Glasshouse Whitefly is a common pest that affects tomatoes, basil 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.
This pest is active year-round under cover: there is no seasonal break.
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.
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, because our tomato guide records that tomatoes are grown under cover almost by necessity — outdoor cropping is marginal north of the Central Belt — and basil is a windowsill or polytunnel crop here too. The cover that makes Scottish growing possible is exactly what makes whitefly a Scottish problem.
Controls: what actually works
Every method below is graded by evidence, not by how often it gets repeated. Where a claim has been tested and failed, we say so plainly.
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.
Sticky traps
Sticky traps are among the prioritised measures, though no effectiveness data is given for them.
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.
This is the site's strongest companion-planting claim landing on the site's most under-cover-specific pest. Both pages should cross-link.
Pesticides
Pesticides are not recommended for glasshouse whitefly. Prevention, biological control and sticky traps come first.
Weaker than the aphid and powdery mildew entries: there the guidance states an explicit 'don't use pesticides' with reasons. Here it simply does not recommend them. Absence of recommendation is not the same as advice against, and the site must not overstate it.
Frequently asked questions
What should I do first if I find glasshouse whitefly in my garden?
Start with biological control. After that, companion planting — french marigolds among tomatoes. The full list of evidenced controls is above, each graded by how much evidence supports it.
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.
Should I just leave glasshouse whitefly alone?
In most gardens, yes. glasshouse whitefly are food for birds, wasps and predatory insects, and a healthy garden carries some without trouble. Tolerance is also the official position now, and the stance box above sets out that reasoning in full.
Sources
1 source, 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, sticky traps, absence of pesticide recommendation