Whiteflies are tiny, snow-white insect pests that (when viewed under a magnifying glass) resemble moths. When viewed without magnification, these insects look more like flying dandruff! Although they might resemble moths, they are actually more related to scale insects. In fact, they are often confused with soft scale insects. Both adult and nymph stages feed by sucking plant juices. Heavy feeding by these pests can give plants a mottled look, cause yellowing and eventually death to the host plant.
Sticky honeydew excreted by these insects glazes both upper and lower leaf surfaces, permitting the development of black sooty mold fungus. Besides being unattractive, sooty mold interferes with photosynthesis, which retards plant growth and often causes leaf drop.
The most common and perhaps most difficult to control insect pests in greenhouses and interior landscapes are whiteflies. Three common species of whiteflies, the greenhouse, sweet potato and banded wing, are potential pests on a wide variety of crops. They attack a wide range of plants including bedding plants, cotton, strawberries, vegetables, and poinsettias. In addition to attacking many different crops, whiteflies are difficult to control. The immature stages are small and difficult to detect. Growers often buy plants, unaware of the whitefly infestation present.
Once adults develop and emerge inside a greenhouse or hothouse, they quickly become distributed over an entire crop or infest other available plants. Chemical control programs directed at the pest often have limited success. Two life stages (egg and pupa) are tolerant of most insecticides. Control measures are also complicated by the insects clinging on to the underside of leaves, making them difficult to reach with chemical or oil sprays.
All species of this plant pest develop from the egg through four nymphal instars before becoming adults. Elapsed time (from egg to adult) varies with species. Eggs are deposited on the undersides of leaves and are often found in a circular or crescent-shaped pattern. The "crawler" hatches from the egg, moves a short distance and then settles and begins feeding -- sucking juices from its plant host. The remainder of the nymphal development is spent in this sedentary condition. The adult whitefly emerges from the pupal case and flies to other host plants to lay eggs and begin the cycle again. Fourth instar nymphs (called pupae) and adults are most frequently used to distinguish one species from another.