Identification
Tiny (0.3–0.5 mm) arachnids, hard to spot with the naked eye, feeding on the underside of leaves. Symptoms: fine, pale speckling (feeding punctures); over time the leaves turn grey, yellow and dry out. Under heavy infestation a fine webbing becomes visible. Warm, dry air favours their development — a common problem for houseplants in winter near radiators. Spider mites are not insects but mites of the family Tetranychidae — adults have 8 legs (larvae only 6). The most common in cultivation is the two-spotted spider mite (Tetranychus urticae): yellowish-green females carry two dark spots on the sides of the body, while overwintering females turn orange-red. In orchards the fruit tree red spider mite (Panonychus ulmi) is frequent, whose red eggs overwinter in clusters on the bark of the shoots. Feeding mechanism: the mites puncture the cells of the epidermis and mesophyll on the underside of the leaf and suck out their contents, and the emptied cells fill with air — hence the characteristic, silvery-grey, fine speckling visible from the upper side of the blade (resembling a mosaic discolouration). At high numbers the leaves turn brown, curl and drop prematurely, and the whole plant weakens and loses its growth. Dense webbing spun between the leaves and shoot tips appears only under an advanced attack and is a sign of the risk of the pest spreading to neighbouring plants. The development cycle is very fast in warmth and drought — at a temperature of 25–30°C a single generation can be completed in 1–2 weeks, hence the numerous generations per season and the rapid, exponential build-up of the population. Females lay spherical, transparent eggs on the underside of the leaves. Fertilised females overwinter (in bark crevices, leaf litter and plant debris) or — in the fruit tree red spider mite — the eggs on the bark. Early infestation is easy to miss; diagnosis is helped by shaking a leaf over a white sheet of paper (moving specks become visible) or examining the underside of the leaf with a magnifying glass.
Treatment
Raising air humidity (misting, a humidifier), washing the leaves with water, potassium soap or oil-based preparations. In biological control, predatory mites are used. Remove heavily infested leaves. Prevention and ecological methods: maintaining higher air humidity (misting, a humidifier, moving houseplants away from radiators in winter) genuinely limits their development, because dry heat is the main driver of the rapid multiplication. Regular, forceful rinsing of the leaves with water — especially from below — mechanically reduces the population and removes the webbing. Newly bought plants are worth quarantining, because spider mites move easily between pots. These are supporting and preventive methods — they slow the pest down, but under an advanced infestation they are not enough on their own. Natural enemy: predatory mites effectively regulate the population under protected cultivation and on houseplants — Phytoseiulus persimilis in warmth and high humidity, Amblyseius (Neoseiulus) californicus in somewhat drier conditions. Introduce them while the infestation is still small, because the predator needs time to get ahead of the pest's development. Chemical control relies on acaricides (mite-killing products) with different modes of action: contact ones, those inhibiting egg hatch and moulting, as well as on sulphur products (sulphur has an acaricidal action) and oil products. Key rule: spider mites develop resistance to products very quickly, so the modes of action must be rotated (do not keep repeating the same group), and the liquid must be directed precisely onto the underside of the leaves, where the mites feed. Broad-spectrum insecticides should be avoided, especially pyrethroids — they do not control mites effectively but wipe out their natural enemies, which leads to a secondary outbreak of the spider mite population. Similarly, sulphur products should not be used alongside the introduction of predatory mites — sulphur also harms predatory mites and would undo the biological control. Hygiene: remove and destroy heavily webbed leaves and shoots, and in autumn clear away the plant debris that serves as an overwintering site for the females.
How to tell it apart from look-alikes
| Feature | Spider mites | Thrips |
|---|---|---|
| Causal agent | Mites (arachnids), 0.3–0.5 mm, 8 legs, on the underside of the leaf | Insects, 1–2 mm, elongated, mobile — adults jump and fly |
| Symptom on the leaf | Fine pale punctures turning into silvery-grey speckling, then browning and drying out | Silvery-grey streaks and patches, deformation of young leaves and flower buds |
| Webbing | Under heavy infestation, fine, spun between the leaves and shoots | None — thrips do not produce webbing |
| Droppings | None distinct | Black, glossy specks (droppings) on the leaf surface |
| Feeding site | Underside of the leaves, usually the lower and older ones first; colonies migrate towards the shoot tips | Young leaves, buds and flowers — favouring sheltered, developing parts of the plant |
Control — organic and chemical methods
Organic and biological methods
| Method | How it works | When to use |
|---|---|---|
| Raising air humidity | Dry heat drives mite multiplication; misting, a humidifier and moving plants away from radiators slow development and make colonisation harder. | Preventively, especially in winter for houseplants |
| Mechanical washing with water | A strong jet of water or a shower rinses the mites and webbing off the underside of the leaves, mechanically lowering the population. | From the first symptoms, repeat the treatment |
| Predatory mites (beneficial mites) | Phytoseiulus persimilis and Amblyseius californicus eat the eggs and mobile stages of spider mites, regulating the population under protected cultivation. | Introduce at a low level of infestation |
Chemical methods
| Method | How it works | When to use |
|---|---|---|
| Potassium soap and oil products (contact) | They act by contact — coating and clogging the mites; they need thorough application to the underside of the leaves and work poorly on eggs, so repeat treatments are necessary. | Every 5–7 days, several treatments |
| Sulphur products (sulphur) | Sulphur has an acaricidal action and limits multiplication; use with care on sensitive species and in hot weather because of the risk of phytotoxicity. | In warmer but not hot weather; do not combine with the introduction of predatory mites |
| Selective acaricide with mode-of-action rotation | Mite-killing products with different modes of action (contact ones, those inhibiting egg hatch and moulting); rotate the groups against resistance and target the underside of the leaves. | Under heavy infestation, do not repeat the same group |
We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.
Sources
- Atlas of ornamental-plant pests
