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. The sprays and predators are under spider mite killers.
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.
The Hotbox Sulfume uses sulphur. Its page says it is designed to stop the spread of fungal infections and "also has an unknown effect on spider mite that seems to put them off their food", so treat it as an extra, not a cure on its own. Keep sulphur and oil-based sprays apart, as our powdery mildew guide explains. Athena IPM's page lists peppermint, lemongrass and geranium oils among its ingredients, so check both labels before using the two on the same crop. Follow the instructions supplied with the Sulfume, and if you use predatory mites, check it is safe for them first.
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.
Quick answers
What do spider mites look like?
Like specks of dust that move. An adult is about half a millimetre long, pale green or yellowish with two dark spots, and can turn orange-red in autumn and winter. You usually see the damage first: pale speckled leaves, then webbing. The white-paper test under Signs of spider mites, above, shows whether the specks crawl.
Are white spider mites the same as red spider mite?
Usually, yes. For most of the year the two-spotted mite and its young are pale enough to look white at a glance, and white specks under the leaves are often its eggs and shed skins. Tiny white insects that fly up when you brush the plant are whitefly, not mites: see our guide to aphids and whitefly.
What about black spider mites?
Probably two-spotted mites seen without a lens: the two dark spots can make one look black at a glance. Close up with a hand lens, the spots sit on a paler oval body with eight legs. Black specks that don't move, on silvery patches, are more likely thrips droppings, and long, thin dark insects are more likely adult thrips - see thrips on plants.
Do spider mites live in soil?
Not as a rule. They live and lay their eggs on the leaves, mostly underneath, not in the soil or compost, so mites you see running over the compost, with none on the leaves, are probably something else: some mites live there, like the predatory mites in our fungus gnats guide. Between crops, spider mites shelter in fallen leaves, old pots and tent seams, which is why you clean the empty room and use an Insecto Fumer, as well as spraying.
What do spider mite eggs look like?
Tiny, round and clear to pale cream, laid under the leaves, often beneath the webbing. In a hot room a mite can go from egg to egg-laying adult in around a week, and the eggs survive many sprays, so new mites hatch days after you spray. Athena IPM's page says it kills all stages, eggs included, on contact, yet still asks for at least three sprays three days apart. With any spray, repeat every few days as the label says, and keep checking until no new ones appear.
Where do spider mites come from?
Usually from outside the room, or from mites still hiding in the room, the tent or the kit from the last crop. They can't fly, so new ones mostly arrive on new plants and cuttings, from garden plants kept nearby, or on your clothes after working outside in summer. Once in, warm, dry air lets them breed fast: see the Keeping spider mites out list above.
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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