Abstract: The optimal conditions for assaying the activity of acetylcholinesterase (AChE) from Odontotermes formosanus (Shiraki) were determined by orthogonal experiment. The concentrations of acetylcholinesterase and substrate, pH and temperature of the reaction system, and reaction time were optimized. L₂₅ (5⁶) orthogonal matrix was adopted, without considering interaction. Through analyzing the data from orthogonal experiment with range analysis and variance analysis, we found that the optimal conditions were as enzyme concentration 12.5 g/L, substrate concentration 8 mmol/L, pH 8.0, reaction temperature 40℃, and reaction time 5 min. Furthermore, the inhibition of 6 insecticides to AChE from O. formosanus was determined.  The IC₅₀ values of methomyl, phoxim, triazophos, profenofos, malathion and omethoate were 3.52×10^-4, 1.86×10^-3, 5.13×10^-3, 9.55×10^-4, 8.81×10^-3 and 1.39×10^-2 mol/L, respectively. The inhibition of AChE activity by six insecticides increased with the increasing of insecticide concentration from 3.3×10^-7 to 5×10^-3 mol/L.

Key words: Odontotermes formosanus, acetylcholinesterase, optimal condition, orthogonal experiment, IC50 values, sensitivity to insecticide