Frequency distributions of esterase activity and insecticide resistance in third-instar larvae in different strains of Helicoverpa armigera Hübner

General esterase activity to α NA in two susceptible laboratory strains (Yanshi and Hubei), two lab selected resistant strains (PCP20 and YG45) and nine field strains of cotton bollworm Helicoverpa armigera Hubner were measured by microtiter plate kinetic assay. Frequency distribution patterns of esterase activity in the Yanshi, Hubei and Shawan susceptible field strains were similar and their mean esterase activities were 991, 1 138 and 1 055 mOD·min -1 ·larva -1 respectively. Two lab selected resistant strains (PCP20, YG45) and three field resistant strains (Yanggu, Handan and Anyang) had many more individuals of high esterase activity (more than 1 800 mOD·min -1 ·larva -1 ) than three susceptible strains and their mean esterase activities ranged from 1 510 to 2 482 mOD·min -1 ·larva -1 . Another two field strains (Xuzhou and Wuhan) had moderate high easterase individuals and their mean esterase activities were 1 208 and 1 404 mOD·min -1 ·larva -1 . The correlation coefficient between mean esterase activity and resistance level was 0 82 to pyrethroids and 0 42 to organophosphates. The same results were obtained when the correlation between the frequency of individuals with high esterase activity and the resistance levels to pyrethriod and organophosphate was analyzed. The results suggested that general esterase activity has some relationship to pyrethroid resistance levels. Because esterase is not a major mechanism for pyrethroid resistance in H.armigera it can only be used as a reference index for the detection of biochemical resistance.