White Fly management under protected cultivation system

Feb 20, 2010 Letters

Whiteflies have begun showing resistance to synthetic insecticides and are very serious shadehouse/greenhouse pests. 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 shadehouse/greenhouse will reduce fruit quality and subsequent economic loss.
Several species of whiteflies attack shadehouse/greenhouse plants, and they typically have a wide host range and resist insecticides. Shadehouse/greenhouse tomatoes and cabbage are especially plagued by whiteflies. The most common whiteflies on shadehouse/greenhouse crops are the shadehouse/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 whitefly 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 shadehouse/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 shadehouse/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 shadehouse/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 shadehouse/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.
Shadehouse/greenhouse plastics themselves may have significant influence on the initial attraction of insects into shadehouse/greenhouses. A study from the late 1990s showed that silverleaf whiteflies preferred to enter shadehouse/greenhouses covered with film that transmitted higher levels of ultraviolet light.
Insect Growth Regulators
Insect growth regulators (IGRs) are one of the least toxic pesticide control options. IGRs typically kill insects by disrupting their development. They have a complex mode of action that precludes insects from rapidly developing resistance. IGRs can work in one of several ways:
(1) They can mimic juvenile hormones, so that insects never enter the reproductive stage of development;
(2) They can interfere with the production of chitin, which makes up the shell of most insects; or (3) they can interfere with the molting process.
IGRs usually work through ingestion, so good spray coverage is essential. They generally don’t affect non-target species, such as humans, birds, fish or other vertebrates. For most IGRs there are minimal re-entry restrictions. IGRs typically take several days to have an effect on pest populations. Because IGRs do not affect mature insects, adult beneficials released into the shadehouse/greenhouse after an IGR application are not likely to be affected. Use of IGRs is generally prohibited by organic certification organizations because the products are synthetic. IGRs can sometimes be used in conjunction with biological control efforts and may provide growers with a safety net, should beneficials fail to keep the pests below economically damaging levels.
Bissasar Chintamanie
Research Scientist

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White Fly management under protected cultivation system

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