Whitefly are 2mm long, sap-sucking insects which lay their eggs on the underside of leaves. Eggs hatch into larvae which soon insert their mouthparts into the leaf and settle into immobile scale-like nymphs. Both adult flies and nymphs are white.
Two species are regularly found in our allotments and gardens. The cabbage whitefly (Aleyrodes brassicae) is found on brussels sprouts and other brassicas and causes little damage unless the infestation is particularly severe. However, infestations of greenhouse whitefly (Trialeurodes vaporariorum) can be more serious.
High temperatures in greenhouses means that greenhouse whitefly multiply rapidly. When plants are infested the leaf surfaces become sticky with honeydew secretions. Black sooty moulds grow on the honeydew secretions and during heavy infestations plants are weakened both because they are losing sap and because the black moulds block light from getting to the leaves. Whiteflies may also spread some viruses.
Whtieflies can survive and breed all winter on indoor plants but will not survive cold winters either outdoors or in greenhouses.
There are a number of ways to reduce whitefly populations in greenhouses
Plant French Marigolds (Tagetes) between your tomato plants to keep whitefly away. It's not that the whitefly don't like these marigolds, but their pungent smell masks the nice smell of the tomatoes that attracts whitefly.
Encarsia formosa and Delphastus pusillus