The predatory mite Phytoseiulus persimilis is effective in controlling tetranychids on many cultivated crops. The LC50 and LC60 values of acequinocyl were determined for P. persimilis using a leaf-disc method and a spraying tower. A laboratory population ACE6 had a 32.75-fold increase in resistance to acequinocyl after six selection cycles. This resistant population also developed low resistance to spinosad, chlorantraniliprole, and deltamethrin and moderate cross-resistance to acetamiprid, indoxacarb, milbemectin and spiromesifen. The synergists piperonyl butoxide (PBO) S-benzyl-O,O-diisopropyl phosphorothioate (IBP) and diethyl maleate (DEM) increased the resistance 2.03-, 2.33- and 41.00-fold, respectively. Crossing experiments revealed that acequinocyl resistance in the ACE6 population was intermediately dominant and polygenic. Additionally, in the ACE6 population, cytochrome P450 monooxygenase (P450) and esterase detoxifying enzyme activities increased 2.40-fold and 1.85-fold, respectively. These results indicated that P450 and esterase were important enzymes in the development of acequinocyl resistance in the ACE6 population. (C) 2014 Elsevier Ltd. All rights reserved.