by Dr. Richard Lindquist
OSU/OARDC Entomology Department
The two most important whitefly pests in floriculture are the greenhouse whitefly, Trialeurodes vaporariorum and the silverleaf whitefly, Bemisia argentifolii. Both whitefly species have been recorded from hundreds of plant genera.
The generalized life cycle (Figure 1) for both whitefly species is as follows: Females live about one month and produce 60 to 100 eggs, which hatch in 7 to 10 days. The newly-hatched immatures, or "crawlers" move for a short distance before settling down to feed. After three molts a "pupal" stage is formed, from which adults emerge in about 6 days. Whiteflies complete their egg to adult cycle in 21 to 36 days, depending on temperatures. All developmental stages are found on leaf undersides.
Adults and immature stages have piercing-sucking mouthparts. Adults on plants are a nuisance. Feeding by large numbers of adults, and especially immatures, can weaken or kill plants. Moderate to heavy silverleaf whitefly infestations on poinsettia can cause stunting, bract deformity and a condition called "white stem" (Figure 2). Whiteflies also produce sticky honeydew, which can be a substrate for black sooty fungus.
To detect whiteflies, inspect plants weekly. Be especially alert when receiving shipments of known whitefly host plants. At first, whiteflies do not occur uniformly throughout the crop. A 10 to 15X hand lens will be needed to see eggs and small immature stages. Yellow sticky traps will attract adults, but will not replace plant inspection as a detection method.
Adult silverleaf whiteflies are usually smaller, more yellow and more active than greenhouse whiteflies (Figures 3 and 4. The wings are held more vertically as well. The immature stages of greenhouse and silverleaf whiteflies are also different, especially in the pupal stage (Figure 5). Silverleaf whitefly pupae are convex or flattened, without spines or fringes. Greenhouse whitefly pupae, have vertical sides with spines.
Whiteflies can be be resistant to one or more organophosphate, carbamate, and pyrethroid insecticides. At this time, there is an effective systemic insecticide (Marathon), representing a different chemical class (chloronicotinyl), that is now providing whitefly control for 6 to 10 weeks with one application. However, other pesticides (including biorationals) will be needed, either before or after Marathon application to obtain control for an entire crop cycle.
Pesticide application technique is very important in whitefly control. For contact insecticides sprays must be aimed at the leaf undersides. Generally, smaller spray drops and/or higher spray pressures are most effective. Whenever possible, combine spray applications with aerosol, fog, or smoke generator applications.
The main beneficial insect used against the greenhouse whitefly is the parasitic wasp Encarsia formosa (Figure 6). This parasite is less effective against silverleaf whiteflies. Other parasite species show some promise as silverleaf whitefly controls, including E. luteola and Eretmocerous spp. A predatory coccinelid beetle, Delphastus pusillus, has been very effective in reducing high populations of silverleaf whiteflies.
Other promising biological controls are naturally-occurring entomopathogenic fungi including Beauveria bassiana and Paecilomyces fumosoroseus. B. bassiana is registered in the USA for use in greenhouses and outdoor crops. P. fumosoroseus is not yet registered.
