The 30-second decision tree
Look at the plant from arm's length. The first thing you spot — webbing, fluff, flies, sticky leaves — narrows the search to one or two pests immediately. Work the list in this order; it sorts the most distinctive visual evidence first.
- 1Fine silk webbing in leaf joints, especially under leaves? → Spider mites. Pale dusty stippling on the upper leaf surface confirms it.
- 2Silvery streaks or scratches on leaves with tiny black dots scattered through them? → Thrips. The black dots are frass (droppings).
- 3White cottony fluff in leaf joints, on stems, or under leaves? → Mealybugs. Push a cotton bud against it — it sticks like wax.
- 4Small black flies hovering around the soil or rising when you water? → Fungus gnats. The adults are harmless; larvae in the soil eat roots.
- 5Hard immobile brown or tan bumps on stems and leaf undersides, scrape off with a fingernail? → Scale insects.
- 6Clusters of small soft green, black, or pale insects on new growth tips and flower buds? → Aphids.
- 7Sticky leaves or sticky surfaces nearby, with no visible pest? → Honeydew. Mealybugs, scale, or aphids are present somewhere — inspect closer.
Spider mites — webbing and stippling
Spider mites (Tetranychus urticae and relatives) are the single most damaging indoor pest, and the smallest. Adults are 0.4–0.5 mm long — the size of a grain of salt — and live almost exclusively on leaf undersides. The first thing you see is usually the damage, not the mites: a fine, dusty, pale stippling across the upper leaf surface where mites have drained chlorophyll cell by cell.
Confirm with the paper test: hold a sheet of white paper under a leaf and tap the leaf hard. Tiny moving dots on the paper are spider mites. Fine silk webbing in leaf joints, especially on the underside between leaf and stem, is a late-stage sign — by the time webbing is visible, the colony is in the thousands.
Spider mites thrive in warm, dry indoor air — peak season is March–August in heated apartments. Treatment, full life-cycle protocol, and prevention are in the spider mites guide.
Thrips — silver streaks and black dots
Thrips are slender, 1–2 mm yellow-to-black insects that scrape leaf surfaces and suck the sap. The damage they leave is unmistakable: silvery, scratched, or 'streaked' patches on leaves, often with tiny black dots (frass) scattered through them. Heavily affected leaves can have papery patches that crumble when touched.
Adult thrips run when disturbed. Tap a leaf over a white surface — they're visible to the naked eye and move in fast skittering bursts. New growth comes in deformed, scarred, or stuck unfurling because thrips feed on the growing tip. Monstera, alocasia, philodendron, and ficus are particularly prone.
Confirmation, isolation protocol, and the four treatment options ranked by toxicity are in the thrips guide.
Mealybugs — white cottony fluff
Mealybugs (family Pseudococcidae) look exactly like small white cotton balls stuck to the plant. Each individual is 2–4 mm and covered in a waxy white coating that protects them from water and most contact insecticides. They cluster in leaf joints, along stem bases, in the crowns of plants like succulents, and on the undersides of leaves.
The waxy coating is the diagnostic feature. Push a cotton bud against a fluff cluster — mealybugs stick like beeswax; mould or fertilizer crust crumbles to powder. A magnifier shows the segmented body underneath the wax.
Mealybugs spread slowly compared to mites or thrips, but they're stubborn. The treatment cascade — alcohol-dip first, systemic insecticide for serious infestations — is in the mealybugs guide.
Fungus gnats — black flies around the soil
Fungus gnats (family Sciaridae) are 2–4 mm dark flies that rise from the soil when watered or disturbed. The adults are mostly a nuisance — they don't bite humans and don't damage leaves directly. The larvae are the real problem: they live in the top 2–3 cm of soil and eat plant roots, particularly fine new roots and germinating seedlings.
Confirm with the yellow sticky-trap test: place a yellow sticky card horizontally over the soil for 24 hours. A heavily infested pot will catch dozens of adults. Soil that stays consistently moist favours the larvae; letting the top 2–3 cm dry between waterings breaks the breeding cycle within 2–3 weeks.
Full breaking-the-cycle protocol — sticky cards, BTI larvicide, top-dressing — in the fungus gnats guide.
Scale insects — immobile brown bumps
Scale insects look like 2–5 mm brown, tan, or black bumps stuck to stems and leaf undersides. They don't move because adult females are sessile — they attach to a feeding spot, secrete a hard protective shell, and stay there for life. New owners often mistake them for natural plant features (bumps on the bark, pollen marks) until the shells start producing sticky honeydew.
Scrape one with a fingernail or a butter knife. Scale lifts off cleanly with a small wet residue underneath; bark and natural marks don't. Crawler-stage scale (immature, before the shell forms) is mobile and looks like tiny pale specks moving on stems — this is the easiest stage to kill.
The scale insects guide covers the soft-vs-armoured distinction (which determines which treatments work) and the systemic approach for established infestations.
Aphids — clusters on new growth and flower buds
Aphids are 1–3 mm soft-bodied insects that cluster on new growth, flower buds, and the underside of fresh leaves. Colours range from green and black to pale yellow and pink depending on species and host. They reproduce extremely fast — a single female can produce 80 offspring in a week — and a small cluster becomes a serious infestation in days.
The visual is unmistakable: a tight cluster of slow-moving small insects, often packed shoulder to shoulder on a stem tip, with deformed or curled new leaves below them. Sticky honeydew and a glossy black sooty mould layer often accompany aphid populations. Ants will sometimes 'farm' aphids for the honeydew.
Treatment is straightforward — physical removal works and a hard water spray knocks down 80% of the population in seconds. The aphids guide covers the residual infestations that need follow-up.
When you see sticky residue but no pest
Sticky leaves or sticky residue on shelves and floor near a plant is honeydew — the sugary excretion produced by sap-sucking pests. It's a reliable sign you have mealybugs, scale, or aphids somewhere on the plant, even if you haven't spotted them yet. Spider mites and thrips don't produce honeydew, so a sticky leaf rules them out.
Inspect leaf joints, stem bases, the new growth tip, and the underside of leaves with a magnifier or phone camera. Dark sooty patches on top of sticky leaves are sooty mould, a fungus that grows on honeydew — it's harmless to the plant directly but indicates an established infestation.
Full follow-up flow in the sticky residue diagnosis guide.
Plants that often have multiple pests at once
Some plants come from the nursery with two or three pests already established. New plants brought home should be inspected on every leaf surface, isolated from other houseplants for 2–3 weeks, and re-checked weekly during quarantine.
- ·Calatheas, marantas, alocasias: spider mites + thrips. Both favour the same warm dry conditions and the same plants.
- ·Citrus, hibiscus, jasmine: scale + mealybugs + sometimes aphids. All three are sap-suckers attracted to the soft new growth on tropical fruiting plants.
- ·Succulents and cacti: mealybugs (especially root mealybugs in soil) + occasionally scale.
- ·Orchids: scale + mealybugs are the most common; thrips also occur on flowers.
- ·Anthurium, philodendron, monstera: thrips and spider mites both favour large leathery aroid leaves.
- ·Herbs (basil, mint, rosemary): aphids dominate, sometimes with whiteflies — small white moth-like flies that lift off in a cloud when the plant is brushed.
Confirm before you treat
The wrong ID wastes weeks. Treating thrips with neem-based contact spray when you actually have mealybugs leaves the colony untouched — mealybugs need alcohol penetration through the wax coating. Isolating a plant and treating it for spider mites when the real cause is fungus-gnat root damage starves the plant of moisture while the larvae continue eating roots.
Two confirmation steps before any treatment: (1) photograph the pest at the highest zoom your phone allows and compare to the species-specific guide linked above; (2) isolate the plant from other houseplants — at least 1–2 m of physical separation — to prevent spread while you confirm. If symptoms are unclear or you can't see a pest at all, work backward through the troubleshooting category to rule out non-pest causes (water, light, nutrients) before reaching for an insecticide.
