Some 200 species of these pests cause problems. Like aphids (their close relatives), they’re sap feeders, sucking plant juices from leaf undersides. The adults, which look like tiny white moths, fly up in a cloud when disturbed. Some of the nymphal stages resemble scale insects. Whiteflies are found the year around in warm climates; in colder regions, you’ll see them only during summer. Warm, still air is the perfect environment for whiteflies, making greenhouses a favored haunt—one species is even called the greenhouse whitefly.