Abstract: A neurophysiological assay was conducted to investigate the effects of five mixtures of cypermethrin stereoisomers on firing of the sensory nerve in the isolated mesathoracic leg of susceptible and resistant houseflies. Each mixture induced an elevation of the firing frequency, then gradual drop and eventually to blockade. Nerve sensitivity was calculated by the blocking time and cyermethrindoses. The sensitivity of the susceptible strain toward five mixtures of cypermethtin stereoisomers was in the order C₄＞ C₁＞C₂＞C₃＞C₅. The sensitivity was dependent on the ratio of trans- to cis-isomer in the mixtures, the higher the ratio,the higher the sensitivity. The results showed that the nerve sensitivity of resistant houseflies decreased markedly. It is suggested that the decrease of nerve sensitivity might be the most important mechanism of resistance in the houseflies to cypermethrin. The resistance levels of RC₁ was the highest among the resistant housefly strains, but its neurosensitivity was also the highest. It was inferred that the main resistance mechanism of RC₁ to cypermethrin is different from that of other strains.

Key words: stereoisomer cypermethrin, house fly, electrophysiology, resistance