Whitefly are the greenhouse gardener's curse: shake a tomato plant and a cloud of tiny white insects rises, settles, and starts sucking the sap out of your crop. Outdoors they are a nuisance; under glass they can build into a plague — breeding year-round in the warmth, with no winter to knock them back.
This Plant Doctor guide covers identifying whitefly, the yellow sticky traps that both monitor and reduce them, the water blast for outdoor outbreaks, the predators that eat them (including the famous Encarsia wasp for greenhouses), and the airflow and hygiene that prevent the next infestation. Written for beginners, fully organic.
| Question | What to know |
|---|---|
| What are they? | Tiny white sap-sucking flies, 1–2 mm, on leaf undersides |
| Sign? | A white cloud rises when you shake the plant |
| Worst where? | Greenhouses — they breed year-round in warmth |
| Damage? | Weakened plants, sticky honeydew, sooty mould, viruses |
| First response? | Yellow sticky traps + blast with water |
| Best long-term fix? | Predators — Encarsia wasps under glass, ladybirds outside |
Step 1: Identify them — the white cloud test
Whitefly are unmistakable once you know the test: gently shake the plant — if a cloud of tiny white insects rises and resettles, it is whitefly. They cluster on leaf undersides, 1–2 mm long, looking like tiny white moths. Sticky honeydew on leaves below, and the black sooty mould that grows on it, confirm a heavy infestation.
The usual victims: tomatoes and aubergines under cover, peppers, brassicas outdoors in late summer, and houseplants. The nymphs — flat, oval, scale-like and motionless on the leaf underside — do most of the feeding damage, while the flying adults spread the infestation from plant to plant. Do not confuse them with aphids — aphids crawl, whitefly fly.
Step 2: Hang yellow sticky traps — monitor and reduce
Whitefly are drawn to yellow: hang yellow sticky traps just above the crop canopy and they catch the flying adults by the hundred. The traps do two jobs — they show you how bad the problem is (check weekly), and they steadily reduce the breeding adults.
Replace traps when covered, and position them close to — not touching — the foliage. Outdoors they catch some beneficials too, so use them mainly under glass, where the trade-off is worth it. Traps alone rarely solve a bad infestation, but they are the essential first move.
Step 3: Blast them off with water
For outdoor outbreaks, a strong jet of water aimed at the leaf undersides knocks whitefly off and disrupts breeding. Repeat every few days — whitefly breed fast, so one blast is never enough. It is free, instant and chemical-free, and it works on the same principle as the aphid water blast.
Under glass, use the hose on a fine-but-firm spray in the cool of the morning so foliage dries by evening — wet leaves overnight invite the fungal diseases tomatoes fear (see our blight guide for why dry foliage matters). Aim especially at the leaf undersides where the nymphs sit; a blast that only wets the tops of leaves misses the colony entirely.
Step 4: Release Encarsia wasps — the greenhouse secret weapon
Under glass, the professional organic answer is Encarsia formosa: a tiny parasitic wasp that lays its eggs inside whitefly nymphs, killing them. You buy them as cards of parasitised nymphs, hang them among the plants, and the wasps emerge and hunt. They are harmless to everything except whitefly — no sting, no bite, invisible in practice.
Release early, at the first sign of whitefly — predators control low numbers far better than plagues. One introduction in spring, topped up through summer, keeps greenhouse tomatoes and cucumbers clean all season. This is how commercial organic growers do it, and it works just as well in a home greenhouse.
Step 5: Recruit outdoor predators — and improve airflow
Outdoors, ladybirds, lacewings and hoverfly larvae all eat whitefly nymphs — the same companion flowers that recruit aphid predators work here too. Be patient with the lag: predators arrive after the pest, and the organic garden balances itself if you avoid broad-spectrum sprays.
Whitefly love still, humid air — ventilate greenhouses aggressively and space plants for airflow. Crowded, steamy plants under glass are a whitefly factory; airy ones stay far cleaner. Remove the worst-infested lower leaves and bin them (never compost) to cut breeding numbers fast.
Step 6: Winter hygiene — break the cycle
In a greenhouse, whitefly overwinter on weeds and leftover plants — clear the greenhouse completely at season's end, weed the borders and paths, and wash down staging. A clean winter break starves the population before next spring.
Check new plants before bringing them under cover: nursery-bought tomatoes and bedding plants are the classic way whitefly arrive. Quarantine newcomers for a week and inspect the leaf undersides — an ounce of inspection beats a season of infestation.
Troubleshooting: whitefly emergencies
Clouds of whitefly on greenhouse tomatoes: Hang yellow traps immediately, release Encarsia wasps, ventilate hard, and remove the worst leaves. Act in spring and you win; wait until midsummer and it is a siege.
Sticky leaves turning black: Sooty mould on honeydew — beat the whitefly and it weathers away. Wipe the worst leaves with a damp cloth in the meantime.
Sticky traps catching bees and hoverflies: Move traps fully under cover (not at open vents or doors) and keep them just above the crop, away from flowering companion plants. Outdoors, rely on water and predators instead.
Whitefly on houseplants: Isolate the plant, wash it thoroughly (leaf undersides especially), hang a small yellow trap nearby, and repeat weekly. Check every nearby plant — indoor colonies spread fast with no predators.
What to do this October
October is greenhouse clean-out month: strip out the last tomatoes and aubergines, bin (do not compost) any whitefly-infested foliage, weed the borders, and wash the glass and staging. This winter hygiene is next year's whitefly prevention.
Order Encarsia cards for spring now if you use them — suppliers sell out of the early-season introductions. See the full October checklist for the month's other jobs.
Frequently asked questions
Are whitefly worse than aphids?
Under glass, yes — they breed year-round in warmth and are harder to wash off. Outdoors, aphids are usually the bigger problem. The controls overlap: water, predators, no broad-spectrum sprays.
Do whitefly bite people or pets?
No — they feed only on plant sap and are harmless to humans and animals. The stickiness is honeydew, not anything dangerous.
Will cold weather kill them?
Outdoors, winter knocks populations right back — which is why they are mainly a greenhouse and late-summer problem. Under glass they never get that check, so you must supply the control.
Can I use soap spray on whitefly?
Insecticidal soap works on contact like it does for aphids — but whitefly adults fly away from the spray, so it mainly hits nymphs. Traps, water and Encarsia are the better primary tools.
Related guides
- Aphids on Plants: How to Get Rid of Them Naturally — the other sap-sucker
- Common Garden Pests and How to Stop Them — the full organic pest playbook
- How to Grow Tomatoes at Home: Beginner's Guide — whitefly's favourite victim
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