Study on Insecticide Resistance in Oriental Fruit Flies, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae)

Oriental fruit flies, Bactrocera dorsalis (Hendel), were treated with ten insecticides, including six organophosphorus insecticides (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. Sub-parental lines of each generation of the oriental flies 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) while the line treated with formothion exhibited the highest increase in resistance (up to 594-fold) compared to the susceptible colony. Synergism bioassays were also carried out. Based on this, when oriental fruit flies were treated with S,S,S-tributyl phosphorotrithioate displayed a synergistic effect for naled, trichlorfon and malathion resistance, whereas the colonies treated piperonyl butoxide displayed a synergistic effect for pyrethroid resistance. All ten 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 organophosphorus-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 of the fruit flies treated with insecticides developed here should provide useful tools and information for designing an insecticide application strategy for controlling this fruit fly in the field. Extended study to explore the biochemical mechanism of resistance to the organophosphorus insecticides, malathion and fenitrothion, in the fly showed that the enzyme activity of glutathione S-transferase was not significantly different between the malathion-resistant line and susceptible colony. However, malathion-resistant line exhibited higher activity in esterase and mixed function oxidase than the susceptible colony did. The target enzyme, acetylcholinesterase (AChE), from the resistant line was less sensitive to the inhibition of organophosphorus inhibitors than that from the susceptible colony. These results suggested that elevated hydrolytic and oxidative metabolic enzymes in conjunction with an altered AChE with poorer catalytic efficiency might contribute to the resistance of this fly to malathion. To fenitrothion, the resistant line exhibited reduced AChE activity compared to that in susceptible colony, while the activities of the other enzymes did not significantly differ between these two fruit fly colonies. The resistant line also exhibited at least a 10-fold reduced sensitivity to a series of AChE inhibitors compared to that of susceptible colony. To investigate the molecular basis of this fenitrothion resistance, cDNAs from the gene encoding AChE were characterized from individuals representing both the resistant lines and susceptible colony. Three point mutations, I214V, G488S and Q643R, resulting in nonsynonymous changes in the amino acid sequence of this gene were detected in the resistant flies. These changes appear to correspond to key catalytic sites affecting the function of AChE.