Spider mites, aphids, scale, and fungal infections hit bonsai hard and fast. Early detection and proper treatment save trees — here's what to watch for.
Before you treat for bugs, check whether there are any: most "sick" bonsai have no pests at all — the usual culprits are overwatering, underwatering, or too little light, and no spray fixes those. If leaves are yellowing or dropping and you can't find insects, webbing, or sticky residue, start with why is my bonsai dropping leaves and the watering guide before reaching for the neem oil.
A healthy tree with good light, correct watering, and moving air rarely gets seriously infested. Most infestations start on a tree already weakened by something cultural — fix the conditions and you prevent 80% of pest problems before they start.
The number-one bonsai pest, bar none — especially indoors. They're nearly invisible, so confirm with the paper test: hold white paper under a branch, tap sharply, and watch. Moving specks the size of a period are mites; a smeared reddish-brown streak settles it. Run it monthly on junipers even when they look fine — mites can bronze half a juniper before you'd spot one.
Mites love warm, dry, stagnant air — most living rooms in winter (the indoor growing guide covers airflow and humidity fixes). First response: blast the entire tree with a strong stream of water, flipping leaves to hit the undersides. Then spray neem oil or insecticidal soap every five to seven days for three consecutive treatments.
Aphids are soft-bodied little green, black, or white bugs clustering on tender new growth. They suck sap and excrete sticky honeydew that grows sooty mold. A strong water spray knocks light populations off immediately; heavier infestations take insecticidal soap every seven days for two to three weeks. Outdoor trees get free help from ladybugs and lacewings, which eat aphids by the hundreds.
Whiteflies favor the same indoor tropicals — ficus, schefflera — and scatter when disturbed, so sprays miss the adults. A yellow sticky card ($5–8 a pack) catches adults; soap every five to seven days for three weeks clears larvae on the leaf undersides.
Check outdoor trees for ants marching up the trunk: they farm aphids for honeydew and defend them from ladybugs. A sticky barrier around the pot feet breaks the escort.
Scale insects look like tiny brown or tan bumps glued to branches and trunks — mistaken for bark texture all the time, which is how they get out of control. Scrape them off with a toothbrush dipped in rubbing alcohol, then follow with neem oil to catch the crawlers (the mobile juvenile stage) you missed. Recheck weekly for at least a month; a few survivors restart the colony.
Mealybugs show as cottony white tufts in branch crotches, favoring indoor tropicals — ficus, jade, and schefflera especially. Dab each with an alcohol-soaked cotton swab, then spray the whole tree with insecticidal soap. Check again in a week; they hide in crevices and almost always need a second round.
Caterpillars are the easiest fix here: hand-pick and relocate — a few chewed leaves won't hurt an established tree. The exception is the bagworm, a 1–2 inch cone of silk and foliage on junipers and other conifers. Pick the bags off in fall or winter; each holds hundreds of eggs.
If your tree came home from a mall kiosk or big-box store in dense black potting soil, expect these. The adults are harmless; the larvae live in constantly wet organic soil, feeding on fungus and, when heavy, fine roots. The soil is the real problem, not the fly.
Break the cycle from both ends. Let the top half inch (13 mm) of soil dry before rewatering — larvae can't survive dry cycles — and add BTI (Bacillus thuringiensis israelensis — mosquito dunks, $10–15 a season): crumble a quarter dunk onto the soil or steep one in the watering can — larvae-specific, harmless to the tree and pets. Sticky cards catch the adults.
The standard neem mix: 1–2 teaspoons of neem concentrate plus a few drops of dish soap (the emulsifier) per quart of warm water, shaken often because it separates fast. A $12–15 bottle lasts a year. Insecticidal soap comes premixed for $8–12; growers genuinely disagree about DIY dish-soap sprays — detergents can strip the leaf's waxy cuticle — so premixed is the safer bet.
Coat every surface, especially leaf undersides, at dusk or on an overcast morning. Indoor trees go to the shower, tub, or outside — spray, drip-dry, return.
Tender species need a test patch: Fukien tea and serissa can drop leaves at full-strength neem. Spray one branch, wait 48 hours, check for burn before treating the whole tree.
When three full rounds of contact spray fail — armored scale is the usual holdout — escalate to a soil-applied systemic: imidacloprid granules ($15–25 a jar), dosed by pot size per the label. It moves through the sap and reaches pests no surface spray can touch.
Fungal issues usually signal a cultural problem: too much moisture, too little airflow. Remove affected leaves and fix the conditions — better drainage (see the soil guide), less frequent watering, a small fan. Persistent cases respond to copper-based fungicide at the exact label dilution. The big three, by species:
Roots need oxygen as much as water; waterlogged soil suffocates them, and rot spreads from dead roots to healthy tissue. This is the one time repotting a sick tree is correct — the repotting guide explains why sick trees otherwise stay in their pots.
Build a 30-second inspection into watering: flip a few leaves, scan branch junctions, glance at the soil surface. Catching five aphids is a two-minute fix. Catching five hundred is a two-week battle with real risk to the tree.
Every new tree gets two to three weeks of isolation from your collection, with close inspection every few days. Nurseries and shows are hotbeds for hitchhiking pests, and one infested newcomer can seed spider mites across a whole bench before anything looks wrong.
The whole guide in ten lines:
