Red Spider Mite: How to Spot It and Get Rid of Spider Mites

Fine pale speckles on the leaves and, later, delicate webbing at the tips: red spider mite is one of the most damaging pests in grow rooms and greenhouses. It breeds fast in warm, dry air, so the sooner you spot it the easier it is to deal with.

What red spider mite looks like

The "red spider mite" of UK greenhouses is the two-spotted spider mite. It's tiny, about half a millimetre, so you need a magnifying glass to see one clearly. For most of the year it's pale green or yellowish with two dark spots on its back; it turns orange-red in autumn and winter, which is where the name comes from. The mites live on the undersides of leaves.

Signs of spider mites

Fine pale or yellow speckling on the upper side of the leaves, where the mites have fed underneath.
Leaves turning dull, yellow or bronze, then drying out.
Fine silky webbing between leaves and around the growing tips when numbers are high.
Tiny moving dots on the undersides of leaves. Tap a leaf over white paper and watch for specks that crawl.

Why they spread so fast

Spider mites love warm, dry air. In a hot room they can go from egg to egg-laying adult in around a week, so a few mites become thousands within weeks. Their eggs also survive many sprays, which is why a single treatment rarely works.

How to get rid of spider mites

Change the conditions. Mites hate humid air. Raising the humidity and bringing the temperature down slows them down. A hygrometer tells you where you are, and humidity control gets you where you need to be. Be careful late in flowering, when high humidity brings a risk of mould.
Clear the worst of it. Remove badly hit leaves and wipe or break up the webbing before spraying, so the spray reaches the mites.
Spray thoroughly. Dr Hydro Spidermite is made for red spider mite: its instructions are 50 ml of concentrate per litre of water, sprayed until it runs off, top and bottom of every leaf, then checked every three days with a hand lens. SB Plant Invigorator and Athena IPM are sprays whose makers say they kill spider mites or soft-bodied insects; Athena IPM's label allows it up to week 5 of flowering, at least three sprays three days apart. A pump sprayer makes full coverage much easier.
Or dust. Desi Dust is diatomaceous earth, which its maker says kills spider mites by damaging their outer shell.
Repeat. Treat again every few days, as the label says, to catch the mites that hatch from eggs the last treatment missed. Keep checking the undersides of leaves for a couple of weeks after you stop seeing them.
Between grows. With the room empty, Insecto Fumers kill spider mites hiding in the room, the tent and the kit. Never use them with plants present.

There is also a predatory mite that eats spider mites and is widely used in greenhouses; specialist suppliers sell it. It needs warmth and humidity, and many sprays harm it, so check before combining the two.

Keeping spider mites out

Check new plants and cuttings closely before they come into your room. Mites often arrive on them.
Keep garden plants and your grow room apart, and change clothes after working outside in summer.
Keep air moving with clip fans and avoid long spells of hot, dry air.
Clean the room, tent and equipment between grows - see our cleaning products.

Whatever you use, read the label first. It gives the dose, how often to repeat it, whether it can go on plants you'll eat, and how close to harvest you can use it.

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