When leaves turn dull and speckled without any fungus or aphid in sight, an almost invisible foe is often behind it: the spider mite. It is barely half a millimetre across, multiplies at breakneck speed in heat, and is often noticed only when fine webbing already coats the shoots.
The key to it lies in its favourite weather. Spider mites are indicators of dryness. Understand that, and with a single move you turn the tables, because against them the very thing helps that you otherwise avoid in the bed: moisture on the leaf.
A pest of dry heat
The two-spotted spider mite, commonly called the red spider, is not a spider but a tiny eight-legged mite. It pierces the leaf cells and sucks them empty, which creates the typical stippled pattern. In heat it runs through its generations in fast motion; single animals become a plague in two weeks.
That is exactly why it appears where it is warm and dry: in the greenhouse, on cucumbers and tomatoes in a hot summer, on roses and fuchsias, and especially on houseplants over the radiator in winter. Damp, cool air, by contrast, it dislikes entirely, and that is your most effective weapon.
How to recognise spider mites
Because the creatures are so small, you usually spot them first by the damage. A few signs lead you safely to the diagnosis.
The telltale signs
- Pale stippling on the leaf
Fine, silvery-pale dots on the upper leaf surface, as if the green were bleached. Later the whole leaf turns dull, yellows and dries up.
- Fine webbing
Delicate silk threads stretch across leaf undersides, in the leaf axils and at the shoot tips. They are the sure sign of an advanced infestation.
- Tiny moving dots
On the leaf underside crawl pinhead-sized yellowish or reddish dots. With a magnifier you can make out the eight legs.
- The paper test
Hold a sheet of white paper under a shoot and tap it. If tiny dots wander about on it, you have your proof.
What helps against spider mites
Raise the humidity
The most important step. Mist the plant regularly with water, especially the leaf undersides. Humid air is hostile to spider mites and slows their breeding at once.
Shower the plant
Shower affected plants down firmly, from below too. That simply washes away most of the mites and webbing. On houseplants, a bag over the plant afterwards keeps the humidity up for a few days.
Remove heavily affected leaves
Cut out fully webbed, yellowed leaves and dispose of them in the residual waste. That takes pressure off the infestation before you go on.
Release predatory mites
In the greenhouse and on houseplants, predatory mites are the most effective biological weapon. They eat spider mites and their eggs and regulate the infestation for good.
Oil or soap sprays as a last step
If that is not enough, a spray of rapeseed-oil products or potash soap helps, coating and suffocating the mites. Wet the leaf undersides thoroughly and repeat after a few days.
Spider mites love it dry and hot. Keep the air humid and shower them off, and you take away their basis of life.
The core rule against spider mites
Frequently asked questions
How do I reliably recognise spider mites?
By the pale stippling on the upper leaf surface and the fine webbing on the undersides and shoot tips. The paper test gives certainty: tap a shoot over white paper, and tiny crawling dots give the mites away.
Why do I get spider mites on houseplants in winter?
Because the heated air is dry and warm, exactly their favourite climate. Keep the plants cooler and more humid, mist them regularly and keep them away from the radiator. That is the best prevention.
Does spraying the plant with water really help?
Yes. Unlike with fungal diseases, moisture is wanted here. Regular misting and showering disturbs the mites hugely and washes them off along with the webbing. On houseplants a bag over the plant provides a few damp days.
What are predatory mites and where do I get them?
Predatory mites are tiny beneficials that eat spider mites and their eggs. You get them from specialist suppliers and release them onto the affected plant. In the greenhouse and indoors they work most reliably.
