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Early indication of infestation may consist of chlorotic spots caused by larval feeding, which may also be disfigured by honeydew and associated sooty moulds. Leaf curling, yellowing, mosaics or yellow-veining may also indicate the presence of whitefly-transmitted viruses. These symptoms are also observed in B. tabaci infestations, however phytotoxic responses such as a severe silvering of courgette and melon leaves, mis-ripening of tomato fruits, stem whitening of brassicas and yellow veining of some solanaceous plants may also be seen (Costa et al., 1993; Secker et al., 1998).
The feeding of adults and nymphs causes chlorotic spots to appear on the surface of the leaves. Depending on the level of infestation, these spots may coalesce until the whole of the leaf is yellow, apart from the area immediately around the veins. Such leaves are later shed. The honeydew produced by the feeding of the nymphs covers the underside of leaves and can cause a reduction in photosynthetic potential when colonized by moulds. Honeydew can also disfigure flowers and, in cotton, can cause problems in lint processing. Following heavy infestations, plant height, the number of internodes, and yield quality and quantity can be affected, for example, in cotton.
Phytotoxic responses in many plant and crop species caused by larval feeding include severe silvering of courgette leaves, white stems in pumpkin, white streaking in leafy Brassica crops, uneven ripening of tomato fruits, reduced growth, yellowing and stem blanching in lettuce and kai choy (Brassica campestris) and yellow veining in carrots and honeysuckle (Lonicera) (Bedford et al., 1994a,b).
A close observation of leaf undersides will show tiny, yellow to white larval scales. In severe infestations, when the plant is shaken, numerous small and white adult whiteflies will emerge in a cloud and quickly resettle. These symptoms do not appreciably differ from those of Trialeurodes vaporariorum, the glasshouse whitefly, which is common throughout Europe.
Intercropping practices using non-hosts have been used in many countries aiming to reduce numbers of whiteflies on specific crops. However, intercropping with susceptible crops can promote whitefly populations, by offering a greater leaf area for feeding.
Weed species can play an important role in harbouring whiteflies between crop plantings and attention should be paid to removing these in advance of planting susceptible crops. Weeds also often harbour whitefly-transmitted viruses (Bedford et al., 1998) and may be a major source of crop virus epidemics, especially where MED species is present, due to its polyphagous nature.
Cultural control is generally much more effective where whiteflies are physical pests rather than virus vectors.
The development of transgenic resistant plant and crop species through genetic engineering must be considered and accepted as a future method of control where whitefly-transmitted viruses are already endemic and causing severe crop losses (Wilson, 1993; Raman and Altman, 1994). Traditional sources of resistance have been used successfully for the control of other whitefly species.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Currently difficult to determine specific economic damage caused for MED species.