Whitefly: how to recognise and treat it

Pest

Symptoms

Clouds of tiny white-winged insects, about 1.5 mm long, fly up when the plant is disturbed.
Flat, oval, creamy-white scale-like nymphs on the undersides of leaves; black nymphs have been killed by a parasitic wasp.
Sticky honeydew on leaves, stems and fruit, followed by black sooty mould.
Leaves yellow, dry or drop where many nymphs feed; some whiteflies cause silvering or spread viruses in vegetables.

Hose adults off with a strong spray of water and remove leaves heavily covered in nymphs.
Under glass, release the parasitic wasp Encarsia formosa before plants are heavily infested; the predatory mite Amblyseius andersoni eats the eggs.
Spray insecticidal soap, plant oil or natural pyrethrum on the undersides of the leaves; they act only on contact and briefly, so repeat as the label directs.
Hang yellow sticky traps among the plants to monitor numbers; they also catch the parasitic wasps, so use them with care alongside Encarsia.

Even strong insecticides only partly control whitefly and kill its natural enemies; avoid pyrethroids. As a last resort flupyradifurone, a systemic insecticide labelled for ornamentals and selected edibles, can be used — follow the label dose and pre-harvest interval.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Check new plants for whitefly and nymphs and keep them apart for a while before bringing them into the greenhouse.
Remove unwanted plants from the greenhouse, clear infested vegetable plants after harvest and clean the greenhouse in winter.
Ventilate the greenhouse well to slow sooty mould.
Under young vegetable plants, a reflective mulch repels whiteflies; remove it when the plants are large and the weather hot.

Figures checked against these sources.