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.
When choosing a product for eliminating whiteflies from
your flowers and plants, remember that each product might kill only specific stages of the
pest. You might also consider that the preferred product can have other uses, such
as indoor or outdoor pest control.
Apply your insecticide when first stage nymphs or adults have emerged. In heavy whitefly populations of mixed life stages, two to three applications per week may be necessary to bring the population under control with a contact insecticide. Read and follow label instructions; each product can have different limits on how often applications can be made.
Proper application of the insecticide is also a key component to a successful pest control program. It is necessary to deliver the insecticide to the undersides of leaves to achieve good control. As many crops mature, a dense canopy of foliage forms that interferes with pesticide delivery. With these crops, it is necessary to control whiteflies prior to the formation of this canopy or to space plants so they can be treated adequately.