Whitefly on cabbage and tomato: how to control without chemicals

What is whitefly and why is it a problem?

Whitefly (Trialeurodes vaporariorum) are tiny whitish insects barely one to two millimeters long with delicate white wings. They hide on the underside of leaves where they breed with astonishing speed. A single female lays about a hundred eggs per week. Within three weeks these hatch into adult flying insects causing enormous damage. The adults suck plant sap and secrete honeydew, a sticky liquid that drips onto leaves below. There black sooty mold grows on it, suffocating the leaf. Your plant weakens, growth stalls, and in the greenhouse a population explodes because no wind, no rain and no natural enemies disturb the pest paradise. In the vegetable garden outdoors whitefly is much less serious because wind, rain and predatory insects naturally control it. But in the greenhouse and conservatory without intervention it becomes a disaster. You have two to three generations per season so waiting does not help. The good news: you can control whitefly without chemicals through a combination of physical measures, beneficial insects and prevention.

Identification: what does whitefly look like?

The first signs are not the fly itself but the damage. Leaves turn yellow, feel sticky, and when you touch them suddenly tiny white specks fly away. These are adult flies. They love the underside of young leaves near the growth point. Look at that spot carefully with a magnifier and you see yellow-green discs without legs, glued to the leaf like scales. These are larvae, the most damaging stage. Eggs are microscopically small so invisible. But larvae you see, and they suck plant sap and secrete honeydew that suffocates useful leaf space.

The first sign is therefore yellowing undersides of leaves, especially in July and August. Sticky surfaces follow quickly. This happens faster in warm humid conditions without air movement. In the greenhouse you can see this accelerate to a plague of hundreds of thousands of insects within three weeks.

Yellow sticky traps: your first warning system

Yellow sticky traps are your first defense and your only way to know when whitefly enters the greenhouse. Whitefly is attracted to yellow and sticks fast as soon as it lands. No chemistry, no gases, purely mechanical. Hang four to six traps per hundred square meters of greenhouse, level with plant tops or just above. Check them daily: count how many flies stick. Up to five per day: no worry yet. Ten per day: start prevention now. More than twenty: active control needed.

Replace your traps every two weeks or as soon as they are full of dead material and mold. Moldy traps no longer work. This system gives you two benefits at once. First, traps catch adult insects and directly lower the population. Second, they signal how fast the pest grows so you know whether other measures work. With yellow traps you are always one step ahead of the plague.

Insect netting: physical barrier against wings

Insect netting with one point five millimeter mesh is the gold standard. It is fine enough that no adult whitefly passes through, but open enough for good ventilation. Place it over all greenhouse openings: windows, air slots, intakes, ventilation openings, even chimneys. Make sure everything is hermetically sealed. A hole of just one square centimeter lets thousands of flies through per day.

Check your netting regularly for damage. A small tear grows quickly. Repair damage immediately with tape or replace the netting. This is old-fashioned technology but incredibly effective. You still have air and water that can pass through, but no insect. Combine this with yellow traps so you see if new flies somehow enter and the netting therefore leaks.

Encarsia formosa: the parasitic wasp as ally

Encarsia formosa is a parasitic wasp of six-tenths of a millimeter, too small to see without a magnifier, but a killer for whitefly larvae. It lays eggs in larvae. The larva dies and a new wasp emerges. This is called parasitism: mutual no benefit, only whitefly loses. This wasp does not work fast, it requires patience. Within four to six weeks the plague declines significantly if you avoid other toxic chemicals that also kill the wasp.

Encarsia needs minimum twenty-one degrees Celsius. Below fifteen degrees it does nothing. You must therefore have summer temperatures. You order cards with pupae through specialized suppliers (about thirty euros). Hang them among infested plants. They find their way to whitefly larvae. Repeat every two weeks with small numbers, ten to twenty wasps per ten square meters. This works best combined with yellow traps because you see if it helps.

Marigolds: discreet but effective

Marigolds (Tagetes) are annual flowers with strong aromatic scent that confuses insects. Plant them along your tomato and cabbage rows, every thirty centimeters. They grow fast, are tough and their scent deters whitefly from your crops. It does not work as sole strategy, the plague grows underneath, but as part of your overall approach it helps. Extra benefit: they attract pollinators so your tomatoes and cabbage are better pollinated. They give you color in the greenhouse every day too. Not sure where marigolds and other companion plants fit best between your rows? On gardenworld.app you see that first in a preview of your own garden.

Ventilation: temperature and humidity against reproduction

Whitefly grows fastest in warm humid air without movement. Above twenty-eight degrees and eighty percent humidity a greenhouse becomes a fly incubator. Ventilation means opening windows, opening doors, running a fan. If you keep greenhouse temperature below twenty-two degrees and humidity below sixty-five percent, whitefly grows slowly. Below fifteen degrees at night and eighteen degrees daytime the plague nearly stops completely.

This costs energy and money, but it is prevention. Start in May already with intensive ventilation as prevention. Open windows every day at least four hours once outside temperature reaches eighteen degrees. Run a small fan on low directed at plant level. This breaks reproduction cycles and puts you two weeks ahead compared to greenhouse growers who only ventilate in July.

Step by step

Step 1: Hang yellow sticky traps in May

Place four to six traps per hundred square meters of greenhouse. Check daily. This gives you the signal.

Step 2: Install insect netting on all openings

One point five millimeter mesh on windows, air slots, intakes. Make sure everything is sealed. Check weekly for tears.

Step 3: Start intensive ventilation

Open windows daily as soon as outside temperature reaches eighteen degrees. Run fan on low. Lowest humidity your crop allows.

Step 4: Plant marigolds between your vegetables

Sow in April, plant in May when ten centimeters tall. Every thirty centimeters between tomatoes and cabbage.

Step 5: Order Encarsia at first warning

As soon as you see ten flies per day on yellow traps, order Encarsia cards. Hang according to instructions. Repeat every two weeks.

Step 6: Check your traps weekly

Do fly numbers decline? Strategy works. Are they rising? More Encarsia needed or netting has leak.

Frequently asked questions

Can I use insecticide if it gets out of hand?

Yes, but it is the last resort. Insecticide kills Encarsia and other beneficial insects too. Pyrethrum (plant-based, from chrysanthemums) is least harmful. But do not combine: choose chemicals or biology, not both. Kaolin dust (chalk) smothers insects without poison and works reasonably well.

Does water spray work against whitefly?

Actually not well. Water removes adults temporarily but damages your plant more. Whitefly actually grows in humidity. Better: dry air, ventilation and yellow traps. Water spray helps against spider mites, not this pest.

How long until you see Encarsia effect?

Four to six weeks. Encarsia works slowly, it is underground labor. Patience is required. If you despair after two weeks, you repeat introduction do not stop. Two to three introductions over six weeks gives better results.

Can I bring whitefly indoors on seedlings from outside?

Certainly. This is the main entry point. Check every new plant undersides before planting. Quarantine three days separately. Recognize first signs. Better prevent than control.

See it first in your own garden

You now recognize whitefly early with yellow traps and you know when Encarsia is needed. Upload a photo of your garden on gardenworld.app and see a free preview of a new design with these plants. The sharp image with plant list costs 7.99 once, no subscription.