Whiteflies are the small, white, moth-like insects that rise in a cloud when you brush against an infested plant. They are a particular problem in greenhouses and on indoor plants, but they attack outdoor vegetables and ornamentals too, especially in warm weather. They suck sap, weaken plants, spread disease and leave sticky honeydew that encourages sooty mould. They are also persistent: the adults fly, the young hide on leaf undersides, and the eggs are hard to reach. Control takes patience and a combination of methods, not a single spray.
This guide covers how to identify whiteflies, what damage they cause, how they live, and how to control them organically.
Identifying whiteflies
Whiteflies are small — about 1–2 mm long — white, moth-like insects with powdery wings. They sit on the undersides of leaves, usually in groups, and fly up in a small cloud when the plant is disturbed. That cloud is the most reliable sign: brush a plant and watch for a puff of tiny white insects.
Look for:
- Tiny white insects on leaf undersides that fly up when disturbed
- Small, flat, oval, scale-like young (nymphs) stuck to the undersides of leaves — these are the feeding stage and do the most damage
- Yellowing, wilting or stunted leaves
- Sticky honeydew on leaves, often with black sooty mould growing on it
- A general loss of vigour, especially in greenhouse plants
Whiteflies are sometimes confused with aphids, but aphids are larger, pear-shaped and do not fly up in a cloud. Spider mites are far smaller and cause stippling with webbing. If the insects are white, winged and fly when disturbed, they are whiteflies. If you are unsure, our pest identifier helper can help you work through the symptoms, and your local extension service can confirm an identification.
The whitefly lifecycle
Whiteflies reproduce quickly in warm conditions. The female lays tiny eggs on the undersides of young leaves, often in a circle or crescent. The eggs hatch into crawlers that move a short distance, then settle down, lose their legs and become flat, scale-like nymphs that feed in one place. After several nymph stages, the adult emerges and flies off to lay eggs on another leaf.
In a warm greenhouse, the whole cycle can take as little as three to four weeks, and there may be several overlapping generations at once — eggs, nymphs and adults all present on the same plant. That is why a single spray rarely solves the problem: it may kill the adults but leave the eggs and nymphs untouched, and a new generation appears within days.
Outdoors, whiteflies are worst in warm summers and die back in cold weather, though they can survive mild winters in sheltered spots. In greenhouses and indoors, they can remain active all year, which is why they are such a persistent problem for protected crops.
Damage signs
Whiteflies damage plants in four ways. They suck sap, weakening the plant and slowing growth — a heavy infestation can stunt a plant and reduce yield noticeably. Their feeding causes leaves to yellow, wilt and drop. Their honeydew coats leaves and encourages sooty mould, which blocks light and looks unpleasant. And they spread plant viruses as they feed, which is the most serious damage — there is no cure for a virus-infected plant.
On tomatoes and cucumbers in the greenhouse, whiteflies are a major pest, reducing yield and spreading viruses such as tomato yellow leaf curl virus. On brassicas such as cabbage and kale, cabbage whitefly forms dense colonies on the undersides of leaves. On houseplants, whiteflies weaken the plant and leave honeydew on leaves and windowsills.
Organic-first control, in order
Work through these steps in order, starting with the gentlest. Whitefly control takes persistence — expect to repeat the steps for several weeks.
1. Inspect regularly. Check the undersides of leaves at least once a week, and more often in warm weather. Shake the plant gently and watch for the white cloud. Early detection is essential, because the lifecycle is fast and overlapping generations make late control much harder.
2. Use yellow sticky traps. Yellow sticky cards attract and catch adult whiteflies, and they also show you how bad the problem is. Hang them just above the plants in a greenhouse, or near affected houseplants. Traps alone will not control a heavy infestation — they catch adults but not the nymphs — but they reduce numbers and give you a way to monitor the population.
3. Spray with water. A strong jet of water knocks adults and nymphs off the leaves. It is less effective than for aphids, because whiteflies fly and the nymphs cling tightly, but it still helps, especially outdoors. Repeat every few days.
4. Remove badly affected leaves. If a few leaves are heavily infested with nymphs, remove them and drop them in a bucket of soapy water or a sealed bag — do not compost them in a cold heap. This removes a whole generation at once.
5. Encourage beneficial insects. The parasitic wasp Encarsia formosa is the classic biological control for whiteflies in greenhouses — it lays its eggs inside whitefly nymphs, and the parasitised nymphs turn black, which is a sign it is working. It is available from biological control suppliers and is highly effective in a greenhouse where temperatures stay warm. Outdoors, ladybirds, lacewings and other predators eat whiteflies, so avoid spraying anything that kills them.
6. Improve growing conditions. Keep plants healthy with steady watering and moderate feeding, and avoid overcrowding — good airflow makes plants less attractive and makes it harder for whiteflies to build up. Mulching helps keep watering steady. Avoid heavy nitrogen fertiliser, which produces soft growth. Our fertiliser calculator can help you avoid over-feeding.
7. Least-toxic sprays, last. If the steps above have not brought numbers down, insecticidal soap or horticultural oil can be used. These work by contact and smothering, so you must cover the undersides of the leaves thoroughly, where the nymphs are. Apply in the evening to avoid harming bees and to reduce leaf scorch, and repeat every five to seven days for at least three weeks to catch each new generation as it hatches. Always read and follow the label, use only products approved for the crop in your area, and wash produce before eating. Never use a broad-spectrum insecticide: it kills the predators and parasites that control whiteflies, and the whiteflies come back worse.
If you are unsure which product is appropriate, check your local extension service for recommendations in your area.
Prevention
Prevention is mostly about inspection and hygiene. Check new plants carefully before bringing them into the greenhouse or the house — whiteflies often arrive on a new purchase. Isolate suspect plants and check them for a week before placing them among your other plants.
Keep the greenhouse tidy: clear away old plant material where whiteflies can breed, and clean the greenhouse in winter. Ventilate well in summer — whiteflies prefer still, warm air. Plant flowers that attract predators, and consider using yellow sticky traps as an early-warning system even when you do not have a problem.
For houseplants, keep them in good light with steady watering, isolate any plant with whiteflies, and check the undersides of leaves weekly. See also our guide to fungus gnats, another common indoor pest.
When is it a serious problem vs cosmetic?
A few whiteflies on an outdoor plant in summer are cosmetic — the plant will usually cope, and predators and weather will keep numbers down. It becomes a serious problem when numbers are high and climbing, when leaves are yellowing and dropping, when honeydew and sooty mould cover the plant, or when greenhouse crops or young plants are affected. In a greenhouse, even a moderate population should be taken seriously, because the warm, sheltered conditions let numbers build fast and there are fewer predators to keep them in check.
Whiteflies on a single outdoor cabbage are a nuisance. Whiteflies covering every leaf of a greenhouse tomato crop are a serious problem. The difference is numbers, speed of increase and the setting — and weekly inspection is how you tell which one you have.
Frequently asked questions
What kills whiteflies instantly? Nothing kills all life stages at once. Sticky traps catch adults, water knocks them off, and insecticidal soap kills on contact — but the eggs and nymphs survive, so you must repeat treatments for several weeks to catch each generation.
Do whiteflies bite people or pets? No. They do not bite people or pets and are not poisonous. They are a plant problem, not a health problem.
Can I use washing-up liquid on whiteflies? It is sometimes recommended as a home remedy, but washing-up liquid is not formulated for plants and can damage leaves, especially in sun. Insecticidal soap, made for the purpose and used at the label rate, is safer and more reliable.
Will whiteflies go away in winter? Outdoors, cold weather kills most of them, though some survive in sheltered spots. In a greenhouse or indoors, they can remain active all year, so winter is the time to clean the greenhouse and break the cycle.
Are whiteflies the same as whitefly on houseplants? Yes — the same insects attack houseplants, greenhouse crops and outdoor vegetables. The control steps are the same, though biological control with parasitic wasps is most practical in a greenhouse.
The bottom line
Whiteflies are persistent because their lifecycle overlaps — eggs, nymphs and adults are present at once — so control takes several weeks of repeated effort. Inspect weekly, use yellow sticky traps, remove infested leaves, encourage predators and parasitic wasps, and use insecticidal soap only if the gentler steps have not worked. For more pest guidance, see the guides hub and the garden tools.