Reach for the spray can and you will lose this fight. Greenhouse whitefly survive contact insecticides because two of their life stages, the egg and the non-feeding pupa, shrug them off completely, so a single spray only clears the adults and rebuilds within days. The control that actually empties a glasshouse of them is a parasitic wasp the size of a pinhead, Encarsia formosa, introduced early while numbers are still low. Sticky traps tell you they have arrived; they do not clear them.
The clouds of tiny white flies that lift off your tomatoes when you brush the leaves are glasshouse whitefly, Trialeurodes vaporariorum, and they rank among the most stubborn pests under glass precisely because their lifecycle dodges sprays by design. Once you understand which stages a spray misses and which stages the wasp attacks, the whole problem changes shape. This guide walks the lifecycle, then the three things that work: biocontrol introduced early at 18°C-plus, monitoring with traps, and isolating bought-in plants before they ever reach the bench.
Why contact sprays do not clear whitefly
Contact insecticides kill the adults and the feeding nymphs, then leave the eggs and the non-feeding pupae untouched. Those surviving stages hatch and mature within days and the cloud comes straight back. University extension research puts it plainly: the egg and the non-feeding pupal stages are generally not susceptible to insecticides that can kill adults and nymphs. One spray never finishes the job because of that single gap.
The lifecycle reads like a relay built to outlast you. A female lays an egg on the leaf underside; it hatches into a mobile crawler; the crawler settles and flattens into a flat scale-like nymph about 1 mm across that sits still and feeds; that nymph stops feeding and seals into a pupa for almost a week before the adult emerges. The RHS lists the same chain of stages, and the immobile scale and the pupa are the two that sprays slide right off.
Numbers make the rebuild relentless. A single female lays somewhere between roughly 150 and 400 eggs across her life, depending on conditions, and those eggs hatch within about 5 to 10 days. A full generation runs in 3 to 4 weeks under favourable warmth, roughly 21 to 36 days end to end. Warmth speeds the whole thing up: the RHS notes the cycle takes several months at 10°C but completes in about three weeks at 21°C, so a hot July greenhouse runs generation after generation. Spray once and you trim one stage; the queue behind it keeps marching.
Resistance makes repeat spraying worse
Because no single application clears the population, growers spray again and again, and that pressure breeds resistance. Greenhouse whitefly already carry documented resistance to insecticides, including a P450-linked resistance to pyriproxyfen recorded in T. vaporariorum. Each repeated dose selects harder for the survivors, so the chemical route tends to get less effective over a season rather than more.
How Encarsia formosa actually clears a glasshouse population
Encarsia formosa is a tiny chalcid wasp, discovered in Britain in 1926, that lays a single egg inside an immature whitefly. It targets exactly the scale and pupal stages a spray misses, the third and fourth nymphal instars before pupation, so between the wasp and the natural death of adults the whole lifecycle gets covered. UC IPM records that it parasitises all whitefly stages except the egg and the mobile crawler, which is the precise complement to what insecticides reach.
The wasp gives no instant knock-down, and that catches people out. The RHS is clear that you must introduce it before plants are heavily affected, because it cannot give instant control. UNH Extension says the same from the other direction: releases must be made early in the infestation, while the whitefly population is still low. Wait until the cloud is thick and you have already lost the timing the method depends on.
What temperature does Encarsia formosa need to work?
Aim for daytime warmth of 18°C and above, with 20 to 25°C as the sweet spot. Below roughly 18°C the adult wasps stop flying, and where the 24-hour average falls under about 17°C the control thins out. Such a temperature floor makes the wasp a spring-to-autumn tool in the UK, viable from late March or April until September, the same window the RHS gives for glasshouse biological controls.
The wasp earns its keep when it is warm. One female parasitises around 5 whiteflies a day across a life of roughly 2 weeks, near 70 hosts in total, and a release keeps breeding inside the greenhouse as long as the warmth and the host supply hold. In a cold snap the maths collapses. There is no point releasing into a chilly April greenhouse that has not yet held 18°C through the day, which is the honest limit of the method.
How the wasp arrives and where to put it
It comes supplied as parasitised black whitefly scales, stuck to small cards or pieces of leaf. Fix the cards securely in a shaded spot directly on the infested plants, and because whitefly congregate on the leaf undersides, that is where the releases should focus. Strip off the most heavily infested lower leaves first, standard hygiene that lowers the pest load the wasp then has to chase.
How do I know the wasp is working?
Watch the scales change colour. As Encarsia develops inside a whitefly nymph, the host turns black, so parasitised scales blacken on the leaf underside while live ones stay pale. The RHS notes this makes progress easy to monitor: black scales appearing among the white ones means the wasp has taken hold and is breeding. A spreading patchwork of black signals you to hold your nerve and let it run.
One rule protects the whole operation. Once the wasp is in, keep broad-spectrum insecticides out, because their residues kill the parasitoid along with the pest and reset you to square one. The same caution applies to sticky traps near release cards, since the RHS notes the yellow sheets catch beneficial wasps too. Choose the wasp or the chemicals; running both at once wastes the wasp.
Yellow sticky traps are for monitoring, not control
Hang yellow sticky traps to detect whitefly and track how the numbers are moving, then accept that they will never clear an infestation on their own. The RHS is explicit that the sheets help monitor whitefly activity rather than give control, and they catch beneficial insects too. As an early-warning system they earn their place. As a cure they amount to a false economy.
Used as a monitor they shape the timing of everything else. University extension guidance suggests placing yellow sticky cards at one per 1,000 square feet and replacing them weekly, which on a home greenhouse means a card or two checked at the weekend. A few catches on a fresh card give you your cue to act, the moment to order the wasp while the population still sits low enough for it to win.
The disturbance test that costs nothing
For a quick read between trap checks, brush a hand across the foliage. Adults fly up in a small white cloud when the leaves are disturbed, so a puff of flies from the canopy is a useful at-home detection cue. It pairs with the sticky cards rather than replacing them. Done as you walk past, it tells you instantly whether the population is building between your weekly card checks, long before a number on a trap would.
Isolating new plants: the prevention almost nobody mentions
Quarantine every bought-in plant before it joins the others, because whitefly usually arrive in a greenhouse on new plants in the first place. The RHS advises isolating new arrivals so that any eggs and nymphs have a chance to develop and be recognised before the plant goes on the main bench. It costs less than any other control, and most guides skip it entirely.
Give a new plant one to two weeks apart from the others and inspect it properly. Turn the leaves and check the undersides with a hand lens, where the eggs and flat scales hide, and look again every few days rather than once. Catching one infested plant at the door saves the whole crop.
What whitefly actually do to your crop
Whitefly damage works in two stages: they drain sap, then they foul the leaves. Feeding on tomato, cucumber, pepper and fuchsia, they pierce the leaf and suck out sap, weakening the plant directly. Then they excrete a sticky honeydew over the foliage and fruit below, and a black sooty mould grows on that honeydew. UNH Extension notes the mould interferes with the plant's photosynthesis, so a heavy infestation starves the plant of light from the outside in.
The host list is wide, which is why the pest moves so freely under glass. Cucumber, melon, tomato, peppers, fuchsia, poinsettia, gerbera and pelargonium are all on the menu, so an ornamental brought in for colour can seed an outbreak that lands on your tomatoes. The sap-sucking saps vigour and the sooty mould blocks light, and between them a thriving colony pulls a crop down well before the plant looks obviously sick. A warm, well-aired greenhouse, covered in our guide to keeping a greenhouse cool, also keeps you in the trap-checking habit through the months whitefly are busiest.
Putting it together on a tomato crop
On greenhouse tomatoes, the sequence is monitor, then release early, then protect the wasp. Hang a yellow sticky card from the moment the plants go in and check it at the weekend. The first catches, paired with a puff of flies when you brush the leaves, are your trigger. Order Encarsia formosa while the numbers are still low, once the greenhouse is reliably holding 18°C through the day.
From there it runs on its own if you let it. Strip the worst lower leaves, fix the wasp cards in shade on the undersides of infested plants, and keep all broad-spectrum sprays away. Watch for the scales turning black, the proof the wasp is breeding, and resist the urge to spray over the top of it. Whitefly travel between plants, so it is worth a glance at neighbouring crops; the same vigilance that catches early tomato blight catches a whitefly cloud before it settles in.
The growing advice here works whatever greenhouse you have. A structure that ventilates well does make the warm, steady conditions Encarsia formosa needs easier to hold, and it keeps you walking the plants where you spot a whitefly cloud early. The NORDIC greenhouse in Swedish pine and twin-wall polycarbonate has a rear opening window opposite the door for cross-ventilation as standard, which helps hold daytime warmth around 18 to 25°C without the air going stale. It is a help, not a requirement; the wasp does the work in any greenhouse that stays warm enough.
The one move that decides the season
Order the wasp early, not late. Hang a yellow sticky card the day your plants go in, check it at the weekend, and the moment you see the first few whitefly or a puff of flies off the leaves, introduce Encarsia formosa while the greenhouse is holding 18°C through the day and the population is still low. Quarantine every new plant for one to two weeks before it joins the others, keep broad-spectrum sprays well clear once the wasp is in, and watch for the scales turning black. Do those four things in that order and the cloud never gets the chance to build.
How to spot whitefly, and the control ladder that actually works
Turn a leaf over. Glasshouse whitefly (Trialeurodes vaporariorum) gives itself away on the underside: flat, oval, creamy-white scale-like nymphs just over 1 mm long, greyish-white cylindrical eggs, and adult white-winged flies about 1.5 mm long that rise in a small cloud when you brush the plant. Sticky black sooty mould on leaves below confirms it.
One quick check saves a misdiagnosis: whitefly stages sit on the underside of leaves. White flecks on the upper surface are more likely to be shed aphid skins, per RHS guidance. Cucumbers, tomatoes, peppers, fuchsias and pelargoniums are among the plants most commonly hit under glass.
Once you have confirmed it, climb this ladder in order rather than reaching straight for a spray.
| Rung | What it does | How to use it | Honest verdict |
|---|---|---|---|
| Yellow sticky traps | Monitor numbers and catch the first adults | Hang one trap near plant tops, check weekly | An early-warning tool for monitoring, not a cure on its own |
| Encarsia formosa (parasitic wasp) | Lays eggs in the whitefly scale nymphs, which turn black as they are parasitised | Introduce 1 to 10 per square metre at first sighting, before plants are heavily affected, and repeat until control is achieved; works best at 20–25°C, with control often insufficient where the 24-hour average stays below 17°C | The established biological control under glass, started early and topped up |
| Broad-spectrum sprays | Kill exposed adults only | Last resort, and incompatible with the wasp | Often disappointing: the egg and scale stages are shielded, so survivors rebuild fast |
The pattern is simple. Catch them early on a trap, release the wasp while numbers are low and the greenhouse stays warm, and you rarely need a chemical at all.
