Greenhouse whitefly

Identification

Tiny (1-2 mm) white insects resembling minuscule moths that rise in a cloud when the plant is disturbed. They feed on the underside of leaves, sucking sap and excreting honeydew. This causes yellowing and weakening. Common on tomatoes, peppers, cucumbers and pelargoniums, especially in polytunnels and greenhouses. The wings of the adults are covered with a white, waxy bloom (hence the name) and at rest lie flat, roof-tile fashion over the body. Females lay their eggs on the underside of young leaves, often in characteristic arcs or circles, on short stalks. The eggs hatch into mobile first-instar larvae (so-called crawlers), which, once they find a feeding site, lose the ability to move and, as flat, oval, translucent greenish 'scales', attach themselves to the leaf until pupation. Feeding symptoms are pale spots, yellowing and wilting of the leaves and - most troublesome of all - copious honeydew, on which sooty mould (sooty mould fungi) develops, disfiguring leaves and fruit and limiting photosynthesis. In heated greenhouses the whitefly develops without interruption, producing many overlapping generations throughout the year; in the Polish climate it does not overwinter outdoors (it is killed by frost), so the source of infestation is protected cultivation and introduced seedlings, while in summer the insects may fly out into the open ground. The whitefly can also be a vector of plant viruses (criniviruses, e.g. tomato chlorosis virus).

Treatment

Yellow sticky traps (whitefly is strongly attracted to yellow), potassium soap, oil-based preparations. In biological control, the parasitic wasp Encarsia. Quarantining new seedlings and ventilating protected crops are important. Control is most effective when it combines monitoring with several methods at once, because no single one covers all developmental stages. Yellow sticky traps serve primarily to detect an incursion and to catch adults - on their own they do not control the larvae, so they remain a supporting tool. Potassium soap and oil-based preparations act solely by contact and must thoroughly cover the underside of the leaves, where the larvae and adults sit; they require several repeat applications every few days. Under cover, the introduction of natural enemies works well: the parasitic wasp Encarsia formosa (which lays its eggs inside whitefly larvae), predatory mites - Amblyseius species (which eat the eggs) and the predatory bug Macrolophus, as well as biopreparations of entomopathogenic fungi (Beauveria, Lecanicillium). Among insecticides it is best to target the larval stages with insect growth regulators, and to apply contact and systemic products in rotation of chemical groups - the whitefly quickly develops resistance, and pyrethroids additionally kill off natural enemies. The foundation remains prevention: quarantine and inspection of new seedlings, mesh screens in the vents, removal of weeds and plant debris that act as a reservoir of the pest, ventilation and avoiding excessive nitrogen fertilisation.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.