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It is important to ensure that plants are not infested with T. vaporariorum before being taken into the glasshouse. This is equally as important when whitefly populations are already present in the glasshouse, due to the risk of introducing novel insecticide resistant strains. A suitable insecticide can be applied as a routine precaution. Recent studies have demonstrated the effectiveness of UV absorbent plastic films at reducing T. vaporariorum infestations on protected crops (Mutwiwa et al., 2005).
Biological control has been widely used in glasshouses, especially since the development of insecticide-resistant whiteflies, and is chiefly based on the chalcid wasp Encarsia formosa and entomopathogenic fungi (Osborne and Landa, 1992).
E. formosa parasitizes T. vaporariorum, each female being capable of laying 50-200 eggs during its lifespan of 10-14 days. Each egg is inserted into advanced nymphal stages which are subsequently killed. Scales containing parasites are highly visible because they are black. Successful control can be obtained if the parasite is established on plants when natural infestations are small. The parasitoid is usually shipped as parasitized scales on strips. For cucumbers, a typical rate of release is 12,500 parasitoids per ha in areas where T. vaporariorum is a habitual pest, starting at the beginning of the season. In tomato, most growers introduce parasitized scales weekly at the rate of 5000/ha as soon as plants are available. Once the parasitoid is established, it is important to avoid removing from the glasshouse leaves with black scales from which adult wasps have not emerged. Empty scales can be spotted by holding leaves up to the light and examining them for emergence holes (Sprau, 1990; Lynch and Johnson, 1991; Ruisinger and Backhaus, 1994; van Lenteren et al., 1996; van Roermund et al., 1997).
The predatory beetle Delphastus pusillus is very effective against greenhouse and sweet potato whitefly (Bemisia tabaci). Both the larval and adult beetles feed on all stages of T. vaporariorum and will eat spider mites when whitefly are not available.
Lacewings (species of Chrysoperla) are also used as general predators of glasshouse pests and will consume whiteflies.
The fungal pathogen Verticillium lecanii attacks whiteflies and thrips and can be a useful control agent in situations where the crop is grown in high humidities (Masuda and Kikuchi, 1993). The disease attacks young as well as adults, taking about 1-2 weeks to develop. Commercial preparations are available (Ravensburg et al., 1990, 1993). Microbial insecticides based on the entomopathogenic fungus Paecilomyces fumosoroseus have also been used for the control of T. vaporariorum (Bolckmans et al., 1995; Sterk et al., 1996).
Soria et al. (1996) showed the participation of both antixenosis and antibiosis resistance mechanisms against T. vaporariorum in an accession of melon (C. melo var. agrestis). Lambert et al. (1995) investigated possible resistance mechanisms in soyabean and discovered that trichome erectness was a factor. Cultivars supporting lower whitefly populations had higher erectness ratings (trichomes flat on leaf surface) than accessions with higher whitefly populations. An investigation of epicuticular lipid composition also revealed that soyabean accessions with low levels of luteol were more susceptible to whitefly colonization (both T. vaporariorum and Bemisia tabaci).
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:
Whiteflies damage plants directly by sucking sap from leaves and indirectly by transmitting viruses and producing a sticky secretion known as honeydew, which prevents crops from functioning normally, as well as acting as a substrate for fungal growth (sooty moulds).
Whitefly adults and nymphs feed by inserting their proboscis into the leaf, penetrating the phloem or nutrient conducting vessels and withdrawing sap. As it feeds, the whitefly injects saliva into plant tissues. Whitefly feeding removes nutrients from the plant which may result in stunting, poor growth, defoliation, reduced yields and even death in extreme cases. The extent of damage caused by feeding is generally directly proportional to the whitefly population and low populations rarely have much impact. On certain plants, stunting or abnormal coloration can be caused due to the physiological stress of the feeding. Plants stressed by the removal of sap due to heavy whitefly feeding may require more irrigation.
Whiteflies secrete significant quantities of honeydew as they feed. When populations of whiteflies are high, honeydew production can be copious, dripping down leaves onto fruit. Honeydew becomes a serious problem when it is colonized by black, sooty mould fungi, blackening leaf or fruit surfaces. This can render fruit unmarketable and block out sunlight, inhibiting photosynthesis.
T. vaporariorum became an economical important insect pest of greenhouse vegetable and ornamental crops in the middle 1970s in Beijing, China. More recently, it has become a serious horticultural pest within areas of southern Europe, where an increase in the incidence of Tomato chlorosis virus may be attributed to it. Within the UK, insecticide resistance has led to increased problems for growers of shrubs such as Ceanothus and soft fruit crops such as strawberries.
The piercing and sucking mouthparts of T. vaporariorum provide an excellent mode for transmitting disease-causing viruses from one plant to another. As immature stages do not move on to new plants, virus transmission is a concern only with adult whiteflies.
Some of the more important viruses spread by T. vaporariorum are Beet pseudo-yellows virus (cucumbers, melons, lettuces and sugarbeet), Tomato infectious chlorosis virus and Lettuce infectious yellow virus. Further viruses are being identified and characterized which include Strawberry pallidosis virus (Tzanetakis et al., 2004).