Abstract:

【Objective】 In this paper, the inhibitory effect of tebufenozide on phenoloxidase (PO) in 5th instar larvae of the common armyworm, Spodoptera exigua, was determined. 【Method】 Fifth instar larvae of S. exigua were supplied with tebufenozide-treated artifical diet to eat for different times and the inhibitory effect of tebufenozide on PO extracted from tebufenozide-treated and non-treated 5th instar larvae were determined by biochemical methods. 【Result】 The results indicated that tebufenozide could inhibit PO activity obviously, and the inhibition concentration of tebufenozide mixed with PO directly leading to 50% activity loss (IC50) was 5.43 µmol•L-1. The inhibitory mechanism of tebufenozide on PO was a mixed-competitive inhibition for oxidation of O-dihydroxybenzene, the dissociative enzmye inhibition constant (K i) and E-S’ inhibition constant (Kis) were 11.223 µmol•L-1 and 34.036 µmol•L-1, respectively. This inhibition may carry out through tebufenozide directly combining with enzyme protein while may be not chelating with Cu2+ in the active center of PO. The PO activity in head, hemolymph and epidermis of 5th instar larvae was also significantly inhibited after the larvae were treated with two sublethal doses (28.41 µmol•L-1 and 85.23 µmol•L-1) of tebufenozide for continuous 24, 48 and 72 h, respectively. The inhibition rates increased with tebufenozide-treated time increasing, and inhibition rates of 85.23 µmol•L-1 were almost higher than those of 28.41 µmol•L-1 in the same tissues at the same length of tebufenozide-treated times. The orders of inhibition rates of PO by tebufenozide in different tissues at the same length of times were head > hemolymph > epidermis. 【Conclusion】 Tebufenozide can significantly inhibited PO activity, and it may be infer that the interference of new cuticle developmemt of larvae by tebufenozide is closely related to the inhibition of PO activity by tebufenozide.

Key words: Spodoptera exigua, tebufenozide, phenoloxidase (PO), inhibitory effect