Something is on my plant — what is it, and what do I do?

Take the plant to a window and turn a leaf over. Use your phone's torch.

• Spider mite. The leaf looks dusty or finely speckled, pale between the veins, and there is a fine web at the leaf joints or across the underside — pull the plant through a shaft of light and the web catches it. The mites themselves are pinpricks, pale or red, moving. They thrive in dry heated air, which is why they appear in the weeks after the heating comes on, and ivy, palms, calathea and crotons get them first.
• Aphids. Soft, green, black or pink, in clusters on new growth and flower buds, with a sticky honeydew on the leaves below and sometimes a black sooty mould growing on that. Easy to see, easy to shift.
• Mealybug. White, cottony tufts in the leaf axils, along stems and at the base of leaves, sometimes in the roots. Slow, sticky, and stubborn because the wax sheds water.
• Scale. Small brown or tan bumps, oval, fixed to stems and the undersides along the midrib; they do not move and look like part of the plant. Sticky leaves below them are the giveaway.
• Thrips. Tiny, slender, dark, fast; silver-grey streaks on the leaf surface where they have scraped it, small black dots of droppings, and new leaves coming in distorted. Tap a leaf over white paper and they fall out and run.

All five feed by piercing the leaf and drinking the sap. The leaf loses colour where it is fed on — the speckle, the silvering — and the sugars the insect cannot use come out as honeydew, which is the stickiness and the food for sooty mould. None of them kills a plant quickly; what they do is weaken it over weeks and spread to its neighbours. A plant already stressed — dry air, low light, erratic watering — is the one they take hold on, which is why a pest is often the second problem, not the first.

Isolate it.

Away from every other plant, today, for three weeks. Most infestations spread by touching leaves.

Wash it.

Under the shower, or in a sink, with a fine spray, undersides especially, every three or four days for a fortnight. This alone controls aphids and mites on most plants; it removes eggs as well as adults.

Wipe or pick off what will not wash.

Mealybug and scale come off with a cotton bud dipped in soapy water, or a fingernail. Check the leaf axils and under the rim of the pot.

Spray with a soap-based insecticide

approved for home use in the UK, if washing has not done it within a fortnight — undersides, stems, axils, repeated at the interval on the label. Soap works by contact and needs to touch the insect, so coverage matters more than strength.

Change the air.

Spider mite is a dry-air pest: raise the humidity, group the plants, move them off the radiator. Aphids and thrips come in on new plants and cut flowers; a fortnight's quarantine for anything new stops most of it.

Cut off the worst.

A leaf that is mostly webbed or scaled is better removed than treated. Bin it, do not compost it.

Keep checking.

Weekly, undersides, for a month after you last saw one. Eggs hatch after the adults are gone.

• Do not use anything not approved for home plant use in the UK. Neem oil is not an approved plant protection product here, whatever the internet says; nor are most “natural” recipes. Approved products say so on the label.
• Do not spray in sun or on a dry plant. Soap sprays scorch leaves in direct light; water the plant the day before and spray in the evening.
• Do not put an infested plant back with the others because it looks better. Three weeks clean, then back.
• Do not mist as a treatment. It raises the humidity for minutes; a shower removes the pest.

The dry-air pests go for the thin-leaved and the palm-fronded first — English ivy, parlour palm, calathea and croton — and anything brought home from a shop in the last month.

• English IvyHedera helix
• OleanderNerium oleander
• Parlour PalmChamaedorea elegans
• Bamboo PalmChamaedorea seifrizii
• Bay LaurelLaurus nobilis
• Column CactusCereus repandus
• False AraliaDizygotheca elegantissima
• Geranium AraliaPolyscias guilfoylei
• GladiolusGladiolus species
• HopsHumulus lupulus
• Japanese AraliaFatsia japonica
• Lemon VerbenaAloysia triphylla
• LupinLupinus polyphyllus
• Madagascar JasmineStephanotis floribunda
• Majesty PalmRavenea rivularis
• Monterey CypressCupressus macrocarpa
• NasturtiumTropaeolum majus
• OliveOlea europaea
• Pygmy Date PalmPhoenix roebelenii
• RoseRosa

All 265 plants in the library →

How do I get rid of spider mites on houseplants?

Isolate the plant, wash it under the shower every three or four days for a fortnight — undersides especially — and raise the humidity around it, because spider mite breeds in dry heated air. If that has not cleared it in two weeks, a soap-based insecticide approved for home use, applied to the undersides at the label interval, finishes the job. Check weekly for a month afterwards.

What are the tiny webs on my plant?

Fine webbing at the leaf joints or across the undersides, with a dusty speckle on the leaves, is spider mite. Ordinary spiders make an occasional web in a corner; spider mite makes a fine mesh on the plant itself, and the leaves under it are pale and speckled.

Why are my plant's leaves sticky?

Sticky leaves are honeydew from a sap-feeding insect above them — aphids, scale or mealybug. Look at the leaves and stems above the sticky ones, and on the undersides: green clusters, brown bumps or white fluff. A black film on the sticky leaves is sooty mould growing on the honeydew; it wipes off once the insects are gone.

Are spider mites harmful to people or pets?

No. They feed only on plants and do not bite; the webbing is a nuisance, not a hazard. The same is true of aphids, mealybug, scale and thrips.

Can I use washing-up liquid on my plants?

It is not a good idea: washing-up liquids contain degreasers and other additives that can damage leaves, and they are not approved for use on plants. A soap-based insecticide sold for the purpose is cheap, approved, and formulated not to scorch.

The RHS's page on glasshouse red spider mite covers the mite itself in detail; this page is the indoor case and the other four. Sabz's Check-up asks what you see — webbing, sticky leaves, bugs on the plant — and what the room has been like, because a spider mite outbreak in November usually has a radiator behind it, and the treatment starts there.