Glasshouse Whitefly is a common pest that affects tomatoes, basil and cucumbers in Scotland. Here is what it looks like, how to spot the damage, and which controls have been tested.
What does it look like?
Adults: Tiny white-winged insects about 1.5mm long, which fly up when plants are disturbed.
Nymphs: Flat, oval, creamy white scale-like nymphs on leaf undersides.
The flying-up-when-disturbed behaviour is the practical field test; the nymphs on leaf undersides are what confirm it.
This pest is active year-round under cover: there is no seasonal break.
🌱 Plants at risk
Glasshouse Whitefly also affects these crops:
Symptoms
Weakens plants through sap feeding, and excretes honeydew which 'allows the growth of black sooty moulds', affecting both appearance and health.
Life cycle
Active all year round on houseplants and in greenhouses.
No overwinteringKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → break under cover means no natural reset — populations build continuously, which is why RHS treats this differently from an outdoor pest.
Why this matters in Scotland
A pest Scotland gets because of how Scotland has to grow, not despite it.
Glasshouse whitefly 'thrives in warm conditions, which is why it is not usually a problem on outdoor plants'. In most of Britain that makes it a niche greenhouse issue. In Scotland it is not niche: tomatoes are grown under cover almost by necessity here, since outdoor cropping is marginal north of the Central Belt, and basil is a windowsill or polytunnel crop too. The cover that makes these crops possible to grow at all is exactly what makes whitefly a problem for them.
Controls: what actually works
Not every method here works equally well: some are backed by real evidence, others are simply traditional and untested.
Biological control: tiny parasitoid wasps that prey on whitefly nymphs
RHS: 'An established method of control for glasshouse whitefly is to introduce tiny parasitoid wasps, Encarsia formosa, which prey on the whitefly scale nymphs.' Available by mail order. Parasitised nymphs turn black, which gives a built-in way to monitor whether it is working.
The blackened-nymph feedback loop is unusual and worth surfacing: most controls give no way to tell if they worked. This one does.
Yellow sticky traps
RHS: 'Hang sticky yellow sheets above or among the plants to trap adult whitefly.' Colour is specific here: the point is yellow sheets, not plain sticky paper. These mainly help with monitoring and catching adults, not full control on their own.
Quarantine new plants before they go anywhere near existing ones
RHS: 'Watch for signs of whitefly on new purchases as the insect is often first brought into a glasshouse on new plants. If possible quarantine new plants in order to give eggs and nymphs a chance to develop and be recognised.' A new plant showing nothing on arrival can still be carrying eggs that haven't hatched yet.
Companion planting: French marigolds among tomatoes
This is the best-evidenced companion pairing we have found, and the only one backed by two independent authorities. RHS attributes it to limonene emission; Garden Organic attributes it to bright petals distracting whitefly. They agree it works and disagree on why.
Two independent authorities agreeing that it works, even while disagreeing on the mechanism, is a genuinely strong result for a companion-planting claim.
Pesticides
Pesticides are not recommended for glasshouse whitefly. Prevention, biological control and sticky traps come first.
This is an absence of a recommendation, not an explicit warning against pesticides, and the difference matters: prevention, biological control and sticky traps are what's put forward as the answer here, with nothing said against reaching for a spray as well.
Natural predators of glasshouse whitefly
These 2 creatures eat glasshouse whitefly, and each has its own page here on how to get more of them in your garden: what to plant, or what to order and when.
Myths about glasshouse whitefly
7 common claims about glasshouse whitefly, checked: 3 busted2 disputed0 untested2 hold up
“Hanging yellow sticky traps in the greenhouse will control a whitefly infestation on its own”
Yellow sticky traps catch adult whitefly that fly onto them, but that is a monitoring function rather than a control measure, tracking activity rather than bringing numbers down. Traps also snag non-target insects, including any Encarsia formosa wasps bought in as a biological control, so relying on traps alone can undercut a control method working alongside them.
Our read The wording is specific: traps help monitor activity rather than give control, which rules out treating a wall of yellow card as a standalone fix for an established infestation.
What to do: Use yellow sticky traps to track whether whitefly numbers are rising or falling, and to catch mobile adults, but pair them with an active control such as Encarsia formosa or thorough regular checks rather than expecting traps to clear an infestation unaided.
In Scotland: Because Scottish tomatoes and basil are almost always grown under cover, an infestation left to traps alone in a polytunnel or greenhouse has no cold snap or predator population to knock it back, unlike an outdoor pest.
1 source
- RHS: Glasshouse WhiteflyYellow sticky sheets hung above or among plants can help monitor whitefly activity rather than give control, and they also catch non-target insects including parasitoid wasps.
“The whitefly on my houseplants is a different pest from the one that attacks greenhouse tomatoes”
Glasshouse whitefly, Trialeurodes vaporariorum, is described as a sap-feeding insect mainly of houseplants and greenhouse plants, treated as one species across both settings rather than two separate problems. It thrives in warm conditions, which is exactly what a heated home and a greenhouse both provide, and is why the same insect turns up on a windowsill basil plant and a polytunnel tomato crop. A genuinely different species, cabbage whitefly, does exist outdoors on brassicasThe cabbage family: cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi, swede, turnip and more, sharing the same pests, diseases and rotation rules. Full definition → and is confirmed as separate, but that is not the houseplant-versus-greenhouse split gardeners usually mean by this claim.
Our read The houseplant and greenhouse pages describe the same insect rather than drawing a species line between indoor and under-cover settings; the real species split in RHS material is glasshouse whitefly versus cabbage whitefly, not indoor versus greenhouse.
What to do: Treat a whitefly sighting on a houseplant and one in the greenhouse as the same threat: check both regularly, and quarantine new houseplant purchases the same way you would a new greenhouse plant, since either can be how it gets in.
2 sources
- RHS: Glasshouse WhiteflyGlasshouse whitefly is a sap-feeding insect mainly of houseplants and greenhouse plants, and thrives in warm conditions, which is why it is not usually a problem on outdoor plants.
- RHS: Cabbage WhiteflyCabbage whitefly is a different species to glasshouse whitefly, restricted to brassicas, and glasshouse whitefly will not feed on brassicas outdoors.
“Whitefly only weakens plants by feeding on sap, it doesn't spread disease between them”
Trialeurodes vaporariorum is confirmed as the whitefly vector that transmits Tomato infectious chlorosis virus from plant to plant, a disease that caused serious tomato losses where it became established in California and southern Europe. The same government pest risk analysis notes this whitefly has not been reported as causing significant virus problems in the UK in recent years, so the mechanism is real even though the practical risk to a Scottish greenhouse right now is low rather than urgent.
Our read The document is a formal pest risk analysis, not general advice, and it draws a clear line between the vector being present and widespread in UK glasshouses and the virus itself not currently being a reported UK problem; both halves matter for an honest answer.
What to do: Don't rely on virus risk as the main reason to control whitefly in a Scottish greenhouse, since it isn't currently a significant UK problem, but do keep controlling whitefly for the sap-feeding and honeydew damage it definitely causes, and quarantine new plants since that is also how a future virus risk would most likely arrive.
1 source
- Defra/Central Science Laboratory Pest Risk Analysis: Tomato infectious chlorosis virusTrialeurodes vaporariorum is the whitefly that transmits Tomato infectious chlorosis virus from plant to plant, and is present in UK glasshouses, though the virus has not been reported as causing significant problems in the UK in recent years.
“Buying Encarsia formosa parasitic wasps will clear a whitefly infestation quickly, however bad it already is”
Encarsia formosa is described as an established biological control for glasshouse whitefly, with parasitised nymphs turning black so progress can be monitored directly on the leaf. The same advice is explicit that it cannot give instant control and needs to be introduced before a plant is already heavily infested, which is the opposite of how most gardeners reach for it, as a rescue measure once whitefly is already obvious. A separate wildlife-focused entry covers Encarsia formosa as a beneficial insect in more depth; this is kept brief here to avoid duplicating that.
What to do: Introduce Encarsia formosa early in the season, at the first sign of whitefly or even before, rather than waiting until an infestation is already heavy, and expect it to take weeks rather than days to bring numbers down.
1 source
- RHS: Glasshouse WhiteflyEncarsia formosa is an established method of control, but it is important to introduce it before plants are heavily affected, as it cannot give instant control.
“A homemade washing-up liquid and water spray is a safe way to deal with whitefly”
Soft-bodied insects including whitefly are among the pests that soap sprays can affect, but the same guidance lists a long run of plants that are prone to leaf damage from soap sprays, plus wider risk factors: spraying in full sun or above roughly 32C, spraying a plant that is already drought-stressed, and using ordinary household detergent rather than a product formulated as an insecticidal soap. Symptoms of damage include yellow or brown scorching and burned leaf tips, which is a real cost against a spray many gardeners assume is harmless because it is not a shop-bought pesticide.
Our read The advice treats soap spray as something to test and use with care, not as an automatically gentle option; the phytotoxicity list is long enough that "safe" is not the right word without qualification.
What to do: Spray in the early morning or evening rather than full sun, make sure the plant is well watered first, test on a small area and check after 24 hours before treating the whole plant, and target the undersides of leaves where whitefly actually feed rather than the top surface.
1 source
- Clemson Cooperative Extension, Home & Garden Information Center: Insecticidal Soaps for Garden Pest ControlWhitefly are among the soft-bodied insects soap sprays can control, but a range of plants are sensitive to damage, application should avoid full sun and high temperatures, and plants should be tested and well watered first.
“Gently shaking or brushing a plant is a reliable way to check whether it has whitefly”
Disturbing a plant is the identification test named directly for this pest: clouds of small white-winged adults fly up when a plant is disturbed, which is what separates whitefly from aphids and other sap feeders at a glance. That covers the flying adults; it does not catch the flat, scale-like nymphs stuck to leaf undersides, which need a closer look rather than a shake to find.
What to do: Give a suspect plant a light shake or brush past it and watch for a cloud of tiny white insects rising off it, then turn a few leaves over to check the undersides for the flatter, paler nymphs the shake test won't reveal.
1 source
- RHS: Glasshouse WhiteflyWhen a plant is disturbed, clouds of small white-winged insects about 1.5mm long will fly up, which is a key identification feature distinct from aphids.
“Planting French marigolds or nasturtiums among tomatoes repels whitefly”
A Newcastle University field trial published in PLOS ONE found that companion planting tomatoes with French marigolds slowed whitefly population development, driven by limonene given off by the marigolds; nasturtiums gave comparable protection. Timing matters — introducing the marigolds early works, but adding them after whitefly numbers have already built up is much less effective.
What to do: Interplant marigolds or nasturtiums with tomatoes from early in the season as one layer of prevention, not as a rescue once whitefly is already established.
In Scotland: This trial used glasshouse/tomato whitefly under controlled conditions similar to a Scottish greenhouse or polytunnel setup, which is where most Scottish growers will actually meet whitefly.
1 source
- PLOS ONE — Companion planting with French marigolds protects tomato plants from glasshouse whiteflies through the emission of airborne limoneneCompanion planting short vine tomatoes with French marigolds throughout the growing season will slow development of whitefly populations... Introducing marigolds to unprotected tomatoes after significant whitefly build-up will be less effective.
Frequently asked questions
What should I do first if I find glasshouse whitefly in my garden?
Start with biological control. After that, quarantine new plants before they go anywhere near existing ones. Work through the rest in order of how well-evidenced they are if the first two aren’t enough on their own.
Will glasshouse whitefly kill my plants?
They rarely kill established plants. The damage is usually cosmetic on outer leaves, and natural predators often bring numbers under control without you doing anything.
How much of this advice is actually proven?
3 of the 5 controls above are evidence-based, not just repeated folklore. Start with the evidence-based ones and treat anything lower down the list as worth trying, not worth relying on.
Sources
2 sources, recorded with what each was used for
- RHS1: no tolerance advice (unlike slugs/aphids/caterpillars), adult and nymph identification, sap feeding and honeydew/sooty mould, warmth requirement and outdoor rarity, year-round activity under cover, Encarsia formosa as an established control with mail-order availability and blackened-nymph monitoring, yellow sticky traps and their risk of catching beneficial insects too, quarantining new plants, absence of pesticide recommendation
- Garden Organic8: French marigolds among tomatoes deterring whitefly by visual distraction, as a second, independent authority alongside RHS's limonene-mechanism explanation for the same pairing