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Pest and Diseases : Whiteflies




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    Yellowing and often disfigured leaves, which are caused by whitefly feeding on the plant cells. Whitefly secrete sticky honeydew deposits as they feed which fall on to the surrounding foliage. This causes dark sooty mould to develop on the leaves.

    Plants affected

    • Whitefly colonies affect a wide range of vegetables, ornamental plants and trees.

    About whiteflies

    • Whitefly adults are tiny, moth-like insects that feed on plant sap. They have two pairs of white wings that fold back over their abdomen when at rest.
    • Over 1400 species of whitefly have been recorded worldwide and 56 of these have been found in Europe.
    • Development of egg to adult takes around three to four weeks, depending on the temperature
    • Females usually lay up to 200 eggs on the underside of leaves, often in neat circles.
    • Eggs hatch into larvae that initially crawl over the leaf surface for a few hours then settle to feed on plant sap.
    • The larvae are small and scale-like in appearance.
    • After the first skin-shed, the larvae become fixed to the leaf by their feeding tubes until they turn into pupae from which the adults emerge.
    • Glasshouse whitefly, Trialeurodes vaporariorum, as their name suggests, usually infest plants grown indoors and under glass such as tomato, cucumber, pepper and many ornamentals.
    • Cabbage whitefly, Aleyrodes proletella, is a common pest in southern England even in the colder winter months. It’s similar to glasshouse whitefly, but has grey spots in the centre of each wing, and attacks only cabbages and other brassicas.
    • Tobacco whitefly, Bemisia tabaci, is a notifiable pest that has to be reported to DEFRA Plant Health if found. It occasionally appears in the UK on imported ornamentals and cut flowers. In southern Europe, the tobacco whitefly causes serious damage to horticultural crops by transmitting plant viruses. It’s similar in appearance to the glasshouse whitefly, although smaller and less triangular in shape when at rest.

    Treatment

    Chemical

    Products containing the following chemical ingredients are all effective on whiteflies

    • Pyrethrum
    • Natural Fatty Acids
    • Surfactant based products

    Organic

    • Inspect your plants every day and thoroughly.
    • Adult whiteflies feed and lay eggs on the new growth of plants, so these areas need to be inspected first. If adults are present, then an organic ,approved natural fatty acid or natural soap product will control them.
    • If you spot adult whiteflies, particularly on older plants, then they’re likely to have emerged from an infestation of larvae and pupae on the lower leaves. Carefully inspect the lower leaves and remove and destroy any that are infested with whitefly larval scales.
    • Nettings and fleeces can be used to cover greenhouse vents and stop whiteflies spreading on to susceptible plants.
    • Use natural enemies such as spiders, which catch large numbers of adult whiteflies in their webs. Ladybirds, hoverfly and lacewing larvae will also eat whitefly.
    • For greenhouse plants use the parasitic wasp Encarsia formosa which is commercially available. This will attack and kill the larvae of glasshouse whitefly.
    • Yellow sticky traps should be placed within a greenhouse to catch adult whitefly.

    Prevention

    • Check plants every day for signs of whitefly infestation and deal with them as soon as they appear.
    • Monitor whitefly activity by hanging yellow sticky traps among greenhouse and conservatory plants.
    • Be wary of weeds that can often host whitefly within or near cultivated plants.
    • Encourage natural enemies into the garden.
    • Don’t use broad spectrum insecticides that kill beneficial insects as well as whiteflies.
    • Destroy leaves that have large infestations of whitefly larvae since many whitefly adults will still develop and hatch from detached and discarded leaves.
    • Encourage insectivorous birds by hanging feeders during the winter months and provide nesting boxes in the spring.

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