WHITE flies have begun showing resistance to synthetic insecticides and are very serious shade house/greenhouse pest. Not only do they feed on plants, but they also produce honeydew, which detracts from the plants’ appearance and attracts other insects and sooty mold. Whiteflies can also transmit plant viruses. The mere presence of whiteflies in a shade house/greenhouse will reduce fruit quality and subsequent economic loss.
Several species of whiteflies attack shade house/greenhouse plants, and they typically have a wide host range and resist insecticides. Shade house/greenhouse tomatoes and cabbage are especially plagued by whiteflies. The most common whiteflies on shade house/greenhouse crops are the shade house/greenhouse whitefly (Trialeurodes vaporariorum), sweetpotato whitefly (Bemisia tabaci) and the silver leaf whitefly (Bemisia argentifolii).
The various whitefly species and biotypes look very much alike, but they have subtle physiological differences. These differences can cause them to respond differently to control strategies. Because control measures must be selected according to the type of whitefly present, accurate identification is critical to successful control.
Plants should be visually inspected for signs of a white fly infestation e.g. off-colour or stunted plants. A hand lens is useful for systematically inspecting a number of individual plants for the presence of eggs, nymphs or adults. Both the upper and lower leaf surfaces should be inspected. It is important to check the shade house/greenhouse in the same pattern on each scouting trip. Locations where whiteflies are found should be flagged so that population development and control efforts can be evaluated. Scouting frequency should be increased during warm weather as whitefly populations multiply faster then. Trapping with yellow sticky cards/boards, both inside and outside the shadehouse/greenhouse, is essential for a successful whitefly management program. The cards are used to detect and monitor population levels. As a general rule, 1 to 4 cards spaced evenly throughout 1000 square feet of shade house/greenhouse are sufficient. A generally acceptable threshold for whiteflies is 0.5 per card per day when the crop is young, and 2 per card per day as the crop reaches maturity. Traps should be hung level with the tops of the plants since whiteflies are most attracted to young foliage. Doors, vents and other openings where whiteflies can enter the shade house/greenhouse are other good sites to hang yellow sticky cards.
Researchers in USA have successfully used silver-painted pot spaces and silver polyethylene mulch to control whiteflies on shade house/greenhouse cabbages. The reflective materials were used in conjunction with yellow sticky cards or tape and resulted in significantly enhanced trapping of whiteflies, relative to controls with sticky traps only. Shade house/greenhouse plastics themselves may have significant influence on the initial attraction of insects into shade house/greenhouses. A study from the late 1990s showed that silver leaf whiteflies preferred to enter shade house/greenhouses covered with film that transmitted higher levels of ultraviolet light.