Every tree hosts a community of insects, but most cause no significant harm. Problems arise when pest populations explode beyond natural control, often because stress has weakened the tree, natural predators are absent, or introduced pests have no natural enemies. Understanding common pests, recognising their damage, and responding appropriately protects your trees without unnecessary chemical intervention.
The Integrated Pest Management Approach
Before reaching for sprays, consider the principles of Integrated Pest Management (IPM). This approach prioritises prevention, uses the least toxic effective treatments, and accepts that some pest presence is normal and not always harmful.
Healthy, unstressed trees naturally resist pest attack better than struggling specimens. Correct species selection, proper planting, adequate water, and appropriate nutrition keep trees vigorous enough to tolerate minor pest damage without intervention. When treatment is needed, start with the least toxic options and escalate only if necessary.
Eighty percent of pest problems can be prevented through proper plant selection and care. Stressed trees attract pests like magnets; healthy trees shrug off most attacks. Focus on tree health before worrying about pest control.
Common Sap-Sucking Pests
These insects pierce plant tissue and extract sap, weakening trees and often transmitting diseases.
Scale Insects
Scale appear as raised bumps on stems and leaves, varying from tiny dots to 5mm or more depending on species. They excrete sticky honeydew that attracts sooty mould, blackening foliage. Heavy infestations cause leaf yellowing, twig dieback, and general decline.
Control begins with horticultural oil sprays that suffocate the insects. Apply during cooler months when trees are dormant and apply thoroughly, covering all surfaces. Multiple applications at two-week intervals may be needed. In severe cases, systemic insecticides can be effective but should be a last resort. Encourage natural predators like ladybirds and parasitic wasps by avoiding broad-spectrum insecticides.
Aphids
Soft-bodied insects clustering on new growth, aphids multiply rapidly in spring when sap is flowing. They cause distorted, curled leaves and excrete honeydew that promotes sooty mould. Most trees tolerate low aphid populations without significant harm.
A strong jet of water dislodges aphids from accessible branches. Horticultural soap sprays kill on contact but have no residual effect. Ladybirds, lacewings, and hover flies are voracious aphid predators; avoid broad-spectrum sprays that kill these beneficials. Most aphid infestations peak in spring then naturally decline as predator populations catch up.
Psyllids
These tiny insects cause distinctive symptoms including pit galls, leaf curling, and stunted growth. Lemon-scented Gum Psyllid and Lilly Pilly Psyllid are common examples. Damage is usually cosmetic rather than threatening to tree survival.
Control is difficult because psyllids are protected within galls or curled leaves. Prune and destroy affected growth if infestations are severe. Systemic insecticides provide some control but rarely eliminate the problem. Healthy trees usually outgrow damage; stressed trees suffer more.
Leaf-Eating Pests
These pests consume foliage, reducing the tree's ability to photosynthesise.
Caterpillars
Various moth and butterfly larvae feed on tree leaves, from tiny leafrollers to spectacular emperor gum moth caterpillars. Most are native insects with natural predators and cause limited damage. Occasional outbreaks can defoliate trees but rarely kill them.
Hand-pick caterpillars if numbers are small. Bacillus thuringiensis (Bt) is a biological control that specifically targets caterpillars without harming beneficial insects or vertebrates. Encourage birds that eat caterpillars by providing habitat and avoiding pesticides. Healthy trees refoliate after caterpillar damage and usually recover fully.
Native caterpillars are food for native birds, parasitic wasps, and other wildlife. Completely eliminating them disrupts food webs. Accept some foliage damage as part of a functioning garden ecosystem.
Citrus Leafminer
The larvae of a tiny moth, citrus leafminer tunnels through new citrus leaves creating distinctive silvery trails. The damage is unsightly but rarely threatens tree health unless infestations are severe on young trees.
Spraying is largely ineffective because larvae are protected within leaves. Oil sprays applied to emerging foliage reduce egg-laying but require precise timing. Healthy, mature trees tolerate leafminer damage without issue. Focus on maintaining tree vigour through proper watering and fertilising.
Christmas Beetles
These large, showy beetles emerge in summer and can defoliate eucalypts in localised areas. Adults feed on foliage; larvae live underground and feed on grass roots. Population explosions occur periodically, followed by years of low numbers.
No practical control exists for home gardeners. Healthy eucalypts regenerate foliage after beetle damage. Severe defoliation in successive years can weaken trees, but isolated events cause only temporary setback.
Boring and Tunnelling Pests
These pests tunnel through wood, potentially causing structural damage.
Borers
Various beetle larvae tunnel into branches and trunks, creating sawdust-filled holes and galleries. Stressed trees are particularly vulnerable. Signs include small round exit holes, sawdust on bark, and branch dieback.
Prevention through tree health is the best defence. Once borers are inside wood, chemical control is nearly impossible. Prune and destroy affected branches. Severely infested trees may need removal. Maintain tree vigour to resist attack.
If you suspect borer damage in large structural branches or the main trunk, consult a qualified arborist. Structural failure can cause property damage or injury. Professional assessment determines whether the tree is safe.
Termites
Subterranean termites attack living trees from underground, hollowing out heartwood and potentially weakening structural stability. Signs include mud tubes on bark, hollow-sounding wood, and unexpected branch failures.
Treatment requires professional pest controllers. Never attempt DIY termite control, as improper treatment can scatter colonies and worsen problems. An arborist can assess whether affected trees remain safe.
Gall-Forming Pests
These pests cause abnormal growths on leaves, stems, or roots.
Citrus Gall Wasp
This introduced pest causes swelling on citrus stems, particularly affecting lemons, grapefruits, and kumquats. Heavy infestations weaken trees and reduce fruiting. No chemical control is available.
Remove and destroy galls before August when adult wasps emerge. Prune affected branches well below visible galls and dispose in sealed bags, not compost. Check trees regularly through winter and remove new galls promptly. Some citrus varieties show greater tolerance than others.
When to Seek Professional Help
Some situations warrant calling a qualified arborist or pest professional:
- Suspected borer or termite damage in main trunk or large branches
- Severe or repeated defoliation affecting tree survival
- Unknown pests or diseases you cannot identify
- Valuable or heritage trees requiring specialist care
- Pest problems affecting multiple trees or spreading through the garden
Professionals have access to diagnostic services, specialised treatments, and experience with unusual problems. The cost of consultation is usually worthwhile for valuable trees or persistent problems you cannot solve alone.