Synergism of Insecticides with DEF in Sweetpotato Whitefly (Homoptera: Aleyrodidae)

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Abstract
Adults of sweetpotato whitefly (SPWF), Bemisia tabaci (Gennadius), from susceptible (S) and field (R) strains were treated in the laboratory with cypermethrin, permethrin, methyl parathion, and sulprofos, alone or in combination with S,S,S-tributyl phosphorotrithioate (DEF). Females were significantly less sensitive than males to all insecticides tested, the difference being greatest (20-fold) with permethrin. The highest level of resistance in females (63.2-fold) was found with permethrin, while a moderate level (13.3-fold) was found with suiprofos. High synergism was obtained with cypermethrin and permethrin. In the S strain, the synergism ratio of cypermethrin/DEF (1:1) was 31.2 for females and 18.2 for males, while that of permethrin/DEF (1:5) was 15.8 for females and 1.8 for males. In the R strain, DEF synergized cypermethrin and permethrin, causing almost complete elimination of resistance to both pyrethroids. This synergistic effect suggests that esterases are involved in the mechanism of pyrethroid resistance in SPWF. The link between arrhenotoky and resistance in SPWF is discussed.