Pests: spotting and stopping spider mite, mealybug and fungus gnats
The three commonest enemies of tropicals indoors, how to recognise them before it is too late, and how to stop them without harsh poisons. Quarantining every new plant is half the work.
Three pests do ninety per cent of the damage to tropicals indoors. Spotting them early is almost everything: by the time you see them with the naked eye without looking, the infestation is already advanced. That is why rule number one is not a treatment, it is quarantine: every new plant that comes into the house spends two or three weeks away from the others, checked every few days. Most infestations arrive that way, on a bought plant, and reach the whole collection before anyone notices.
Spider mite
The most insidious because the smallest: dots the size of dust, usually on the underside of the leaves. The first signal is not the animal but the damage — a leaf stippled with tiny pale specks, as if sprinkled with sand, losing its shine. In advanced cases very fine webbing appears between stem and leaves. It loves heat and dry air: exactly the air of a flat in winter. You fight it by raising the humidity (it hates that), washing the leaves down with lukewarm water, and in serious cases with a specific miticide or neem oil, repeated every five to seven days three times over, because the eggs hatch in waves.
Mealybug and scale
Two forms. The mealy one: white tufts, like small flecks of cotton, tucked into leaf axils and along the veins. The scale: brown shells, round and flat, stuck to the stems, which look like part of the plant until you scrape them off. Both suck sap and leave a sticky honeydew on which sooty mould then grows. On a few plants you remove them by hand, with a cotton bud soaked in alcohol that dissolves the waxy shield. On widespread infestations, neem oil or a systemic, always repeated because the young escape the first treatment.
Fungus gnats
The little black flies that lift off when you move the pot. The adults are harmless and irritating; the problem is the larvae in the substrate, which in bad cases chew the young rootlets. But the real news they bring is different: fungus gnats thrive in permanently damp substrate. If you have them, you are almost certainly watering too much. Let the top layer dry properly between waterings and half the problem goes away on its own; yellow sticky traps catch the adults and tell you whether the population is falling.
The principle that holds for all three
Isolate the affected plant at once, so it does not reach its neighbours. Treat several times a few days apart, because no treatment kills the eggs, and eggs become adults in a week: a single treatment is a wasted treatment. And always check the underside of the leaves and the axils — that is where they hide, not on top where you look. I prefer the gentle remedies, neem oil and potassium soap above all, keeping the aggressive systemics for emergencies: in a collection at home, where you live and where there are sometimes animals and children, gentleness is worth more than power.