Fine webbing on my houseplants: is it spider mites?

Spider mites are the pest people find last, because everything about them is small. They are arachnids rather than insects, well under a millimetre long, and they live on the undersides of leaves where nobody looks. What you notice first is usually that a plant has gone dull and slightly grey, as though it needs a wipe. By the time you see actual webbing, the population has been building for weeks.

The most likely reason you have them: your air conditioning

This is the part that surprises people who moved here from somewhere drier. Spider mites reproduce fastest in warm, dry conditions, and an air-conditioned house in Southwest Florida is warm and dry indoors regardless of what the weather is doing outside. Cooling air removes moisture from it; that is the mechanism, not a side effect. A home sitting in 85% humidity outdoors can easily run at 40% in the living room, which is roughly what a heated northern house runs at in January. Mites do not care that Florida is humid. They care what the air around that leaf is doing.

Two things reliably make it worse. A plant standing in the direct airflow of a vent is living in the driest, most mite-friendly microclimate in the house. And a plant already stressed, by dry soil, by too much sun, or by being rootbound, is a much easier target than a plant in good condition.

The other things it might be, ranked

• Nothing at all. Fine cobwebs from a house spider are wider, structured, and not attached to stippled leaves. Spider mite webbing is dense, messy, and concentrated in leaf joints and along stems, with damaged leaves under it.
• Dust and hard-water spotting, which dull a leaf but wipe clean and leave no stippling pattern behind.
• Mealybugs. Cottony white clusters in leaf axils, sticky honeydew, sometimes black sooty mould growing on the honeydew. Visible to the naked eye; mites effectively are not.
• Thrips. Silvery scarring and tiny black specks of frass, with distorted new growth. Slim dark insects that dart when disturbed.
• Scale. Hard or waxy fixed bumps on stems and leaf undersides that do not wipe off.
• Dry-air leaf damage without pests. Crispy edges and tips with no stippling and nothing moving on white paper: that is an air problem, and the treatment is completely different.

How to confirm it in five minutes

• The paper test. Hold a sheet of white paper under a suspect leaf and tap the leaf firmly. Specks that crawl are mites. Specks that stay still are dust.
• Look underneath, magnified. A phone camera at maximum zoom works well. You are looking for tiny moving dots and for fine strands between the veins.
• Read the stippling. Mite feeding punctures individual cells, so damage reads as fine pale dots rather than solid patches. Heavy infestation turns the whole leaf bronzed or greyish.
• Check the neighbours. Whatever was touching the infested plant is infested too until proven otherwise.

What to do this week

1. Isolate the plant today, in a different room, not just a different shelf.
2. Rinse it physically. Take it to the shower or sink and wash both leaf surfaces thoroughly with tepid water. This alone removes a large share of the population and costs nothing.
3. Prune out the worst leaves and any heavily webbed growth, and bag the trimmings rather than carrying them through the house.
4. Treat with insecticidal soap or horticultural oil, covering leaf undersides completely; contact products only work on what they actually touch.
5. Repeat every five to seven days, at least three or four times. This is the step that decides whether you win, because sprays do not kill eggs.
6. Raise the humidity around the plant with a humidifier, grouping, or a pebble tray, and move it out of direct AC airflow. Do not rely on misting: it raises humidity for minutes and then stops.
7. Keep inspecting for two weeks after the last treatment before you reintroduce the plant to the collection.

When to call a professional

Call when mites have moved through several plants, when a valuable specimen is badly hit, or when you have already treated and they came back, which almost always means the schedule slipped rather than the product failed. A pest management visit means I identify what you actually have under magnification, treat the whole collection on one coordinated schedule instead of plant by plant, and fix the conditions that made your house hospitable in the first place. That last part is what stops it recurring next dry season.

Related: white fuzzy spots covers the pest people confuse these with most, and crispy calatheas share the same root cause in the air.