Abstract: 【Objective】To investigate the effects of hexaflumuron on phenoloxidase （PO） activity in the common cutworm, Spodoptera litura（Fabricius） larvae.【Methods】The activities of phenoloxidase directly reacted in vitro with hexaflumuron and that from hexaflumuron-treated 5th instar larvae of S. litura were assayed with enzyme kinetics methods. 【Results】 The optimum reaction conditions of PO were pH 6.5 and best working temperature 30℃ by using catechol as substrate. Hexaflumuron could directly combine with enzyme protein, but it was not chelated with Cu2+ in the active center of PO. The PO activity in vitro increased when hexaflumuron concentrations were below 1 153 mg/L, reached its highest level at 138 mg/L and was inhibited above 1 153 mg/L. After the 5th instar larvae were continuously treated with two sublethal concentrations（46 mg/L and 92 mg/L） of hexaflumuron for different time, the PO activity was significantly higher than that of the control at the same treatment time and the activation rates at high concentrations were larger than that at low concentrations. The PO activity in the hemolymph, cuticle and head of the 5th instar larvae increased with treatment time. The activation rate of PO at the same treatment time was the highest in the hemolymph and the lowest in the head. After the 5th instar larvae were continuously treated with hexaflumuron, PO activities in prepupae and pupae were also significantly enhanced. 【Conclusions】 Hexaflumuron can activate phenoloxidase activity in S. litura.  

Key words: Spodoptera litura, insect growth regulator, hexaflumuron, phenoloxidase, enzyme activation