One of the most common greenhouse pests is the whitefly; white white-winged insects 2-3 mm long that rest beneath young leaves, flying around energetically when disturbed. The immature nymphs look like clear scales and may also be found on the undersides of leaves. Whitefly damage soft-tissued plants, particularly fuchsias and tomatoes, by sucking the sap and leaving behind a sticky honeydew (excrement), encouraging the growth of sooty moulds. These moulds can reduce the amount of light reaching the leaves, which may lead to foliage yellowing and plant death.

Control

Whitefly can be controlled biologically in greenhouses or conservatories by using their natural enemies. Encarsia formosa is a parasitic wasp that lays its eggs into whitefly scales, whilst Delphastus is a small black beetle from the ladybird family that feeds voraciously on whitefly eggs and larvae.

Alternatively, spray greenhouse plants with dimethoate/permethrin regularly in summer, or use permethrin insecticidal smokes. However, pesticide resistance is a frequent problem with this greenhouse pest.

Whitefly may also be found outdoors on fruits and vegetables such as brassicas, settling underneath leaves where they are difficult to spray. These plants may be treated with dimethoate/permethrin, although you must make sure that the undersides of leaves are well drenched. Insecticidal soap may also be used on three or four occasions at five-day intervals, or the lower leaves where the larvae are usually to be found may simply be picked off.