PlantBy the time you see webbing, the colony is several generations old. Check leaf undersides weekly with a 10× lens and treat every 5–7 days for at least three rounds.
Why mites explode indoors
The two-spotted spider mite (Tetranychus urticae) is an arachnid, not an insect. Adults are 0.4–0.5 mm long, barely visible as moving specks, with two dark patches on the body. At 25–30 °C a mite goes from egg to egg-laying adult in about 7–10 days, and generations overlap, so a few mites on a new plant can become thousands within a month.
Warm, dry air suits them. Heated rooms in winter often sit at 20–35% relative humidity, which is ideal for mites and stressful for the plant. Frequent hosts include fiddle-leaf fig (Ficus lyrata), Alocasia, calatheas, English ivy (Hedera helix), Schefflera, croton (Codiaeum) and indoor palms. Because mites are arachnids, many general-purpose insecticides do little against them.
Read the early signs
Mites feed by piercing single leaf cells and emptying them, and each emptied cell shows as a pinprick of pale yellow or white on the upper leaf surface. This stippling is the first sign. As damage builds, leaves take on a dull, greyish or bronzed cast, then yellow and drop. Fine silk webbing between the leaf stalk and stem, or across new growth, only appears once numbers are high.
- Turn leaves over and scan along the midrib and veins with a 10× hand lens.
- Tap a branch over white paper; mites show as moving dots that smear green or reddish.
- Check the newest leaves and growing tips, where colonies often start.
- Inspect every plant whose foliage touches an affected one.
- Rule out dust, old sunburn and thrips, whose damage is silvery streaks with black specks.
Knock the numbers down physically
Move the plant away from the rest of the collection at once; mites spread by crawling across touching leaves and drift on air currents. Cover the pot surface with a plastic bag and shower the foliage with lukewarm water, directing the spray at the undersides. On large-leaved plants, wipe both sides of each leaf with a damp soft cloth.
Cut off leaves that are mostly bronzed and bag them before they leave the room. Wipe down the shelf, windowsill and pot. A thorough rinse removes a large share of the active mites, but the eggs stay glued to the leaf, so it is only the first step.
Treat on the mite’s calendar
Insecticidal soap (potassium salts of fatty acids) and horticultural oil kill by contact and leave no residual activity. Coverage of the leaf undersides is everything. Because eggs survive, repeat the treatment every 5–7 days for at least three applications so each new hatch is caught before it can lay. Test on one leaf and wait 24–48 hours first, avoid spraying in direct sun or above about 30 °C, and follow your local product label. Household dish detergent is not a formulated insecticidal soap and can scorch foliage.
For persistent or large infestations, the predatory mite Phytoseiulus persimilis eats only spider mites and can clear a collection biologically; it works best in moderate warmth and humidity and should not be sprayed with soap or oil after release. Raising humidity to 50–60% slows mite reproduction but will not eradicate a colony on its own. If you use a labelled miticide, rotate active ingredients, because mites develop resistance quickly.
Decide when it is really over
Old stippling never greens up again, so judge progress by new leaves and live mites. Declare the plant clean only after two consecutive weekly checks with a lens show no moving mites or fresh stippling, and keep checking its neighbours for another 3–4 weeks.
Afterwards, make a quick underside check part of your weekly watering round for mite-prone species, especially from late autumn, when the heating comes on and the air dries out.
