Selection for fenpropathrin and fenpropathrin plus acephate resistance in the silverleaf whitefly (Homoptera : Aleyrodidae)

A large, genetically diverse pool of Bemisia argentifolii Bellows & Perring was collected in 1994 from different crops, and a mixed colony was establi shed in the laboratory. Subsets of this colony were reared on cotton plants held in large Plexiglas cages, and adult whiteflies were selected for resi stance to fenpropathrin and to fenpropathrin+acephate (1:5). Selection was per formed by exposing adults to treated glass vials at doses sufficient to give 60-80% mortality, Thirteen generations of adult selection with fenpro pathrin+acephate yielded 856.3- and 1,289.3-fold tolerance (using lethal co ncentration ratio), respectively, to fenpropathrin and to fenpropathrin+ace phate, indicating additive genetic variation for resistance in the source p opulation. Selection with fenpropathrin alone yielded only a 10.9-fold incr ease in tolerance to fenpropathrin at the end of the selection period. Ther e was no significant change in tolerance to fenpropathrin+acephate in this strain. Contrary to expectations, the early onset and the magnitude of resi stance attained on selection with fenpropathrin+acephate compared with fenp ropathrin alone indicates that this mixture evidently possesses a high degr ee of selectivity for development of resistance in B. argentifolii. Estimat es of realized heritability of resistance to fenpropathrin and to fenpropat hrin+acephate in B. argentifolii (in the fenpropathrin+acephate-selected st rain) showed that they were significantly higher in the first half of selec tion (six generations), in both instances. Rearing of the fenpropathrin+ace phate-resistant strain under conditions free of insecticides for six genera tions did not result in any significant decline in resistance, indicating t hat resistance is fairly stable.