You notice tiny white insects flying up when you touch a leaf — spray with a pyrethroid. A week later they are back in force. Spray again. The effect lasts three days. The problem is not the product or the dose — at that moment there are hundreds of eggs and day-eight scale-like "puparia" on the leaves that a contact insecticide simply cannot reach. Each new generation emerging from the hidden stages restores the population faster than a single treatment can knock it down.
Quick glossary: Whitefly — a small insect in the order Hemiptera; the main greenhouse species are Trialeurodes vaporariorum (greenhouse whitefly) and Bemisia tabaci (tobacco/silverleaf whitefly); feeds on phloem sap and excretes honeydew that serves as substrate for sooty mould. Adult (imago) — the winged form; easily spotted and easily disturbed into flight. Puparium (fourth-instar larva) — the immobile "scale" attached to the leaf surface; has a waxy coating and is resistant to contact insecticides — this stage is what saves the population during spray applications.
The Life Cycle and Why a Single Treatment Fails
Whitefly passes through five stages: egg → three mobile larval instars → puparium (immobile scale) → adult. Eggs and first-instar larvae are on the underside of leaves. Second and third instars are in the same location — small and nearly transparent. The puparium is immobile, covered by a waxy shield, fixed to the leaf.
A contact insecticide applied during treatment kills adults and partly mobile larvae. The puparium survives — the waxy coating protects it from most products. Eggs survive too. Within 5–7 days, new adults emerge from the puparia, lay eggs — and it looks as though "the product stopped working."
The full cycle at +24°C is approximately 3–4 weeks. A single female lays 150–300 eggs. In warm conditions without control, the population grows exponentially.
Tobacco vs Greenhouse Whitefly: Why the Species Matters
Trialeurodes vaporariorum — the greenhouse whitefly, more common in cooler conditions. Bemisia tabaci — the tobacco (silverleaf) whitefly, more aggressive and more dangerous: it transmits more viruses, is more resistant to insecticides, and develops faster above 25°C.
They can be distinguished by wing position: in Trialeurodes, the wings are held flat and horizontal, parallel to the leaf surface — "roof-shaped." In Bemisia, the wings are angled along the body — more streamlined. The difference matters because Bemisia has greater resistance to neonicotinoids — the primary insecticide class used against whitefly — and requires a different rotation strategy.
Conditions That Trigger an Outbreak
Whitefly reproduces fastest at 22–28°C and low humidity. Greenhouse overheating in summer is the most common precondition for an outbreak.
Introduction of infested plants is the primary vector for initial infestation. Seedlings, mother plants, new crop introductions without quarantine — the standard source. Even a few eggs on purchased transplants produce a visible population within two weeks.
Weak monitoring is the reason for late detection. Yellow sticky traps reveal the presence of adults before the population becomes noticeable during leaf inspection. The alert threshold is 1–5 individuals per trap per week — any appearance warrants a thorough inspection of leaf undersides.
Control: What to Do and in What Order
Detection and isolation. At first detection — identify the focus. Place yellow traps around the outbreak zone, intensify scouting. Plants with heavy infestation — remove or isolate.
Biological control at low population levels. Encarsia formosa — a parasitic wasp that lays eggs inside puparia of the greenhouse whitefly. Effective against T. vaporariorum, significantly less so against Bemisia tabaci. Eretmocerus eremicus — broader host range including Bemisia. Beneficials are introduced at low population density — at heavy infestation, use chemical treatment first to reduce numbers.
Chemical control with rotation. A series of applications at 5–7 day intervals to catch new generations emerging from puparia. A minimum of 3 applications per series.
Key IRAC groups: Group 4A (neonicotinoids — imidacloprid, thiamethoxam), Group 23 (tetraniliprole), Group 9 (pymetrozine — mobile stages only), Group 28 (diamides). Rotate groups between series — neonicotinoids in particular develop resistance rapidly under mono-application.
Treat the underside of leaves. The majority of the population — eggs and larvae — is on the leaf underside. Spraying top-down does not cover them effectively. Coverage of the leaf underside is mandatory.
Three Mistakes That Cost the Most
One application and "let's wait and see." Even an effective insecticide does not reach the puparium. Without a follow-up application in 5–7 days to catch the emerging generation, the population recovers and it looks as though "the product did not work."
Not checking leaf undersides during scouting. Flying adults are obvious. Eggs, larvae, and puparia on the leaf underside are not. Assessing the true population only by adults in the air or on traps means seeing the tip of the iceberg. For the overall IPM strategy and decision logic, see the relevant article.
Using only neonicotinoids. Both major greenhouse whitefly species have documented resistance to this group in many populations. If three imidacloprid applications produce minimal effect — this is not a dosing question, it is a resistance question. Switch groups.
How to Know Whitefly Is Under Control
Counts on yellow sticky traps decline for three consecutive weeks. Inspection of leaf undersides shows mostly darkened puparia (parasitised by beneficials) or an absence of fresh eggs. No new plants showing feeding damage appear. Honeydew and sooty mould are not spreading.
For deeper understanding: Beneficial Insects: Which Predators Target Which Pests and How Not to Destroy Them with Chemistry — explains how Encarsia and Eretmocerus fit into the chemical rotation and which products can be applied without eliminating the beneficials.