Development and characteristics of resistance in the susceptible WHO/SRS house fly (Musca domestica) strain subjected to selection with beta-cypermethrin

The standard reference WHO/SRS house fly (Musca domestica L.) strain was selected with beta-cypermethrin ('Chinmix') by topical application for 25 consecutive generations. By the F-25 generation the LD(50) (expressed as ng per fly) increased to 164.9 x and 190.2 x initial values in the female and male flies, respectively, of the selected strain (CHXSEL). Developing rates and patterns of resistance seemed to be similar in both sexes and showed four characteristic phases: I. stagnant, II. quickly rising, III. slowly increasing and IV. plateau. Due to high mortality and low fertility of survivors after F-25 the selecting dosages could not be increased further, indicating that a selection limit had been reached. Upon termination of selection pressure the resistance reverted almost completely in the NOSEL substrain. The LD(50) decreased continuously and by the F-60 generation the resistance ratios (RR) were only 6.9 and 10.2 in the female and male flies, respectively. Synergist studies with different metabolic inhibitors, SKF-525a, piperonyl butoxide, 1-naphthyl N-propylcarbamate and NIA-16388 suggested that both enhanced oxidase and esterase activity would contribute to the resistance of the CHXSEL strain. Moreover the analysis of the cross-resistance pattern among pyrethroids makes the presence of a nerve insensitivity factor likely. The pattern of cross-resistance within and outside the pyrethroid group is discussed in detail. The relationship of contributing resistance mechanism(s) to similarities of chemical structures is also discussed.