This guide shows you how to prevent, diagnose and manage the most common issues on hobby bonsai—using an Integrated Pest Management (IPM) approach first, and only resorting to compliant home-and-garden products where truly necessary.
Key takeaways
Start with IPM: quarantine new trees, improve airflow and watering, clean tools, remove infected tissue, hand-pick pests;
Support natural enemies and use biological controls where possible. Chemical products are a last step.
In Ireland, amateur/home gardeners may only use products authorised for amateur use.
Check the DAFM Plant Protection Products (PPP) Database before buying or applying anything. Using professional-only PPPs without certification is illegal.
The biggest pot risk is often vine weevil (adult notches on leaves; grubs in the soil eat roots).
Biological nematodes work well in light, open composts, especially late summer to early autumn.
Indoors/under cover - watch for mealybugs, scale and glasshouse red spider mite;
Isolate new plants, wipe/flush, use horticultural soaps/oils as labelled for home use, and consider beneficial mites in summer.
High-risk diseases include powdery mildews, Phytophthora root rot, juniper tip blights (Phomopsis/Kabatina), Dothistroma needle blight on pines, and Verticillium wilt on some broadleaf species.
Airflow, dry foliage overnight, and sanitation are your first lines of defence.
Integrated Pest Management (IPM) for bonsai—what to do first
Quarantine new trees (2–4 weeks); separate tools/sprayers. Regularly inspect with a hand lens.
Hygiene: clear fallen leaves/needles; disinfect blades after cuts; avoid sharing soil between pots.
Watering & airflow: water in the morning, keep foliage dry overnight, improve spacing/ventilation—especially for junipers and pines.
Monitor weekly: check leaf undersides, shoot tips, root zone (weevil grubs). Record what you see.
Escalate stepwise: mechanical (wash off/hand-pick) → cultural/biological (beneficials, nematodes) → amateur-authorised products as per label only if needed.
Legal note (Ireland): before using any plant protection product, verify it’s authorised for amateur/home & garden use in the DAFM PPP Database; follow the label exactly.
Common bonsai pests (symptoms → actions)
Aphids
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Signs: distorted soft growth, sticky honeydew → sooty mould, clusters on new shoots.
Actions: blast off with water; tolerate low levels to support predators; under cover, introduce biological controls if needed; avoid broad-spectrum sprays that also kill beneficials.
Mealybugs
Signs: white “cottony” clusters at nodes/roots; plant weakening; honeydew/sooty mould.
Actions: isolate; wipe small colonies (cotton bud), prune heavily infested parts; use horticultural soaps/oils labelled for indoor/ornamental use; in warm months consider parasitoids/predators from reputable suppliers.
Scale insects (soft/armoured)
Signs: waxy shells on stems/leaf undersides; honeydew and blacsooty mould.
Actions: scrape off light infestations; improve plant vigour and airflow; soaps/oils as labelled; check for parasitised (already dead) scales before spraying; biologicals under cover where feasible.
Glasshouse red spider mite
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Signs: stippled/pale foliage, fine webbing; thrives in hot, dry conditions.
Actions: raise humidity and improve ventilation; wash foliage; consider predatory mites (e.g., Phytoseiulus persimilis)—effective in glasshouses/summer.
Vine weevil (Otiorhynchus)
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Signs: adults make “notches” in leaves; larvae in compost destroy roots → wilting, sudden collapse.
Actions: inspect rootball; replace infested media; drench with entomopathogenic nematodes per label (work best in open, moist composts); use traps for adults in summer.
Slugs & snails (outdoor benches)
Signs: ragged holes, slime trails on soft growth/seedlings.
Actions: water in mornings, encourage predators (frogs, beetles, birds), hand-pick at night; many home “barriers” have poor evidence—focus on habitat and targeted protection of vulnerable plants.
Pine sawflies (on pines)
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Signs: gregarious “caterpillar-like” larvae stripping needles.
Actions: hand-pick/shake into soapy water; prune small colonies; maintain airflow; (note: Bt for caterpillars doesn’t work on sawflies).
Common diseases & physiological problems
Powdery mildews (often on maples)
Signs: white powdery coating on leaves/shoots; worsens with drought stress + humid canopies.
Actions: improve airflow/light, water consistently (don’t let trees swing between drought and soak), avoid high N flushes; remove infected tissue.
Phytophthora root/collar rot (oomycetes)
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Signs: wilting despite moist soil, chlorosis, crown/root necrosis; major cause of woody plant losses.
Actions: drainage, drainage, drainage—free-draining mixes and careful irrigation; don’t reuse contaminated compost; if confirmed, remove infected material, clean tools/surfaces, and improve site drainage.
Juniper tip blights (Phomopsis / Kabatina)
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Signs: browning/dieback of shoot tips; Kabatina often late winter/early spring, Phomopsis in cool/wet spells.
Actions: prune out infected tips in dry weather; keep foliage dry overnight; increase sun/airflow; avoid wounding and heavy shearing.
Dothistroma (red-band) needle blight on pines
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Signs: dark bands/spots, tip death on older needles; thinning canopy.
Actions: maximise air circulation, avoid overhead wetting, remove infected needles/branches; keep crowns dry overnight.
Verticillium wilt (broad hosts; maples vulnerable)
Signs: one-sided wilt/dieback; vascular disease from soil—no curative treatment.
Actions: prune dead wood, avoid stress and waterlogging; consider species/rootstocks less susceptible if site has history.
Note on fireblight (hawthorn, apple/pear, cotoneaster): if you suspect Erwinia amylovora, act fast—sanitation and safe disposal are key; keep up with DAFM plant-health updates for Ireland.
Species snapshot: what commonly goes wrong?
Junipers (Juniperus)
Typical issues |
Tip blights (Phomopsis/Kabatina), red spider mite; pear rust in mixed plantings |
Dry foliage overnight; prune in dry weather; increase sun/airflow; check mites. |
Pines (Pinus)
Typical issues |
Dothistroma needle blight; sawfly larvae |
Airflow, no overhead wetting; remove infected needles; hand-pick sawflies. |
Japanese maples (Acer palmatum)
Typical issues |
Powdery mildew; Verticillium wilt risk on some sites; aphids |
Steady moisture, shade from hot winds; sanitation; avoid soil stress. |
Hawthorn (Crataegus)
Typical issues |
Fireblight risk; aphids |
Sanitation and tool hygiene; follow DAFM guidance if suspected. (data.oireachtas.ie) |
Beech (Fagus)
Typical issues |
Honeydew/sooty mould from sap-feeders; generally cosmetic |
Wash foliage, support predators; avoid unnecessary sprays. |
Indoor figs & co.
Typical issues |
Mealybug, scale, red spider mite |
Quarantine, wipe/flush; humidity/ventilation; home-labelled soaps/oils; beneficials under cover. |
Step-by-step
A) Aphids on spring flush
Hose off with water; 2) pinch remaining clusters; 3) avoid broad-spectrum sprays—let predators build; 4) re-check in a week.
B) Vine weevil grubs in a pot
Check roots (white C-shaped grubs) and repot into clean, open mix; 2) apply nematodes per label; 3) repeat late summer; 4) place adult traps in summer.
C) Red spider mite under cover
Raise humidity and improve airflow; 2) wash foliage; 3) introduce Phytoseiulus where suitable.
D) Juniper tips browning
Prune back to healthy wood in dry weather; 2) keep canopies dry overnight; 3) disinfect tools each cut.
E) Suspected Phytophthora
Fix drainage and media; 2) don’t reuse contaminated compost; 3) clean benches/tools; 4) focus on cultural control.
When (and how) to use products—Ireland-specific
Use chemicals only after cultural/mechanical/biological measures.
As a home gardener, you may only use products authorised for amateur use in Ireland; confirm in the DAFM PPP Database. Follow the label exactly (crop/ornamental use, dose, timing, PPE, disposal).
Biologicals (e.g., nematodes for vine weevil; predatory mites for spider mite) are widely used in home settings when conditions suit.
Spring: aphids; pine sawfly hatch; early powdery mildews—prioritise monitoring and sanitation.
Summer: spider mites under glass; keep foliage dry at night to deter leaf/needle diseases.
Late summer–autumn: apply vine weevil nematodes; clean up fallen leaves/needles.
Winter: tool disinfection, bench cleaning, plan media/drainage improvements.
FAQ
"Bonsai pests diseases guide in Ireland"
Do I always need a spray for powdery mildew?
Often no—airflow, steady watering (avoid drought stress), and pruning infected tissue are the core fixes.
Are slugs/snails always “pests”?
Only a few species eat living plants; many are part of a healthy garden food web. Protect vulnerable trees, but focus on habitat-based control and hand-picking.
Can I use professional agricultural products on my bonsai?
No—unless you’re certified and the product is authorised for that use. Home gardeners must use amateur-authorised products only. Check DAFM’s PPP database.
Helpful resources & references
RHS (Royal Horticultural Society) — IPM and problem profiles:IPM without chemicals; Aphids & predators; Mealybug; Scale; Glasshouse red spider mite; Powdery mildews; Phytophthora (root rot & managing outbreaks); Slugs & snails; Pine sawfly; Conifers: brown patches.
University/Extension — disease specifics:Juniper tip blights (Iowa State; Morton Arboretum); Dothistroma needle blight (Univ. Minnesota; Wisconsin Extension). (Yard and Garden, The Morton Arboretum, extension.umn.edu, Wisconsin Horticulture)
Ireland (DAFM) — legal & plant health:Amateur/Home & Garden users; PPP Database; Responsible pesticide use (amenity & garden); Fireblight factsheet/updates.
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