Insensitivity of acetylcholinesterase and increased activity of esterase in the resistant cotton aphid, Aphis gossypii Glover.
The resistance levels of cotton aphid strains collected from five different districts, Taian (Shandong), Laiyang (Shandong), Nanjing (Jiangsu), Beijing and Anyang (Henan) in China, designated as strain I, II, III, IV and V, respectively, were determined with leaf-dip bioassay. The results indicated that strain V had the highest resistance while strain I had the lowest resistance to conventional organophosphorus and carbamate pesticides, such as methamidophos, monocrotophos, pirimicarb and thiodicarb. Generally, the resistance 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 strain II, IV and V apparently decreased and that from strain I and 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. The 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 low-level 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.