Spider mites: how to spot them and eliminate them from plants

What spider mites are

The common spider mite (Tetranychus urticae, often actually yellow-greenish despite the popular name) is a plant-feeding mite that sucks sap by piercing leaf cells. It multiplies at temperatures above 25°C and air humidity below 50%. Generations follow every 7-14 days, so a small infestation can explode in weeks.

Typical symptoms

• Yellowish or silvery stippling on the upper leaf surface, later turning brown.
• Overall leaf discolouration, as if faded.
• Fine white webbing between petiole and leaf, or in branch axils, in more advanced infestations.
• Leaves that dry from the edges and drop.
• Tiny moving dots on the underside of the leaves when viewed with a magnifying lens.

Most affected plants

Spider mites attack almost anything, but some plants are particularly vulnerable: calathea, maranta, ficus, dracaena, hibiscus, roses, tomatoes, beans, ivy. Thin-leaved plants with a large surface area are their favourites. Calatheas, especially, develop lightning-fast infestations when kept in air that's too dry.

How to eliminate them: step-by-step protocol

• 1) Isolate the affected plant immediately to keep the mites away from other plants in the house.
• 2) Give the plant a shower: under lukewarm water, lathering well with mild soap or potassium soap (5-10 g/L). Focus on the underside of the leaves where mites hide. Mechanical cleaning alone removes 70-80% of the mites.
• 3) Over the next 2-3 days treat with diluted neem oil (2-5 ml/L of water, with a teaspoon of soap as an emulsifier) or with potassium soap alone, spraying the whole plant — leaves and stems, upper and lower surfaces.
• 4) Repeat every 5-7 days for at least 3 cycles. Mites cycle every 7-14 days: stopping earlier means the eggs restart the infestation.
• 5) Sharply raise the air humidity around the plant: mist twice a day, keep the plant away from radiators, consider a humidifier in winter.
• 6) Manually remove severely damaged leaves (more than 60% yellowed or dry).

Natural remedies that work

• Potassium soap: effective, biodegradable, safe on the plant at the right dose.
• Neem oil: insecticidal and repellent action. Apply in the evening or out of direct light to avoid burns.
• Water + denatured alcohol (10-20%): useful for very localised infestations, applied with a cotton swab in branch axils.
• High humidity: spider mites don't tolerate air humidity above 60%. Raising humidity is an ongoing form of preventive control.

Prevention

• Keep air humidity at 50-60% around sensitive plants, especially in winter with heating on.
• Regularly inspect the underside of calathea, ficus and tropical plant leaves.
• Always isolate new plants for two weeks after bringing them home: mites almost always arrive from the nursery.
• Don't crowd plants together without air circulation: mites move easily from leaf to leaf.
• Periodically mist the leaves with water to create an environment mites dislike.

When a stronger intervention is needed

If after 3-4 cycles of natural treatment the infestation persists and the plant continues to decline, you can turn to specific acaricides (abamectin or bifenthrin based) available at specialist nurseries. Warning: these are chemical products to be used with gloves, outdoors and strictly following the instructions, and should be considered a last resort after natural options fail. If the acaricide doesn't work either, the plant is probably too compromised and it's wiser to save healthy cuttings and start over.