The greenhouse whitefly, Trialeurodes vaporariorum, can be a common insect pest in greenhouses and on houseplants and garden plants during the summer. This insect has a host range of more than 250 ornamental and vegetable plants. Poinsettia, begonia, nicotiana, aster, calendula, cucumber, lantana, tomato, grape, ageratum, bean and hibiscus are commonly infested. The greenhouse whitefly sucks sap from the plant, primarily from the phloem. Heavy infestations cause decline of plant vigor. Stunting, yellowing of foliage and premature leaf drop are among the symptoms of injury. Whiteflies excrete sticky honeydew during feeding. This detracts from plant appearance and allows gray sooty mold fungi to grow on the foliage. In recent years, a second whitefly has become established in some Colorado greenhouses: the silverleaf whitefly, Bemisia argentifolii. This species also is sometimes known as the sweetpotato whitefly Strain B or the "poinsettia whitefly." It has become endemic in the southwestern United States and in Florida. There it causes serious damage to crops and ornamental plants. It is far less common on houseplants than the greenhouse whitefly. Prevention of whiteflies is the best management tool. Infestations originate only from infested plant materials, so all new plants should be checked carefully and quarantined before moving them into a room with susceptible plants. Once the insects are established in a greenhouse, a host-free period can starve them. This might mean removing for at least two weeks any plants on which the whitefly can survive. Adult whiteflies are highly attracted to yellow and white, and they are often found on lighter-colored foliage. To capture the winged whiteflies appropriately colored sticky traps can be placed just above susceptible plants or around edges of plantings. These traps usually are flat sheets or tape coated with a sticky material that can hold the insects. Insecticidal control can be difficult because the insect is resistant to most insecticides. Furthermore, egg and the nonfeeding pupal stages are generally not susceptible to insecticides that can kill adults and nymphs. Thus repeated applications are usually required. The most effective insecticide for whitefiles is imidacloprid, which has systemic properties and is usually used as a soil application, taken into the plant via the roots. It is not registered for most home-grown vegetables. Horticultural oils (e.g., SunSpray) are one of the most effective whitefly controls available for use on houseplants. Applied as dilute sprays (e.g., 2 percent), they smother the immature forms and eggs. Other products effective against nymphs are neem-based insecticides and some insecticidal soaps. Because of the potential for plant injury, they should be tested first on a few plants or on a small area of a plant. Adult whiteflies are usually best controlled with pyrethrins or various pyrethroid insecticides labeled for use on houseplants (e.g., permethrin, tetramethrin, bifenthrin, resmethrin). For more information see the fact sheet Greenhouse Whitefly.

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