Spider Mites: Spot Them and Kill Them

Spider mites are the most damaging common houseplant pest, and the most easily missed. They are about 0.4 mm long — you can see them, but only if you know to look. By the time webbing is obvious, the population is in the thousands.

Confirming it

The stippling. Look at the top surface of affected leaves. Spider mites feed by piercing individual cells and draining them, leaving a scatter of tiny pale dots. Early on it looks like fine dust or a faint speckling; later the whole leaf goes bronze or greyish and dull.

The paper test. The definitive check. Hold a sheet of white paper under a leaf and tap the leaf sharply two or three times. Mites fall off and show up as specks the size of ground pepper, moving slowly across the paper. Dust does not move.

The webbing. Fine silk strands where the leaf meets the stem, between leaves, or across new growth. This means an advanced infestation — act immediately.

Where to look: undersides of leaves, leaf joints, and new growth. Use your phone camera zoomed in if your eyes will not resolve them.

Act on isolation first

Before you treat anything, move the plant away from every other plant. Spider mites travel on air currents, on your sleeves, and along touching leaves. A single infested plant in a collection becomes five infested plants in a fortnight.

Check the neighbours it was touching too — they are probably already carrying mites.

The treatment

Three weeks, no shortcuts. The reason infestations come back is almost always stopping early.

Step 1: physical removal

Take the plant to a shower or sink and rinse it thoroughly, paying particular attention to leaf undersides and joints. Lukewarm water, moderate pressure. This removes the bulk of the adult population immediately and is the highest-value single action you can take.

For a large plant, wipe every leaf on both sides with a damp cloth.

Step 2: spray, every five days for three weeks

Choose one:

• Insecticidal soap — effective, gentle, widely available. Coats and desiccates soft-bodied mites on contact.
• Neem oil — also disrupts feeding and reproduction, with some residual effect. Mix per the label with a drop of dish soap as an emulsifier.
• Rubbing alcohol solution — one part 70% isopropyl to three parts water. Cheap and effective. Test on one leaf first, as some plants react.

Spray until the leaves are dripping, especially the undersides, plus the stems and the soil surface. Anything you miss is where the survivors will be.

Repeat every five days for three weeks. This is the part that matters. Eggs are unaffected by these sprays, and they hatch every few days — the schedule is designed to kill each new generation before it can lay.

Spray in the evening or out of direct sun; oil and soap on sunlit leaves can scorch them.

Step 3: change the conditions

Spider mites explode in hot, dry air, which is exactly what a heated home in winter provides.

• Raise humidity above 50% with a humidifier
• Move the plant away from radiators and heating vents
• Space plants so their leaves do not touch
• Wipe leaves monthly — dust is shelter

What not to bother with

• Misting alone. Helps prevent, does not cure.
• A single spray. Kills the adults you hit and nothing else. The eggs hatch a week later and you are back where you started.
• Systemic granules for indoor plants. Overkill for most homes, and many are not labelled for indoor use.

Preventing the next one

• Quarantine new plants for two to three weeks, checking undersides weekly. This is where most infestations come from.
• Inspect when you water. Turn over two or three leaves each time. Thirty seconds a week catches problems while they are small.
• Keep humidity up in winter. The single biggest environmental lever.
• Check plants coming in from outdoors in autumn especially carefully.

If leaves are yellowing and you are not sure whether mites are the cause, work through the other possibilities in why plant leaves turn yellow — stippled yellowing looks quite different from the even yellowing of overwatering.