How to control greenhouse whitefly

Confirm the insect first

Glasshouse whitefly, Trialeurodes vaporariorum, is a common sap feeder on greenhouse vegetables and ornamentals. The adults are small and white and may rise in a cloud when a plant is disturbed. The more persistent stages are on the undersides of leaves: eggs and pale, flat, oval nymphs that can look like tiny scales. The RHS notes that some nymphs turn black after parasitism by Encarsia.

Honeydew is another clue. Both adults and nymphs excrete this sticky substance, which can support dark sooty mould on foliage and fruit below. Do not assume every white speck is whitefly: shed aphid skins can sit on upper leaves, and whiteflies on outdoor brassicas may be a different species. Inspect the insect and host plant before choosing a response.

Why a few adults can become a large problem

Whitefly breed rapidly under warm greenhouse conditions, and several stages can be present at once. Removing flying adults does not remove eggs and nymphs. The RHS explains that life-cycle speed depends on temperature and that whitefly can remain active through winter in a greenhouse if suitable host plants survive.

That is why an early inspection routine matters more than waiting for leaves to turn black with sooty mould. Check the lower surfaces of a few leaves on tomatoes, cucumbers and peppers during ordinary watering. Look at newly bought plants before they join the crop; quarantine where feasible to allow hidden eggs or nymphs to become visible.

Start with isolation and removal

Move a small infested potted plant away from clean seedlings if its needs allow. Remove badly affected leaves carefully where the plant can tolerate it, placing them directly into a container rather than shaking them through the greenhouse. Remove weeds and spent host plants that do not need to remain under glass. Wash hands and tools before handling clean material.

Do not strip a tomato so aggressively that you damage the crop or expose fruit abruptly to strong sun. Your response should match plant health and infestation level. Good ventilation can help limit sooty mould, but simply opening a vent will not eradicate whitefly. Our greenhouse hygiene guide covers the daily clean-to-dirty routine.

Use yellow cards with a clear purpose

Hang a small number of yellow sticky cards near susceptible plants if you need an early-warning signal. Check them regularly and confirm any capture against insects and nymphs on the plant. A rising count tells you where to inspect more closely; a clean card does not prove every leaf is clear.

The RHS is explicit that sticky yellow sheets mainly help monitor whitefly activity rather than provide full control. It also warns that they can catch non-target organisms, including parasitoid wasps. If you plan an Encarsia release, review card placement or remove cards according to the biological supplier's instructions. More cards are not automatically a better integrated-pest-management plan.

Consider timely biological control

The RHS identifies Encarsia formosa as an established biological control. The tiny wasp parasitises whitefly scale nymphs; parasitised nymphs darken, helping you monitor progress. It is available from specialist biological-control suppliers and works best before an infestation is heavy. It cannot give an instant knockdown of all life stages.

Order only after confirming the pest, checking the supplier's temperature and release requirements and considering any recent or planned pesticide use. Biological agents are living organisms, so storage, timing and compatible greenhouse conditions matter. Do not buy a release simply because one adult appeared on a card; monitor whether the population is establishing.

Be cautious with pesticide claims

There is no universal whitefly spray suitable for every tomato, cucumber, pepper and ornamental crop. UK authorisations, labels and permitted uses can change. If considering a chemical control, check the HSE pesticide databases and the exact current product label for the crop, greenhouse use, dose, protective measures and harvest interval. An online marketplace category or generic description does not establish legal suitability.

Chemical products may also harm beneficial controls or undermine a biological release. Follow current RHS and supplier guidance for compatibility. This article does not recommend a named pesticide because selection requires the specific crop, pest, current authorisation and situation.

Prevent the next outbreak

Inspect incoming plants and greenhouse leaf undersides regularly. Remove exhausted crops promptly, keep weeds from acting as hosts and avoid crowding plants so tightly that no one can see their lower leaves. Clean staging and glazing when crops are cleared; the RHS says winter glasshouse cleaning can help reduce overwintering whitefly.

Keep a note of where the first insects appeared and which plants were brought in shortly before. That evidence can reveal an introduction route and improve the next season's quarantine. If whitefly is one of several problems, return to the UK greenhouse pest guide rather than treating every symptom as the same insect.

Sources and related reading

Identification, symptoms, sticky-card limitations and Encarsia guidance come from the RHS glasshouse-whitefly advice. Use the HSE pesticide database for current legal product details and follow biological supplier instructions for any release.