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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 Brassica crops and yellow veining of some solanaceous plants are only caused by MEAM1 (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.
Cultural Control
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 MEAM1 species is present, due to its polyphagous nature.
Cultural control is generally much more effective where whiteflies are physical pests rather than virus vectors.
Host-Plant Resistance
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.
Chemical Control
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:
MEAM1 species of B. tabaci can have a serious impact on the production of certain field crops as well as a wide range of protected horticultural crops. In the majority of cases, this is due to viruses that the whitefly transmits between susceptible crops or acquires from indigenous host reservoirs. MEAM1 is also able to induce a phytotoxic response from a number of plant species that could cause yield loss or reduced quality produce. This includes squash silver leaf (Bedford et al., 1994b), pumpkin white stem (Costa and Brown, 1991), white streaking of cole crops (Brown et al., 1992), reduced growth and stem blanching of kai choy (Costa et al., 1993) and uneven ripening of tomato (Maynard and Cantliffe, 1989). All of these can affect the yield and quality of a crop and thus its market value. In 1991, MEAM1 alone caused an estimated $500 million loss to the 1991 winter harvest in California, USA, mainly through virus damage. However, in other areas of the world where MEAM1 has appeared, it is found alongside an indigenous non-B biotype, so it is extremely difficult to determine specific economic damage. For example, MEAM1 is found alongside the K biotype in Pakistan where both biotypes transmit a disease of cotton, Cotton leaf curl virus. Around 2 million tonnes of cotton are grown in Pakistan and between 30 and 40% crop losses can be expected through whitefly-transmitted viruses based on figures in the mid-1990s. An estimate of 2.4 billion dollars damage was caused by the virus between 1993 and 1994 (Bhatti and Soomro 1996). In 1994, the cotton virus spread to India as did a whitefly-transmitted virus of tomato, Tomato leaf curl virus (Colvin et al., 2002), which caused a number of complete crop failures. This tomato virus was then reported to have spread to potato (Gard et al., 2001). Again MEAM1 was present within the epidemics although indigenous biotypes G, H and I were also recorded from India, so specific damage attributed to MEAM1 alone, could not be calculated.
Within Israel around the Mediterranean Basin, North Africa and on the Canary Islands MEAM1 is present alongside the indigenous Mediterranean (MED) species (formerly known as biotype Q). As seen in Pakistan, it is impossible to calculate the economic impact of MEAM1 alone in these areas. The economic impact of more recent appearances of MEAM1 within Africa, South and Central America and Australasia currently remains unknown.
