Whiteflies are closely related to aphids, mealybugs and scale, all of which feed by sucking sap from plants. Whiteflies can be found on the undersides of leaves and are active during the daytime when the temperature is warm.
Thanks to the University of Missouri Extension for this information on whiteflies.
When a heavily infested plant is disturbed, white clouds of winged adults fly into the air.
Some species of whiteflies can become serious pests of certain vegetable crops, greenhouse plants or ornamental plants. Two of the most important species are the greenhouse whitefly, Trialeurodes vaporariorum, and the sweet potato whitefly, Bemisia tabaci. In colder climates, whiteflies die outdoors, but in warmer climates, as well as indoors and in greenhouses, they can reproduce throughout the year with several overlapping generations
Adult whiteflies are about 1⁄10 to 1⁄16 inch long and look like tiny moths. They have four broad, delicate wings that are held rooflike over the body and covered with a white powdery wax. Adult females usually lay between 200 and 400 eggs. Sometimes the eggs are deposited in a circular pattern in groups of 30 to 40 because the female will often keep her mouthparts in the plant to feed while moving her abdomen in a circle.
Within about a week, the eggs hatch into flattened nymphs, called crawlers, that wander about the plant. Soon, they insert their mouthparts into the plant and begin to feed. After their first molt, the nymphs lose their legs and antennae. They attach themselves to the undersides of leaves with several waxlike rods coming from their bodies, giving them the appearance of small white oval scale. The nymphs remain fixed to the plant and feed for about four weeks. After a pupa stage, the adults emerge and live for about one month. Within a population, all life stages are present, and generations often overlap.