Abstract: The effects of pyrethroids fenvalerate and permethrin were studies upon isolated abdominal nerve cords from the susceptible (HD-S) and resistant (HJ-R) strains of Helicoverpa armigera (Htibner). 10^-5 mol/L fenvalerate and 10^-5 mol/L permethrin induced increases in spontaneous firing frequency primarily and then led to cessation in both strains; 10^-4 mol/L permethrin blocked the neural activity directly without producing repetitive discharges. On the basis of the time of excitation or cessation, there was a consistent pattern showing that the nervous system of HJ-R is 2- 3 times more resistant to pyrethroid actionshan that of HD-S. This experiment implied that there is a resistant mechanism of decreased nervous sensitity to pyrethroids in the field strain of Heliothis armigera in China and the mechanism conferred resistance to type I or II pyrethroids.

Key words: Helicoverpa armigera (Hubner), electrophysiology, spontaneous discharge, fenvalerate, permethrin