Abstract: A strain of organophosphate (OP) resistant cotton aphid, Aphis gossypii was selected by exposure to omethoate insecticide in the laboratory. The resistance factor of this strain was 17 times that of an OP-susceptible strain of cotton aphid. Four substrates, α-naphthyl acetate (α-NA), α-naphthyl butyrate (α-NB), α-naphthyl phosphate (α-NP) and β-naphthyl phosphate (β-NP), were used to determine carboxylesterase activity, Michaelis constants (K_m) and maximum velocities (V_max) in OP-susceptible and resistant strains of cotton aphids. Carboxylesterase activity was significantly lower in the OP-resistant strain than the OP-susceptible strain when α-NA and β-NB were used as substrates and the same trends were obtained by native polyacrylamide gel electrophoresis (PAGE) analysis. However, no significant difference was found between OP-susceptible and resistant strains when α-NP and β-NP were used as substrates.

Key words: Aphis gossypii, insecticide resistance, carboxylesterase