Why Plants Die
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Spider mites: pale speckled leaves and fine webbing

Check your plant with the questions

The first sign is usually a leaf that looks faintly sandblasted. Seen up close, the dull patch is made of hundreds of pale pinpricks, each one a place where a mite has drained a few cells. Spider mites are not insects but arachnids, kin to spiders and ticks, and they are small enough that the speckling tends to be noticed well before the animals doing it. Warm, dry air suits them, which is why a heated greenhouse or a dry room indoors can turn a handful into a serious infestation within weeks.

Reading the leaves

The mites feed from below, yet the evidence shows on the upper face first: a fine mottling of light dots, sometimes with a bronze cast. Keep feeding going and the leaf yellows or reddens, fades to almost no green, then dries and drops. On a plant that has been left alone for a while, threads of silk run between leaves and along stems, and mites often show up traveling along them.

Turn a speckled leaf over. What looks like dust on the underside may be a colony, and one colony can hold hundreds of mites. Look for round, glassy eggs like minute droplets, and pale shed skins left behind as the young grow. Under a 10x lens the specks resolve into eight-legged animals with oval bodies. For most of the year the common two-spotted spider mite shows as a yellowish green marked with a darker blotch on each flank; it turns orange-red when it rests over winter or when the plant it is on can no longer feed it. That late color explains why British guides call the species red spider mite.

No lens handy? Hold a sheet of white paper under the leaf and give the leaf a sharp tap. Any mites that land will start scurrying across the paper within moments.

Telling them apart

Webbing is the clearest mark. Several small pests crowd onto leaf undersides, but spider mites are the ones that spin silk, and that separates them from other mites and from insects of a similar size. Thrips also leave pale, silvery patches, yet they scatter tiny black dots of excrement over the damage and spin nothing. Aphids coat the leaves below them with sticky honeydew; a mite-damaged leaf feels dry.

Why they build up

Heat speeds everything. When conditions suit them a whole generation can pass from egg to egg-laying adult inside a week, while in cool weather the same cycle stretches to nearly two months. Plants short of water are hit hardest, both because more mites settle on them and because a thirsty plant copes worse with the feeding. Dust on the foliage is linked with outbreaks as well. Crowded, hot, dry growing conditions are the worst case.

Mites also arrive. As a heavily fed-on plant declines, females drift on air currents to whatever is nearby, and new plants can bring a colony home with them. The host range is so broad that hardly any plant is out of reach; orchids, impatiens, poinsettias, fuchsias and pelargoniums all feature in the RHS list.

How much harm they do

A few mites are not a crisis, and many woody plants carry light numbers without much effect. The risk rises with the count. Very large populations hurt soft-stemmed plants most and can kill annuals outright, and a badly damaged plant of any kind is left weak. Because the mites sometimes leave before the speckling is spotted, check that live ones are still on the plant before you treat; a plant whose mites have gone often recovers without help.

Bringing numbers down

  1. Rinse the undersides. A firm spray of water aimed at the lower leaf surfaces, repeated daily, knocks mites off and raises the humidity they dislike. On its own this seldom finishes them.
  2. Keep the plant watered. Letting it dry out to the point of stress feeds the problem.
  3. Use a soap or oil spray if rinsing falls short. Sprays of insecticidal soap, neem oil or a horticultural oil kill only the mites they wet, so wet every underside and plan on several rounds. Try the product on a small patch first and wait a few days for any leaf damage. Skip these sprays on a plant that is dry at the roots or in hot weather.
  4. Release predatory mites in a greenhouse. Phytoseiulus persimilis, the usual choice, eats every stage of the pest; it is pear-shaped, orange-red and noticeably quicker than its prey. Amblyseius californicus is weaker against a heavy attack but can live on pollen and other prey when spider mites are scarce, so it suits prevention. Lower heavy numbers with a soap spray first, and release predators onto sheltered parts of infested plants. Most pesticides will kill them.
  5. Discard a plant that is mostly bare. Its mites would otherwise keep moving to the plants around it.

Wiping leaves by hand achieves little against animals this small. Broad-spectrum insecticides tend to backfire: several pyrethroids and organophosphates, and carbaryl, wipe out the natural enemies and can even make the mites breed faster, so outbreaks commonly follow their use.

Stopping a return

Mated females shelter through winter in crevices: in stakes, canes, plant ties, fallen leaves and the potting mix. Clearing dead leaves and cleaning or replacing supports before spring removes many of them. Check new plants on arrival and keep them apart for a while, so any colony has time to show itself before it can spread. Tap-test suspect plants every week or so through the warm months.

Plants whose sources name spider mites

The list below comes from the site's data: every plant on it is one that RHS or NC State Extension lists spider mites against.

If the leaves are silvered and dotted with black rather than webbed, compare with thrips. To check for other causes at the same time, start the diagnosis.

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Sources

This page is written for this site from the sources above; it quotes none of them. It has not seen your plant.