Fine webbing between the leaves
Spider mites. What the webbing means, how to be sure with a sheet of white paper, and the routine that actually clears them.
This one is worth acting on today rather than at the weekend. A week of waiting changes what is left to save.
What it usually is
1 Spider mites
| How to tell | Hold a sheet of white paper under a leaf and tap it. Spider mites fall as specks the size of a full stop and then walk, which distinguishes them from dust. The leaf above will be faintly speckled with pale dots where they have fed, and fine webbing runs between leaf and stem rather than being a spider's tidy geometric web. |
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| The fix | Take the plant to a sink or shower and wash both sides of every leaf with tepid water and a little soap, paying attention to the undersides where they live. Repeat every five days for three weeks, which covers the egg cycle. Isolate the plant while you do it. |
2 Dry warm air letting a small population explode
| How to tell | The outbreak began within weeks of the heating going on, or after a spell on a hot sunny sill, on a plant that was clean before. |
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| The fix | Raise the humidity around the plant and keep it off the radiator shelf. Mites breed fast in warm dry air and slowly in damp air, so washing plus humidity does more than washing alone. |
3 A new plant brought them in
| How to tell | The first infested plant is one bought or given in the last month, and the plants next to it are starting to speckle. |
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| The fix | Quarantine every new plant for three weeks away from the others and look at the undersides of its leaves weekly. It is the least glamorous habit in plant keeping and it prevents most of this. |
Through the year. Spider mites are a winter problem indoors, because central heating gives them exactly the warm dry air they breed fastest in. Outdoors they are a hot dry summer pest.
Webbing is late evidence. By the time you can see threads running between a leaf and a stem, the population has been building for weeks and has probably reached the plants either side. That is why this symptom is marked urgent: not because a mite kills a plant quickly, but because a fortnight of delay turns one plant into a shelf.
The mites themselves are barely visible, under half a millimetre, and they live on the underside of the leaf where they pierce cells and drink the contents. The first sign is not webbing but stipple: a fine pale speckling across the top of the leaf, as though someone had gone over it with a very sharp pencil. Held up to the light it looks slightly translucent in those spots. A leaf like that, on a plant near a radiator in February, is worth a proper look underneath before anything else.
The white paper test settles it. Hold a sheet under a leaf, tap the leaf firmly, and look. Dust does not move. Mites do.
Treatment is mechanical and repetitive, and the repetition is the part people skip. Wash the whole plant, undersides especially, with tepid soapy water. That removes most of the adults. It does not remove the eggs, which hatch over the following week, so wash again after five days, and again five days after that. Three washes over three weeks clears an ordinary infestation. One wash never does, and the population is back to where it started within a month.
While you are treating, keep the plant away from the others, raise the humidity around it, and move it off any hot dry spot. Mites complete a generation in about a week at twenty seven degrees and in nearly three weeks at fifteen, so a cooler damper plant is a plant that is winning.
If a plant is badly infested and not precious, the honest answer is often to throw it away. A heavily webbed plant sheds mites onto everything it touches, and three weeks of washing a plant you did not much like is three weeks of risk to the ones you do.
Entries that name it
What the journal says about this symptom on each plant.
| Alocasia | Spider mites, which find it quickly in dry air. Rinse both sides of every leaf, raise the humidity, and repeat weekly. |
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| Areca palm | Spider mites, which find a palm in a dry heated room within weeks. Rinse both sides of every frond in a shower, raise the humidity, and repeat weekly until no new webbing appears. |
| Boston fern | Spider mites, which arrive when the air is dry. Rinse the whole plant under a shower, raise the humidity, and repeat weekly until no new webbing appears. |
| Calathea | Spider mites, which take hold quickly on a plant kept too dry. Rinse both sides of every leaf, raise the humidity, and treat weekly with an insecticidal soap until no new webbing appears. |
| Cape leadwort | Mites, more troublesome in hot, dry, still conditions. Hose the foliage down regularly to raise humidity around the leaves, and treat with insecticidal soap or horticultural oil if webbing is already visible. |
| Chinese fan palm | Red spider mite in dry heated air. Rinse the fronds thoroughly, undersides included, and raise the humidity. Check weekly through the winter. |
| Croton | Spider mites, which find a croton in dry indoor air very quickly. Rinse both sides of every leaf, raise the humidity, and repeat weekly until nothing new appears. |
| Dragon tree | Spider mites in dry indoor air. Wipe both sides of every leaf, raise the humidity, and treat weekly until nothing new appears. |
| Dumb cane | Spider mites, encouraged by dry warm air. Wipe both sides of every leaf, raise the humidity, and repeat weekly until nothing new appears. |
| English ivy | Spider mites, which take hold on an ivy in a warm dry room faster than on almost any other houseplant. Rinse the whole plant under a shower, move it somewhere cooler and more humid, and repeat weekly until nothing new appears. |
| Fiddle-leaf fig | Spider mites, which thrive in dry indoor air. Wipe both sides of every leaf, raise the humidity around the plant, and treat weekly with an insecticidal soap until the webbing stops appearing. |
| Fishtail palm | Red spider mite, which this palm attracts as readily as any. Rinse the fronds and undersides, raise the humidity, and check weekly in a heated room. |
| Kentia palm | Spider mites in dry heated air. Rinse the whole plant under a shower monthly, which prevents them as much as it cures them. |
| Lady palm | Spider mites in dry heated air. Rinse the whole plant under a shower and repeat weekly until nothing new appears. |
| Parlour palm | Spider mites, which find any palm in a dry heated room. Rinse the whole plant under a shower, raise the humidity around it, and repeat weekly until nothing new appears. |
| Prayer plant | Spider mites, encouraged by dry air. Rinse the leaves top and bottom, raise the humidity, and treat weekly with an insecticidal soap until nothing new appears. |
| Pygmy date palm | Red spider mite, which loves a palm in a dry heated room. Rinse the fronds thoroughly including the undersides, raise the humidity and check weekly. Palms are the plant this pest finds first. |
| Weeping fig | Spider mites in dry warm air. Rinse the whole plant under a shower, raise the humidity around it, and repeat weekly. |
| Zebra alocasia | Red spider mite, which finds alocasias before almost anything else. Rinse the undersides thoroughly, raise the humidity and check weekly. Dry heated air is what brings it on. |
Questions from the margin
- What is the fine webbing on my houseplant?
- Spider mites. They are under half a millimetre and live on the undersides of the leaves, and the webbing appears once the population is well established. The first sign is a fine pale speckling across the top of the leaf, weeks before any web shows.
- How do I get rid of spider mites?
- Wash the whole plant, undersides especially, with tepid soapy water, and repeat after five days and again five days after that. Three washes over three weeks covers the egg cycle. Isolate the plant, raise the humidity around it, and keep it off any hot dry windowsill.
Checked against RHS, Glasshouse red spider mite; Colorado State University Extension, Spider mites.