| The housefly, a worldwide human and animal health pest, is the probable carrier of more than 65 human and animal intestinal diseases. For decades of years, efforts for the control of houseflies have been focused on the use of insecticides. Pyrethroids were initially highly effective on the housefly control. Unfortunately, very high levels of resistance to this class of insecticides have developed worldwide due to intensive use over the last 30 years.In this papaer, the original susceptible strain of houseflies was anti-selected with propoxur in the laboratory to get a real susceptible strain; Discriminating concentration of insensitive acetylcholinesterase (AChE) was found in accordance with AChE sensitiveity of susceptible strain; and LD50, AChE activity, CarE activity and kinetic constants were compared and studied for each generation in the establishment of near-isogenic line.The sensitivity of housefly was increased to 13.87-fold in susceptible strain and the resistant index was increased from 85.73-fold to 1189.48-fold after six times of anti-selection. The slope of toxicology regression curve in the susceptible strain was reduced from 2.58 down to 1.54. It showed that the heterogeneity in the susceptible strain was high, and there was still a part of carrying resistance gene heterozygotes in the susceptible strain.With the I90 values of the susceptible and resistant strains in houseflies were 1.65×10-4mol/L and 1.75×10-4mol/L, respectively. And the dose of AChE to distinguish the susceptible and resistant strains was 1.65×10-4, the sensitive test to the single showed that those individual contain not sensitive ache genes took 12% of susceptible strains, and those individual which contain not sensitive ache genes took 42% of resistant strains. That means this population had the resistance to propoxur. The susceptible strains of housefly contained no-sensitive AChE, and needed to be anti-selected repeatedly; the resistant strains were collected in the field of Beijing. This area should stop using propoxur and replace it by other types of pesticides to avoid or delay the development of resistance.The test result of NIL showed that since BC4F2, resistance with propoxur became stable gradually, after backcross with susceptible recurrent parents again, the resistant multiple of the offspring of self-cross were same as resistant donor parents, which was bigger than 1189.48.The resistant multiple of BC5F3 and BC5F4 which were the self-cross offspring were not change any more (>1189.48), that meant the resistance became stable. The establishment of resistance near-isogenic line with propoxur in the housefly was complete.The AChE activity and the kinetic constants of near-isogenic line showed that the Km of resistant strain was 8.3 times than susceptible strain, the Vmax is 2.3 times than the susceptible strain, the differences of susceptible and resistant strains in the Km and Vmax showed that there were two different types of AChE between the two trains, which implied allosteric in AChE. In the near-isogenic line, the Km and Vmax of F2 were higher than the F1. With the increase of backcross generations, there were the differences of Km and Vmax between F1 and F2. This indicated that the near-isogenic lines reserved AChE which was same as resistance strain. AChE activity ratios of different generatons showed a positive relationship with the multiple resistance.The carboxylestere (CarE) activity and the kinetic constants of near-isogenic line showed that the Km values usedβ-NA as the substrate were significantly higher than which usedα-NA in susceptible, resistant strains and the generations of NIL. That showed the affinity of CarE to theα-NA was higher than which to theβ-NA. The Vmax values usedβ-NA as the substrate were all higher than which usedα-NA in all the tested CarE, that meant the hydrolysis rates toβ-NA were higher than toα-NA. Described above, in near-isogenic lines with propoxur in housefly there were two different CarE. The CarE activity of the resistant strain toα-NA andβ-NA were 1.21 and 1.66 times higher than the susceptible strain, however, there was no linearly relationship between the CarE activity and resistant level. All above showed that CarE might be one of the reasons which leaded to resistance mechanism with propoxur, but it was not the main reason.
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