Abstract: Tebufenozide acting in a similar fashion of ecdysone was found to produce phenotypic effects that finally interfere the formation of new larval cuticle in insects. To explore the effects of tebufenozide on the enzyme activities related to the formation of new cuticle in insects, high performance liquid chromatography (HPLC)-fluorescent assay was used to determine the activities of dopa decarboxylase (DDC) and tyrosine hydroxylase (TH) in 5th instar larvae of Spodoptera exigua which were continuously treated with tebufenozide for different time. The results indicated that after the larvae were treated with two sublethal concentrations (LC₁₁ 28.41 μmol/L and LC₃₃ 85.23 μmol/L) of tebufenozide, the activities of DDC and TH were significantly inhibited, and the inhibitory effect was more marked when the larvae were treated with tebufenozide at the high sublethal concentration than at the low sublethal concentration. The inhibition rates of DDC and TH activities were raised with treatment time at the same sublethal concentration. Tebufenozide had the similar inhibitory effects on DDC and TH activities in the midgut, head, fat body, hemolymph and cuticle of the 5th instar larvae as on those in the whole larval body. It is so concluded that the interference in the formation of new larval cuticle is likely related to the inhibition of DDC and TH activities by tebufenozide.

Key words: Spodoptera exigua, tebufenozide, dopa decarboxylase (DDC), tyrosine hydroxylase (TH), enzyme activity, inhibitory effect