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The whitefly Bemisia argentifolii Bellows & Perring has been a majorpest of crops in the southwestern United States since 1991. However,there has been no published information on whitefly populations on alfalfa, Medicago sativa L., a major crop found throughout this region. In this study, adult and immature Bemisia argentifolii were collected every 2 or 4 wk in 22 commercial fields of alfalfa within localized sites in the Imperial Valley, CA, and in Yuma, AZ, from June 1993 to January 1996. Populations of all stages of whiteflies on alfalfa were low or nonexistent from December to May at both sites. In the Imperial Valley, adult populations increased in June and peaked during September, and decreased from October to November. In Yuma, adult populations peaked during August, and decreased late October to November. Populations of eggs, 1st to 4th instars, late 4th instars, and exuviae at both sites increased in June, July, and August. Compared with densities of eggs and nymphs, densities of exuviae were low on alfalfa.In the Imperial Valley, adult populations were significantly higher in 1993 than in 1995, and in Yuma, adult populations were higher in 1993 than 1994. Greater populations of whiteflies were found in Imperial Valley than in Yuma in 1993 and 1994. Densities of whiteflies of all stages were similar among fields within sites during the season. Eggs were distributed evenly among leaves from top, middle, and bottomstrata of plants, whereas nymphs were found mostly on those from themiddle and bottom strata. Low percentages of parasitized whitefly nymphs (< 1%) were found on alfalfa, and only 2 adult whitefly parasitoids were collected during the 3 yr. The results of this study suggestthat control measures for whiteflies on alfalfa in southern California and Arizona may be necessary only from June or July through September. Controlling whiteflies on alfalfa during these months may reduceany damage the whiteflies cause to alfalfa, but it probably would not have [ABSTRACT FROM AUTHOR]