Abstract: In this paper the microplate assay was used to determine AChE inhibition by different concentrations of organophosphate and carbamate insecticides. The results showed that there were no inhibition of organophosphate insecticides, such as methyl parathion, monocrotophos, omethoate and acephate to the AChE of cotton aphid. By measuring the AChE activity of individual cotton aphids on cascara and on cotton seedling in Beijing and Gaomi, Shandong Province, along with the inhibitory frequency of methomyl to it, it was found that the distributive frequency among individual AChE activities was similar among the three populations of cotton aphid tested. The individuals with inhibitory rate of methomyl to individual AChE below 30% in the three different populations were amounted to 2.4%,16%,29%,respectively.The individuals with inhibitory rate over 70%, accounted for 72%, 33% and 1% of the three populations respectively. The difference among these three populations was noticeable, and this result was in accord with the outcome of biological measurement.Consequently the method of microplate titer assay would be a better forecast skill for determinating insensitivity of AChE to carbamate insecticides in cotton aphid. It might be helpful for releasing pest resistance.

Key words: cotton aphid, microplate assay, acetylcholinesterase, insecticide