The effect of the Synergists on the Permethrin Resistance in Dengue Fever Mosquito, Aedes aegypti Linya Strain (Diptera: Culicidae)

Aedes aegypti is the vector of dengue fever, and dengue hemorrhagic fever in many tropical countries, including the southern areas of Taiwan. LYPL strain was collected from Lingya in Kaohsiung in 1990 and was selected with permethrin at larval stage for 35 generations. Three sub-strains;LYPPL, LYPA, and LYPPA were established by laboratory selection from LYPL strain. LYPPL strain was selected with a mixture of permethrin and piperonyl butoxide (PBO, a cytochrome P450 monooxygenase inhibitor) at larval stage for 5 generations. LYPA strain was selected with permethrin at adult stage for 5 generations. LYPPA strain was selected with a mixture of permethrin and PBO at adult stage for 5 generations. The resistance ratio of the four strains (LYPL, LYPPL, LYPA, and LYPPA) to permethrin at larval stage was 322.27, 148.18, 182.73, and 296.82, respectively. The resistance ratio at adult stage was 46.57, 40.51, 98.08, and 40.30, respectively. In the synergistic effect test, PBO, s,s,s-tributyl phosphorotrithioate (DEF, an esterase inhibitor), diethyl maleate (DEM, a glutathione S-transferase inhibitor), and triphenyl phosphate (TPP, an esterase inhibitor) were used and only PBO and DEF had effect on increasing permethrin toxicity at larval stage, and PBO, TPP, DEM, DEF were all had effect on increasing permethrin toxicity at adult stage. All the four strains had moderate to high level of cross-resistance to cypermethrin, phenothrin, and tetramethrin at larval and adult stage. In the cytochrome P450 monooxygenases and b5 content analysis, at larval stage the resistance was only associated with the higher content of cytochrome P450 of LYPL strain. At adult stage the resistance was associated with the higher activity of cytochrome P450 of LYPL strain and LYPPA sub-strain. And the resistance was associate with higher content and activity of cytochrome P450 of LYPA sub-strain. In addition to cytochrome P450, there should be other resistant mechanisms involved with resistance, such as esterase, glutathione-S-transferase, target site insensitivity and penetration resistance.