A 3-yr project was initiated in 1993 to examine the effects ofinsecticides and sustained whitefly, Bemisia argentifoliiBellows & Perring [aka. B. tabaci Gennadius (Strain B)],feeding on alfalfa plant growth and vigor in greenhouse cage studies,and to determine the impact of natural Bemisia whiteflypopulations on alfalfa forage yields and quality in a large-plot fieldexperiment. Alfalfa plant growth and vigor after exposure toimidacloprid and a mixture of fenpropathrin and acephate insecticidesdid not differ from untreated plants in the greenhouse. Consequently,foliar and soil applied insecticides were used to manipulate whiteflydensities on alfalfa plants to measure whitefly feeding effects onplant growth and forage yield. Heavy whitefly densities on untreatedalfalfa plants in the greenhouse resulted in significant reductions inrelative growth rates and net assimilation rates as compared withimidacloprid-treated plants that were maintained relativelywhitefly-free. Reductions in alfalfa plant growth measured betweeninfested and treated plants were proportional to whitefly densities.Field plot results derived from three crop seasons were relativelyconsistent with our greenhouse trials. Both experimental approachesclearly showed that alfalfa plants exposed to high densities ofwhitefly immatures and adults grew at a significantly slower rate andproduced less foliage. As a result of reduced growth rates, alfalfamaturity in the naturally infested plots was estimated to be ≈7–10 dbehind managed plots. Delays in maturity resulted in significantreductions in forage yields of 13–18% during August–Septemberharvests when whitefly populations reached peak abundance. Whiteflyfeeding stresses also effected hay quality through the reduction ofcrude protein content and contamination of foliage with honeydew andsooty mold. The status of the Bemisia whiteflies as aneconomic pest to alfalfa is clearly evident from these studies, but thedamage potential of whiteflies in the southwestern United Statesappears to be restricted to one or two harvest periods during thesummer coinciding with peak adult populations and their dispersal fromalternate host crops.