Spider mites
Tetranychus urticae
also called Red spider mite, Two-spotted mite
Near-invisible mites that stipple leaves yellow and spin fine webbing in hot, dry conditions. Raise humidity, mist and rinse foliage, and introduce predatory mites under glass.
How to know it
Specks barely 0.5 mm long on the leaf undersides — straw-yellow with two dark shoulder spots in summer, orange-red in the overwintering form, despite the name. Hold a leaf against the light or tap it over white paper and the dots walk.
What you'll see
- A fine pale stippling or sand-blasted mottling over the upper leaf surface.
- Leaves turn dull yellow-bronze, dry, and drop early from the bottom of the plant up.
- Fine silk webbing between leaves, over shoot tips and along stems in bad attacks.
- Dust-fine specks moving on the leaf underside when you look with a hand lens.
- Whole plants under glass reduced to bare stems and webbing in a matter of weeks.
Where it comes from
A generation takes as little as a week at 30 °C, and each female lays over a hundred eggs — the reason an outbreak seems to appear from nowhere. Mites disperse on air currents, on clothing and on tools, and overwinter as red females in greenhouse crevices, under bark and in debris.
Hot, dry, still air is everything: greenhouses in high summer, conservatories, house plants over a radiator, and drought-stressed plants against a sunny wall. Humidity and rain knock populations back hard.
Dealing with it
- Raise the humidity: damp down floors, mist foliage morning and evening, and rinse leaf undersides with plain water.
- Introduce Phytoseiulus predatory mites early, while the infestation is still light — they clear it completely and cost nothing thereafter.
- Cut off and bin the worst webbed shoots rather than trying to save them.
- Water thoroughly and mulch; a plant that is not drought-stressed shrugs off far more mite feeding.
- Avoid broad-spectrum sprays entirely — they kill the predators and mite numbers rebound higher than before.
Keeping it out
- Ventilate and shade glasshouses in summer, and never let plants dry out between waterings.
- Clean out greenhouse crevices, canes and debris in winter, where the red overwintering females sit.
- Quarantine new plants and inspect leaf undersides with a lens.
- Do not stand houseplants over radiators or in hot dry window bays.
- Introduce predatory mites at the start of the season under glass, not after the webbing appears.
What it goes for
Cucumbers, tomatoes, aubergines, peppers, French beans and strawberries under glass; outdoors on fruit trees, roses and conifers in hot summers.
- Abiu
- Alocasia
- Anthurium
- Apple
- Areca Palm
- Asparagus Fern
- Atemoya
- Australian Native Ginger
- Avocado
- Banana Plant
- Blue Eyed Grass
- Breadfruit
- Breath of Heaven
- Brugmansia
- Caladium
- Calathea
- California Pipevine
- Canna Lily
- Capers
- Cast Iron Plant
- Cherimoya
- Chilean Guava
- Chilean Potato Vine
- Chinese Evergreen
and 93 more in the catalog
Who hunts it for you
- Big-eyed bugGeocoris spp. A stubby 3 mm bug with enormous wrap-around eyes, working the soil surface and the low foliage for aphids, mites, thrips and insect eggs. It is the main natural check on chinch bugs in a lawn and it is killed outright by insecticide. Grow to recruit it: Calendula,Clover,Cosmos,Goldenrod,Sweet alyssum
- Green lacewingChrysoperla carnea Its bristly larvae — "aphid lions" — impale aphids, mealybugs, thrips, and mite colonies, eating hundreds a week. Adults feed on nectar and pollen from small open flowers and overwinter in hollow stems left standing. Grow to recruit it: Cilantro,Cosmos,Dill,Sunflower,Yarrow
- Ladybird (lady beetle)Coccinellidae The gardener’s favorite ally: adults and their alligator-shaped larvae each eat thousands of aphids, plus scale, mealybugs, and other soft-bodied sap-suckers. Grow flat, open flowers for the adults and never spray a colony that ladybird larvae are already working. Grow to recruit it: Calendula,Cilantro,Cosmos,Dill,Fennel,Yarrow
- Minute pirate bugOrius spp. A pinhead-sized true bug that is the best natural check on thrips, and happily spears aphids, mites, and insect eggs too. It needs pollen when prey is scarce, so keep something flowering nearby. Grow to recruit it: Cosmos,Fennel,French marigold,Sweet alyssum
- Predatory mitesPhytoseiulus & Amblyseius Fast orange mites that hunt spider mites egg-by-egg, and cousins that clear thrips and fungus-gnat larvae. Sold as a biological control for greenhouses; outdoors they follow humidity — mist and mulch, don’t dust with sulfur.
- Rove beetleStaphylinidae Slim, quick beetles with short wing cases that hunt in soil and litter, taking root-fly eggs and larvae, fungus-gnat larvae, and mites. Mulch and undisturbed ground keep them resident year-round. Grow to recruit it: Mint,Thyme
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com