12 Common Houseplant Pests and How to Get Rid of Them

Indoor pests arrive on a new plant, in a bag of soil, or through an open window, and they spread quickly because a warm room with no predators is close to ideal for them. The good news is that the list of things you will actually meet is short, and each leaves a distinct mark.

Identify it before you treat it

1. Fine pale speckling on the leaf surface and delicate webbing where the leaf meets the stem. Thrives in dry, warm air.Spider mites
2. White cottony fluff tucked into leaf joints and along stems, and a sticky residue underneath.Mealybugs
3. Small brown or tan bumps on stems and leaf undersides that do not brush off and look like part of the plant.Scale insects
4. Clusters of soft green, black or pink insects on the newest growth and flower buds.Aphids
5. Tiny white insects that fly up in a cloud when you disturb the plant.Whiteflies
6. Slim dark insects that scurry when disturbed, with silvery streaks and black specks on the leaves.Thrips
7. Small black flies drifting around the pot and the room, harmless to leaves but a sign of permanently wet soil.Fungus gnats
8. Pale winding tunnels traced through the inside of a leaf.Leaf miners
9. Tiny white jumping specks on the soil surface when you water. Harmless, and a sign the soil is too wet.Springtails
10. Irregular holes chewed from leaf edges overnight, with a slime trail. Common on plants summered outdoors.Slugs and snails
11. Dense colonies on stems that leave leaves sticky and later sooty and black.Soft scale and its honeydew mould
12. Distorted, cupped or unusually narrow new growth with no visible insect at all.Broad or cyclamen mites

The four that cause the most damage

Spider mites are the most destructive because they are nearly invisible until the population is large. They are arachnids, not insects, they multiply in days in warm dry air, and by the time you see webbing the plant has lost a great deal of leaf surface. Hold a sheet of white paper under a leaf and tap it: if specks fall and then walk, you have mites.

Thrips rasp the leaf surface and drink what comes out, leaving silvery streaks and tiny black droppings. They breed in the soil as well as on the plant, which is why they come back after a single leaf treatment.

Mealybugs hide in exactly the places you do not look: leaf axils, the underside of the crown, and down among the roots. Any plant that keeps declining after a clean-up should be tipped out of its pot and the roots checked for white fluff.

Scale is easy to mistake for part of the plant. The adults sit under a hard shell that shrugs off most sprays, so they have to be removed physically or treated at the crawler stage.

Treatment that works, in order

Isolate first. Move the plant to another room before you do anything else. Pests spread between touching leaves faster than any treatment works.

Then physically remove what you can. Rinse the plant thoroughly in the shower or at the sink, undersides included. For mealybugs and scale, a cotton bud dipped in rubbing alcohol kills on contact. This single step removes most of the population and makes everything after it more effective.

Then treat, and repeat. Insecticidal soap or a horticultural oil such as neem smothers soft-bodied pests. Cover every surface including the undersides and stem joints, because these products only work where they land. Repeat every five to seven days for at least three cycles: almost nothing kills the eggs, so you are catching each generation as it hatches. Stopping after one application is the most common reason an infestation returns.

Fungus gnats are a watering problem and do not need any of this. The larvae live in the top few centimetres of permanently damp soil. Let the surface dry properly between waterings, add a layer of coarse grit or sand on top, and use yellow sticky traps for the adults. Within a generation they are gone.

Prevention, which is mostly two habits

Quarantine new plants for a fortnight, checking leaf undersides twice in that time. And look at your plants when you water them: turn one leaf over on each plant, check the growing tip, and feel for stickiness. Every infestation is trivial to fix in its first week and a serious project by its fourth.

If you cannot tell what you are looking at, photograph the affected area up close and run it through Identium's diagnosis. It names the likely problem, tells you how urgent it is, gives you a treatment and, importantly, tells you whether it spreads to the rest of your plants, which decides whether you can put the plant back on the shelf.