The overexpression of acetylcholinesterase compensates for the reduced catalytic activity caused by resistance-conferring mutations in Tetranychus urticae

► A Tuace gene duplication in T. urticae resulted in the overexpression of acetylcholinesterase (TuAChE). ► The resistance mutations (G228S and F439W) increased the insensitivity of TuAChE to monocrotophos. ► The A391T mutation did not change the kinetic properties of TuAChE but mitigated the negative impacts of F439 mutation. ► The resistance mutations (G228S and F439W) decreased the catalytic efficiency of TuAChE, likely causing a fitness cost. ► The overexpression of TuAChE compensated for the reduced catalytic activity caused by the resistance mutations.