Red spider mite

What you are looking at

The first sign is a change in the surface of the leaf rather than anything you can point at. A monstera or a fig that was glossy goes dull, and close up the upper side carries a fine pale speckling, as though someone has dusted it. The dust does not wipe off, because each speck is a group of leaf cells that has been emptied from below.

Turn the leaf over. An adult is about half a millimetre long, oval, pale yellowish-green with a dark blotch on each side of the body. Those two blotches are gut contents showing through a translucent animal, and they are what gives it the name two-spotted spider mite. At that size the naked eye reports specks of grit.

The test that settles it costs nothing. Hold a sheet of white paper under a shoot and tap the stem hard. Wait five seconds. Grit stays where it lands and mites start walking. Drag a finger through them and the smear is green.

Webbing comes later and means the population is already large. Fine silk runs between the leaf stalks and the growing point, easiest to see against a window, and in a bad case the whole shoot tip is wrapped and the mites are moving over the outside of it.

Why the name misleads

A spider mite is an arachnid, closer kin to a tick than to anything with six legs, and it does spin silk, so half the name earns its place. The colour is where it goes wrong. Adults are yellowish-green through spring and summer, and the fertilised females turn orange-red in autumn as they leave the plant to overwinter. Someone looking for red mites in July finds nothing and concludes the plant has something else.

Eight legs rather than six puts it with the spiders, ticks and scorpions rather than the insects. That is a practical point rather than a taxonomic one, since products developed against insects are frequently poor against mites, and resistance in this species is among the worst documented in any plant pest.

Why it explodes in a heated room in winter

Temperature runs the whole thing. At 30 degrees C a mite goes from egg to egg-laying adult in about a week; at 21 degrees it takes a fortnight; below about 12 degrees it stops. A female lays somewhere around a hundred eggs.

A centrally heated sitting room in January sits at 21 degrees with humidity down near 30 per cent, which is closer to the conditions this mite evolved in than a British garden ever gets. That is why the houseplant most likely to be ruined by red spider mite is ruined in February, while everything else on this site is dormant, and why the plant nearest the radiator goes first.

Dry air is half of it. The mite loses water quickly at high humidity and its eggs do badly there, so raising the moisture around a plant is a control measure rather than a comfort measure.

What the damage adds up to

Each mite empties individual cells with a pair of needle-like mouthparts, and one puncture is nothing. The speckling merges as the count rises, the leaf loses the ability to photosynthesise across whole regions, and it goes bronze and falls. A plant can drop most of its foliage in a month.

Outdoors this rarely ends a plant, because predators find the colony and rain knocks it about. On a fig or a calathea in a warm room, with no predators and no rain, the same population runs unchecked until the plant has nothing left. Take an indoor infestation more seriously than an outdoor one.

What to do about them

Change the air before you change anything else. Move the plant off the radiator shelf, stand it on damp gravel, group it with others, and run a humidifier through the heating season if you keep a large collection. A colony that was doubling every week slows to something the plant can carry.

Then shower it. Tepid water over the leaf undersides, weekly for a month, removes most of the adults each time. The eggs stay, which is why one wash achieves nothing and four in a row achieve a great deal. Take off the bronzed lower leaves at the same time and bag them.

Under glass, buy predators. Phytoseiulus persimilis eats nothing else, breeds faster than its prey above 21 degrees C, and will clear a greenhouse. Order it when you find the first stippling rather than when the leaves start dropping, because it needs a living population to work through and a plant that has already collapsed gives it nothing to eat.

Why no spray is listed here

This mite has developed resistance to more classes of pesticide than almost any other plant pest, and a treatment that kills 95 per cent of a colony leaves the 5 per cent that shrug it off to rebuild in a fortnight. Repeated spraying selects for exactly that.

The fatty acid and plant oil products sold for it work by contact and by smothering, so they need to reach the underside of every leaf, and they kill introduced predatory mites as efficiently as they kill the pest. Releasing Phytoseiulus and then spraying is money spent twice to no effect.

What it gets mistaken for

Thrips also silver the leaf, but they leave small black dots of excrement among the damage and the injured tissue often has a metallic sheen. No webbing.

Underwatering and leaf scorch both dull a leaf, and both affect it evenly rather than as a scatter of tiny pale points. Look at the leaf against a light: mite damage is stippled at a scale of tenths of a millimetre, and stress damage is not.

Fine webbing with no stippling and no mites is a spider that has taken up residence, which is a different animal and does no harm at all. The webbing to worry about is dense, dusty and stretched between the leaf stalks and the growing tip rather than spanning a gap.