Abstract: The LD50 values and the knockdown effects of deltamethrin, permethrin and DDT to Dec-R, 2C1-R and susceptible (SP) strains of house flies were investigated. The resuits indicate that the resistances of the Dec-R, 2C1-R and DDT-R to deltamethrin, permethrin and DDT were 120 912, 6 032 and 112 folds respectively in comparison with Spstrain. Significant cross-resistance to the above insecticides as found in these three resistant strains, which also demonstrated resistance to the knockdown by pyrethroid and DDT. Theparameters of synergism of PB to deltamethrin and permethrin in Dec-R and 2CI-R were determined. The relative percentages of synergism (R%S ?) from these two resistant strains were much higher than that of the SP (R%S (S)). When crossing Dec-R femals with SP males and Dec-R males with SP femals, the resistance of F₁, adults to deltamethrin decreased 16 860 and 26 319-fold respectively as compared with that of the Dec-R. The results suggested that there is a recessive gene responsible for the deltamethrin resistance in theDec-R strain. The sensitivity of the central nervous system (CNS) in the houseflies to DDT, permethrin and deltamethrin was studied with electrophysiological technique. The results show that the CNS insensitivity is an important factor of resistance to DDT andpyrethroid in the housefly, and it is also an important cause of cross-resistance among DDT and pyrethroid analogues. These results suggested that the mechanism for pyrethroid-resistance in housefly may be caused by Kdr genes.Pretreatment with 10^-7 mol/L deltamethrin and pulse-depolarization by high K+(55 mmol/L) for synaptosomes enhanced the release of³H-choline in the susceptible houseflies. But in similar experiments using synaptosomes from Dec-R house flies and pretreated with 10^-4 mol/L deltamethrin and K⁺, the enhancment of ³H-choline release was minor or absent. These results are consistent with the hypothesis that the knockdown (Dec-R) resistance is most likely due to a decreased affinity of voltagegated sodium channel for pyrethroids.

Key words: houseflies, pyrethroid-resistance, resistance mechanism, neurotransmitter re- lease