Spider mites: how to recognise and control them naturally

Spider mites are pests of dry, warm air. That is why they hit greenhouses, conservatories and windowsills first, and cucumbers, aubergines, beans and chillies hardest. Outdoors they usually appear only during long hot spells.

The tricky part: by the time you see the fine webbing, the infestation is already well advanced. Checking the upper leaf surface early saves you half the work.

In short

Spider mites are under 0.5 mm and sit on the underside of leaves.
The first sign is fine pale stippling on the upper leaf surface, not the webbing.
The trigger is dry, warm air, often combined with drought stress at the roots.
Raise humidity, hose the plants down, and use soft soap with a little rapeseed oil if needed.
In a greenhouse the predatory mite Phytoseiulus persimilis is the most reliable answer.

What are spider mites?

Spider mites are not insects but arachnids. The most common species in our gardens is the two-spotted spider mite (Tetranychus urticae). Adults measure 0.3 to 0.5 mm, are greenish yellow to reddish, and are barely recognisable as animals with the naked eye — you see something more like dust that moves.

They suck out individual leaf cells. At 25 to 30 °C in dry air a generation takes barely a week, and a female lays around 100 eggs. That speed explains why an infestation seems to explode overnight.

How do I recognise spider mites?

Stippling: fine pale to silvery dots on the upper leaf surface, as if pricked with a very fine needle. This is the earliest and most important sign.
Discolouration: later the dots merge, and the leaf looks dull, bronzed or dusty.
Webbing: fine threads in the leaf axils and at shoot tips — already a sign of heavy infestation.
Paper test: hold a leaf over white paper and tap it gently. The specks that fall start moving after a few seconds.
Hand lens: check the leaf underside at 10× — you will find animals, eggs and moulting skins.

How bad is the damage?

Every emptied cell drops out of photosynthesis. In a heavy infestation the plant loses so much leaf area that growth and fruit set collapse. Cucumbers and aubergines can break down completely within two to three weeks.

Stress comes on top: a weakened plant is more vulnerable to everything else, and the fine feeding wounds are entry points for fungi. Spider mites are rarely the only problem once it has gone that far.

Prevention: humidity is the lever

Keep humidity up. In the greenhouse, wet the paths in the morning and mist the plants occasionally. Above 60 per cent relative humidity the mites find it distinctly uncomfortable.
Avoid drought stress. Plants whose root ball dries out regularly are almost always hit first. Water evenly and mulch.
Ventilate and space out. Still, hot air between closely packed plants is ideal for mites.
Check new plants. Bought house and container plants bring spider mites with them. Keep them apart for two weeks and inspect the leaf undersides.
Do not overfeed. Plenty of nitrogen makes soft tissue that is easier to pierce.

Control without chemicals

Hose down: spray the plant thoroughly with water, explicitly including the leaf undersides. This washes off a large share of the animals and raises humidity as a side effect. Repeat after two to three days.
Soft soap solution: about 20 g of pure soft soap per litre of water plus a teaspoon of rapeseed oil. Spray leaf undersides until dripping wet, three or four times at four to five day intervals — only then do you also catch the ones hatching afterwards.
Remove badly infested leaves and dispose of them in household waste, not on the compost heap.
Humidity chamber for container plants: water the plant, then wrap it loosely in a clear bag for three to four days. The high humidity brings the population to a halt.

Tip

Never spray in full sun or onto hot leaves — you will scorch them. Early morning or evening is right. With sensitive species, test the soap solution on a single leaf first and wait a day.

Beneficials: the most reliable answer under glass

Phytoseiulus persimilis is a predatory mite that lives exclusively on spider mites. It arrives on cards or in carrier material and is placed into the affected plants. From about 20 °C and with sufficient humidity it works very fast — a moderate infestation is usually cleared in two to three weeks.

In cooler or drier conditions Amblyseius californicus is the more robust choice. It eats more slowly but persists when prey is scarce, which makes it suitable for prevention.

Caution

Beneficials and soap solution do not mix. If you release predatory mites, you must stop spraying afterwards — including natural preparations. Decide on one route, or hose down thoroughly first and release the predators a few days later.

Frequently asked questions

How do I tell spider mites from thrips?

Thrips damage shows as silvery glossy patches with tiny black specks of excrement in them; spider mite damage is even, fine stippling without those specks. And only spider mites produce webbing.

Can spider mites overwinter in the greenhouse?

Yes. The females overwinter red-coloured in crevices, under stakes and in soil material. Clear out thoroughly in autumn, remove plant debris and wash down the structure.

Does a nettle or garlic brew help?

Not reliably on its own. It may strengthen the plant and act as a mild deterrent, but it replaces neither hosing down nor soap solution nor beneficials.

Are the fruits still edible?

Yes. Spider mites are entirely harmless to humans, and the fruit can be picked and eaten as usual. The damage is purely horticultural.

Our young plants grow in the greenhouse under regular inspection — so no infestation travels into your garden and you start with a clean stock.