Abstract: Resistant levels of cotton aphid strains collected from five different districts, Taian, Laiyang, Nanjing, Beijing and Anyang in China, designa ted as strainⅠ,Ⅱ,Ⅲ,Ⅳ and Ⅴ respectively, were determined with leaf-dip bioassay. The results Indicated that strain Ⅴ was the highest resistant while strainⅠ was the lowest resistant to conventional organophosphorus (OP) and carbamate pesticides, such as methamidophos, monocrotophos, pirimicarb and thiodicarb. Generally, the resistant levels of the five resistant strains were: Ⅴ>Ⅳ>Ⅲ,Ⅱ>Ⅰ. Compared with the sensitive strain 81-171B, the sensitivity of acetylcholinesterase (AChE) from strainⅡ, Ⅳ and Ⅴ apparently decreased and that from strainⅠand Ⅲ were relatively sensitive. The apparently decreasing of Km value indicated that AChE from strain Ⅳ and Ⅴ have been changed in quality. Esterase (total esterase and carboxylesterase) activity increased dramatically in different resistant strains. Esterase activity of strain Ⅲ is two times that from stain Ⅱ. Specially, Km value of carboxylesterase from strain Ⅴ reached 2460.4 μmol/L and that from strain Ⅳ decreased to 84.4 μmol/L It could be deduced that both increased esterase activity and reduced target sensitivity existed in all resistant strains. For lowlevel resistant cotton aphid strain, increased esterase activity plays a much more important role. However, either apparently increased esterase activity or reduced AChE activity in cotton aphid may result in middle-level resistance. Both dramatically increasing of esterase activity and reduction of AChE sensitivity are presented in high-level resistant strains of cotton aphid.

Key words: Aphis gossypii; resistance, acetylcholinesterase, carboxylesterase, median inhibitory concentration, Michaelis constant