Whiteflies may be small--the insects measure less than 2 mm long--but they can pose a significant health threat to your tomato crop when they're present in significant numbers. Common symptoms of an infestation include leaf yellowing and leaf curl. Cultural and chemical control options are available to the backyard gardener to reduce and eradicate this troublesome garden pest.
Plant your tomato plants as far from whitefly-attracting plants as possible. This is the best way to control whiteflies, according to the University of California. Common problem plants include cole crops like cabbage, kale and broccoli, and all types of melons. If you cannot plant your tomatoes away from these plants, like if you are restricted to a small backyard garden, rotate your crops so that you're never growing tomatoes at the same time as the problem plants.
Spray your tomato plants with a vigorous stream of water. This knocks most whiteflies and their larvae off your plants and, when repeated daily, discourages colonization of your plant.
Treat your tomato plants with a standard insecticidal soap, available from most garden stores and nurseries. Spray the soap onto all exposed surfaces of your tomato plant, including its stems. The product smothers and kills any whiteflies that are present.
Avoid reintroducing whiteflies into your garden. When purchasing seedlings or plants from a nursery, always inspect them for pests before bringing them home. If you grow melons or cale crops, remove all vestiges of the plants after harvest, including stems and fallen leaves, to minimize the risk of attracting more whiteflies.