Question. What is the best method to rid my greenhouse of whiteflies? Is there a prevention I should implement? Are there certain plants that attract whiteflies?

Answer. Whiteflies remove plant fluids with their piercing-sucking mouthparts. They feed primarily on leaves, which may result in plant stunting and leaf distortion. Whiteflies produce a clear, sticky liquid called honeydew. Honeydew serves as a medium for growth of black, sooty mold fungi. These unsightly fungi can reduce photosynthesis and the plant's appearance.

Whiteflies come in through openings in the greenhouse, or on new plants that you bring into the greenhouse. In your case, you're probably bringing whiteflies in on your plants when you bring them in for winter. Inspect each plant for all whitefly life stages, including eggs. Look especially on the undersides of the leaves.

Apply contact insecticides when whitefly numbers are building up. Be sure to thoroughly cover leaf undersides. Appropriate insecticides, including insectidal soap, horticultural oils and others, are available at http://www.hort.purdue.edu/hort/ext/Pubs/HO/ID218.pdf .

Monitor with yellow sticky cards placed 1 to 2 inches above the plant canopy. These are available from some mail-order garden supply companies. Place two sticky cards per 1,000 square feet. Count the number of whiteflies on the sticky cards with a 10x hand lens. Record the number of whiteflies trapped on the cards before and after insecticide applications to determine efficacy. Make decisions about spraying by actually inspecting plants for pests, and spray only when necessary.

Reduce the introduction of whiteflies by putting up screens to exclude them. Growers who have installed screens report their use of pesticides declined by 50-90 percent. Several species of parasitic wasps are quite effective against whiteflies. See http://www.biocontrol.ucr.edu/bemisia.html for more information.

-- Beverly Shaw, Advanced Master Gardener, Purdue University

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