You know whiteflies are present when you touch a plant and a mass of tiny, white, winged insects fly out of it. Turn over a leaf and you'll see other adults as well as stationary pupae and nymphs. Both adults and nymphs suck a plant's juices, debilitating it moderately or severely depending on the size of the pest population. Symptoms range from yellow stippling on leaves to browned, curled foliage. The pests also excrete honeydew, which attracts ants and fosters sooty mold.
Natural enemies such as parasitic wasps normally help keep whiteflies under control. For severe infestations, you can purchase wasps and introduce them into the garden; the most common is Encarsia formosa. Or check with your local Cooperative Extension Office to find out if a different wasp might better manage the problem. You can also hose off plants with strong jets of water.
Chemical control is not recommended. Insecticides don't easily penetrate the pupae and nymphs, and more adults fly away than are caught by the spray. You'll kill more helpful wasps than you will whiteflies, and the pest population will simply increase.