The two main species of whitefly which may infest hydroponic greenhouse crops are the greenhouse whitefly (T. vaporariorum) and the tobacco whitefly (Bemisia tabaci) (Perdikis et al., 2008). While both species are pests of a wide range of crops, the greenhouse whitef ly is the most common worldwide under protected cultivation. Whitefly adults are small (2 mm), moth-like in appearance and will fly up from the foliage when disturbed (Fig.1). In crops such as tomato, cucumber and capsicum, greenhouse whitefly adults will congregate within the tops of the plants to feed and lay eggs. Whitefly develop through three larval instars and a pupal stage to adult. Eggs are laid on the undersides of the newest leaves in the crop and these take 4–10 days to hatch into the f irst instar. Larval instars are flat, oval in shape and transparent. Development time from egg to adult is dependent on temperature and host plant with rapid population increases occurring under warmer conditions. The first instar is the only mobile nymphal instar and selects a location on the underside of the leaf to establish, where it remains stationary (Perdikis et al., 2008). Later nymph stages and pupae are typically located on the older leaves. Removal of these lower, older leaves of tall plants such as tomatoes is one means of slowing population increase by removing foliage containing late instar nymphs or pupae.

Whitefly damage crops by both sucking sap from the foliage and depositing honeydew, a sticky substance which contaminates produce and is rapidly colonized by black sooty mould. The presence of honeydew and sooty mould not only reduces photosynthesis and assimilate production by the plant, but also necessitates the washing of fruit after harvest to remove this contamination. Heavily infested plants become weakened by whitefly feeding and may exhibit yellowing leaves, lack of plant vigour, reduced fruit size and yields. Whitefly may also transmit virus diseases in many crops and are an efficient vector of the tomato yellow leaf curl virus (Mehta et al., 1994).

It has become increasingly difficult to control greenhouse whitefly using insecticides as this pest has developed genetic resistance to a variety of pesticide compounds (Kamikawa et al., 2018). Classes of pesticide must be rotated regularly and persistent chemicals avoided to prevent spray resistance. Other spray options such as soaps and oils may be used to a limited extent on pests such as whitefly; however, frequent, long-term use has been known to cause extensive spray damage in many greenhouse crops. Control of greenhouse whitefly often requires an integrated approach. Screening of vents and double-door entries can assist with exclusion of flying adults. Removal and control of host plants such as weed species in the immediate vicinity of the greenhouse can reduce the source of infestations. The use of yellow sticky traps placed close to vents, doors and other entry points can serve as a monitoring aid to determine when the pests may have entered a crop (Lemic et al., 2020). Monitoring pest numbers by counting the number of nymphs found on the undersides of leaves can be used to determine when control measures need to be applied, or the effectiveness of IPM programmes. Biological, botanical and chemical control options are available for whitefly. T hese Hydroponics and Protected Cultivation a number of predator and parasites which are commercially available for release into greenhouse crops. Parasitic wasps, Encarsia formosa and Eretmocerus eremicus, are the most commonly utilized species; however, long-term control requires an environment conducive to survival of the parasite as well as maintaining a balance between pests and control agents. Botanical control agents, such as extract from the seed kernel of the neem tree (Azadirachta indica), which contains insect growth regulatory, antifeedant and other activities, have been developed into a number of spray products that are used for greenhouse whitefly control.

Reference : A Practical Guide Hydroponics and Protected Cultivation by Lynette Morgan

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