SEASONAL POPULATION TRENDS OF BEMISIA-ARGENTIFOLII (HOMOPTERA, ALEYRODIDAE) ON ALFALFA IN SOUTHERN CALIFORNIA AND ARIZONA

The whitefly Bemisia argentifolii Bellows & Perring has been a major p ests of crops in the southwestern United States since 1991. However, t here has been no published information on whitefly populations on alfa lfa, Medicago sativa L., a major crop found throughout this region. In this study, adult and immature Bemisia argentifolii were collected ev ery 2 or 4 wk in 22 commercial fields of alfalfa within localized site s in the Imperial Valley, CA, in Yuma, AZ, from June 1993 to January 1 996. Population of all stages of whiteflies on alfalfa were low or non existent from December to May at both sites. In the Imperial Valley, a dult populations increased in June and peaked during September, and de creased from October to November. In Yuma, adult populations peaked du ring August, and decreased late October to November. Populations of eg gs, 1st to 4th instars, late 4th instars, and exuviae at both sites in creased in June, July, and August. Compared with densities of eggs and nymphs, densities of exuviae were low on alfalfa. In the Imperial Val ley, adult populations were significantly high in 1993 than in 1995, a nd in Yuma, adult populations were significantly higher in 1993 than i n 1995, and in Yuma, adult populations were higher in 1993 than 1994. Greater populations of whiteflies were found in Imperial Valley than i n Yuma in 1993 and 1994. Densities of whiteflies of all stages were si milar among fields within sites during the season. Eggs were distribut ed evenly among leaves from top, middle, and bottom strata of plants, whereas nymphs were found mostly on those from the middle and bottom s trata. Low percentages of parasitized whitefly nymphs (< 1%) were foun d on alfalfa, and only 2 adult whitefly parasitoids were collected dur ing the 3 yr. The results of this study suggest that control measures for whiteflies on alfalfa in souther California and Arizona may be nec essary only from June or July through September. Controlling whiteflie s on alfalfa during these months may reduce and damage the whiteflies cause to alfalfa, but it probably would not have a large impact on whi tefly emergence because it appeared that relatively few whiteflies eme rged from alfalfa fields subjected to normal 30- to 45-d harvesting sc hedules.