely available treatments. It works by smothering scale insects and their eggs. Look for products labelled as "white oil" or "spraying oil" at Bunnings or Mitre 10. You can also make a basic white oil at home (mix dishwashing liquid and cooking oil with water), though commercial formulations are more reliable.
- Apply as a thorough spray, covering all stems, branches, and the undersides of leaves
- Do not spray in hot weather (above 30°C) or in direct sun, as this can cause leaf burn
- Repeat applications may be needed every 2–3 weeks during the crawler season
2. Insecticidal Soap
Insecticidal soap sprays are effective against crawlers and soft-bodied scales. They break down quickly and have low impact on beneficial insects when used carefully. Apply directly to affected areas and repeat as needed.
3. Systemic Insecticides
For severe infestations, a systemic insecticide containing imidacloprid or similar active ingredients can be applied as a soil drench or trunk injection, allowing the tree to absorb the chemical and poison feeding insects. Always read the label carefully, follow all safety directions, and check that the product is registered for use on your tree species in NZ. Be cautious using systemic insecticides on flowering trees, as they can affect bees and other pollinators.
4. Physical Removal
For light infestations on small trees or accessible branches, you can physically remove scale with a soft brush, old toothbrush, or a cloth dampened with methylated spirits. This is labour-intensive but chemical-free.
5. Encourage Natural Predators
Several beneficial insects — including ladybirds, parasitic wasps, and lacewings — naturally prey on scale. Reducing broad-spectrum pesticide use and planting a diverse garden helps support these natural controls.
Safety Tips
- Always wear gloves, eye protection, and appropriate clothing when handling pesticides
- Keep children and pets away from treated areas until the spray has dried
- Store chemicals in their original containers, out of reach of children
- Dispose of unused chemicals responsibly — check with your local council for hazardous waste drop-off options
- Refer to WorkSafe NZ guidelines for safe handling of garden chemicals
Preventing Scale Insects Coming Back
- Control ants — ants protect scale from predators; use ant barriers or sticky bands around trunks
- Prune affected branches — remove heavily infested wood and dispose of it (don't compost)
- Avoid over-fertilising with nitrogen — lush, soft growth attracts scale
- Maintain tree health — a vigorous, well-watered tree is more resilient; check with your local council or a qualified arborist for general guidance on maintaining healthy outdoor environments
When to Call a Professional
Some situations are best left to a qualified arborist:
- Large or mature trees where safe access requires climbing or specialist equipment
- Severe or repeated infestations that aren't responding to DIY treatment
- When you're unsure of the pest species or the correct treatment
- Trees close to power lines, structures, or waterways where chemical use needs careful management
- If significant pruning or removal of infested branches is needed at height
A certified arborist can accurately diagnose the problem, recommend the most appropriate treatment, and carry out any necessary work safely. You can find qualified local arborists on tradedirectory.co.nz.
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