Red spider mite

A fine sprinkle of pale, silvery-white or yellow dots — called stippling, as if the leaf was dusted with tiny pin-pricks — shows up on the upper side of the leaves. As it worsens the whole leaf goes dull and bronzed or rusty, then yellows, dries at the tips and edges, and falls early. It almost always starts on the older, lower leaves and along the hot, dusty edge of the field.

— about the size of a speck of ground pepper, or the full-stop at the end of this sentence. Turn a stippled leaf over and look underneath, if you can with a hand lens (a magnifying glass): you'll find slow-moving oval mites, pale green to yellow with two dark spots on the back (they turn orange-red in the hottest, driest weather), sitting beside clear round eggs like tiny glass beads. In a heavy attack you'll also see fine silk webbing — like a thin, dusty cobweb — over the leaf undersides, in the leaf joints and over the growing tips, with mites crawling inside it.

Drought or hunger (a plant short of water or feed) also yellows the leaves, but leaves no dots and no mites. Thrips scratch silvery patches too, but they leave tiny black specks of dung and no webbing. Real spiders spin coarse, open webs and are your friends — mite webbing is fine, lies flat on the leaf, and is full of moving specks. When you are unsure, tap a leaf over a sheet of white paper: mites fall as 'dust' that then crawls — real dust just lies still.

Start scouting once the crop is growing well (the development stage) and keep it up through flowering, fruiting and pod-fill, right into the late, drying-down season — mites do the most harm in the hot, dry middle and end of the season, and on plants that are short of water or standing in dust. Look at least once a week, and twice a week during a hot, dry run.

so you cover the four corners and the middle — not just the row beside the path. Make about 10 stops. Begin at the hot, dusty edges and the driest corners: that is where the mites arrive first.

At each stop, pick a plant and check the older, lower leaves first. Hold a leaf up so the light shines through and look at the top for pale stippling; then turn it over and look underneath for the mites and any fine webbing. Tap the leaf over white paper or your palm — moving specks confirm the mites are alive and feeding.

Don't wait for webbing to cover the leaves: once the webbing is general the colony is huge, sprays can no longer wet the mites hiding underneath, and the crop is already losing ground. The first patches, caught early, are your cheap win.

Mites explode on thirsty, dusty, over-fed plants. Water enough that the crop never wilts, and in the heat wash the dust off the leaves with plain water — a dusty leaf is a mite nursery. Go easy on nitrogen fertiliser, because soft, over-lush growth feeds mites. And pull out the weeds in and around the field, since they carry mites over from one crop to the next.

The worst mite attacks come right after a broad-spectrum insecticide (a general bug-killer) that wipes out the mites' natural enemies. Avoid blanket insect sprays. Where you already see a few mites, pick off and destroy the worst-infested leaves to slow the spread before it takes hold.

Tiny predatory mites, ladybird beetles and lacewings hunt and eat spider mites for free. Protect them by not spraying broad poisons, and where you can buy them, release predatory mites onto the first infested patches. A firm spray of plain water onto the leaf undersides knocks the mites and their webbing off and raises the humidity that mites hate — a cheap first strike on a small patch.

If a patch keeps spreading despite all the above, use a proper acaricide (a spray made specially to kill mites) — never an ordinary insecticide, which kills the mites' enemies and makes things worse. The mites hide underneath, so drench the underside of the leaves until they are wet; the spray only kills the mites its droplets actually touch. Safety: cover your skin, eyes, nose and mouth; spray in the cool of the morning or evening with the wind at your back; keep it off food, water, animals, children and bees; wait the full number of days on the label between spraying and picking (the pre-harvest interval); and wash yourself and your clothes afterwards.

Spider mites build resistance fast — they survive a chemical used too often until it no longer kills them — so change to a different group of acaricide each time and never spray the same one twice in a row. If it keeps spreading after two rotated sprays, or webbing has already blanketed the plants, that block is beyond saving: cut and burn or bury the worst plants to protect the rest, keep the healthy crop well-watered, and start clean next season with weed-free, well-watered fields so the mites cannot carry over.