A large, genetically diverse pool of Bemisia argentifolii Bellows & Perring was collected in 1994 from different crops, and a mixed colony was established in the laboratory. Subsets of this colony were reared on cotton plants held in large Plexiglas cages, and adult whiteflies were selected for resistance to fenpropathrin and to fenpropathrin acephate (1:5). Selection was performed by exposing adults to treated glass vials at doses sufficient to give 60–80% mortality. Thirteen generations of adult selection with fenpropathrin acephate yielded 856.3- and 1,289.3-fold tolerance (using lethal concentration ratio), respectively, to fenpropathrin and to fenpropathrin acephate, indicating additive genetic variation for resistance in the source population. Selection with fenpropathrin alone yielded only a 10.9-fold increase in tolerance to fenpropathrin at the end of the selection period. There was no significant change in tolerance to fenpropathrin acephate in this strain. Contrary to expectations, the early onset and the magnitude of resistance attained on selection with fenpropathrin acephate compared with fenpropathrin alone indicates that this mixture evidently possesses a high degree of selectivity for development of resistance in B. argentifolii. Estimates of realized heritability of resistance to fenpropathrin and to fenpropathrin acephate in B. argentifolii (in the fenpropathrin acephate-selected strain) showed that they were significantly higher in the first half of selection (six generations), in both instances. Rearing of the fenpropathrin acephate-resistant strain under conditions free of insecticides for six generations did not result in any significant decline in resistance, indicating that resistance is fairly stable.
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Vol. 93 • No. 3