Resistance and Synergistic Effects of Insecticides in <I>Bactrocera dorsalis</I> (Diptera: Tephritidae) in Taiwan

Oriental fruit flies, Bactrocera dorsalis (Hendel), were treated with 10 insecticides, including six organophosphates (naled, trichlorfon, fenitrothion, fenthion, formothion, and malathion), one carbamate (methomyl), and three pyrethroids (cyfluthrin, cypermethrin, and fenvalerate), by a topical application assay under laboratory conditions. Subparental lines of each generation treated with the same insecticide were selected for 30 generations and were designated as x-r lines (x, insecticide; r, resistant). The parent colony was maintained as the susceptible colony. The line treated with naled exhibited the lowest increase in resistance (4.7-fold), whereas the line treated with formothion exhibited the highest increase in resistance (up to 594-fold) compared with the susceptible colony. Synergism bioassays also were carried out. Based on this, S,S,S-tributyl phosphorotrithioate displayed a synergistic effect for naled, trichlorfon, and malathion resistance, whereas piperonyl butoxide displayed a synergistic effect for pyrethroid resistance. All 10 resistant lines also exhibited some cross-resistance to other insecticides, not only to the same chemical class of insecticides but also to other classes. However, none of the organophosphate-resistant or the methomyl-resistant lines exhibited cross-resistance to two of the pyrethroids (cypermethrin and fenvalerate). Overall, the laboratory resistance and cross-resistance data developed here should provide useful tools and information for designing an insecticide management strategy for controlling this fruit fly in the field.