Abstract: Malathion susceptibility in the two populations of the grasshopper Oxya chinensis, collected from Linyi of Shanxi Province and Xuzhou of Jiangsu Province, China, was determined. General esterases from the two populations were characterized and compared. LD₅₀ of the Xuzhou population (13.00 μg/g body weight) was 2.80-fold higher than that of the Linyi population (4.64 μg/g body weight). Inhibition studies of general esterases using four inhibitors, including paraoxon, malaoxon, eserine, and carbaryl, indicated that most general esterases In the two populations were B-type. Kinetic studies showed that the Michaelis Menten constant (K_m) and the maximal velocity (V_max) of general esterases from the Xuzhou population were higher than that from the Linyi population, using α-naphthyl acetate (α-NA), α-naphthyl butyrate (α-NB), β-naphthyl acetate (β-NA) as substrates. The esterase activities in females of the Xuzhou population were 2.02, 1.58, and 1.28-fold higher than those of the Linyi population, using α-NA, α-NB and β-NA as substrates, respectively, and in males they were 2.71, 1.67, and 1.33-fold higher in the Xuzhou population than in the Linyi population. The spectrum of esterase activities showed that O. chinensis individuals with high esterase activities were more in the Xuzhou population than those in the Linyi population using the three selected substrates. We speculated that esterases in the Xuzhou population may be biochemically different from those in the Linyi population, and it might be attributed to the different geographic distributions, ecological environment and nutrition resources in the two localities. In addition, the biochemical differences might also be due to the difference in insecticides selective pressure on the two populations of O. chinensis.

Key words: Oxya chinensis, general esterases, malathion susceptibility, enzyme kinetics, esterase inhibition