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Glasshouse whitefly is a common sap-feeding insect, mainly of house-plants and greenhouse plants. They excrete a sticky substance (honeydew) on foliage, which allows the growth of sooty moulds.
Glasshouse whitefly. Image: ©www.gardenworldimages.com
Glasshouse whitefly is a sap-sucking true bug that can reduces the vigour of plants and excretes a sticky, sugary substance, called honeydew, on the leaves, stems and fruits of its host plants.
It attacks many vegetables and ornamental plants grown in greenhouses as well as house-plants. These include: cucumber, melon, tomato, peppers, Chrysanthemum, Gerbera, Pelargonium, Fuchsia, Lantana, poinsettia and Verbena. Outdoor plants can also be attacked but not to such a damaging degree. Note that whiteflies seen on brassicas, Viburnum tinus, honeysuckle, evergreen azalea and rhododendron are other species of whitefly specific to those plants.
Glasshouse whitefly thrives in warm conditions, which is why it is not usually a problem on outdoor plants. This insect is active all year round on houseplants and in greenhouses.
Due to this insects rapid reproductive rate and the widespread occurrence of pesticide-resistant strains, biological control often gives better results than insecticides on greenhouse plants.
This involves introducing tiny parasitoid wasps, Encarsia formosa, which prey on the whitefly scale nymphs. The parasitoid is available by mail order from the suppliers of biological controls. It is important to introduce the parasite before plants are heavily infested as it cannot give instant control. Parasitised nymphs turn black so it is easy to monitor the progress of the control. As Encarsia is killed by most insecticides, avoid spraying with products other than fatty acids, plant invigorators, plant extracts or plant oils (see below) prior to its introduction.
Other non-pesticide controls
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
Biological control suppliers (Adobe Acrobat pdf document)
This sap-sucking insect breeds rapidly and produces many generations in warm greenhouse conditions. The adults and their whitish scale-like nymphs live on the underside of the leaves where they feed on sap, weakening the plants. They lay greyish white cylindrical eggs either singly or in circles on the underside of the leaves.
Each female can lay more than 200 eggs. Males are rare and reproduction takes place without the need for fertilization. The eggs hatch into small crawler scale-like nymphs which crawl around for a while before they begin feeding and become immobile. The nymphs are a flat, oval shape, whitish-green in colour, and just over 1mm in length when fully developed. The final nymphal stage is called a pupa and the adult whitefly eventually emerges through a slit in the dorsal surface. The length of the life cycle varies according to the temperature. At 10ºC (50ºF) the life cycle takes several months, but can be completed in about three weeks at 21ºC (70ºF). The pest can remain active during the winter in an unheated greenhouse, provided suitable host plants are present. Glasshouse whitefly does not usually survive winter out of doors.
Image: © GWI/John Swithinbank. Available in high resolution at www.gardenworldimages.com
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