Effects Of Tebufenozide And Hexaflumuron On Tyrosine Hydroxylase And Decarboxylase Activity Of Spodoptera Litura Larvae

In this paper, we studied the effects of tebufenozide and hexaflumuron on enzyme activity of Spodoptera litura related to larvae epidermization process, and further clarifies the role of epidermization mechanism of S. litura,which provide theoretical basis for the development of new high efficiency, low toxicity, selective pesticides and the resistance management.(1) Toxicity of tebufenozide to different instar larvae of S. litura was determined by artificial diet dipping method. The results showed that the toxicity of 2th instar larvae was highest, LC₅₀ was 79.3 mg·L^-1, 3th instar larvae′s LC₅₀ was 79.3 mg·L^-1, 4th instar larvae′s LC₅₀ was 226.7 mg·L^-1, 5^th instar larvae has lowest toxicity, its LC₅₀ was 441.8 mg·L^-1. The results indicated that the toxicity of tebufenozide to S. litura was reduced with the larvae growth.(2) After the 5^th instar larvae were continuously treated with five sublethal concentrations (20.40 mg/L, 30.63 mg/L, 42.30 mg/L, 55.80 mg/L and 71.56 mg/L) of tebufenozide for different times, the tyrosine hydroxylase (TH) and dopa decarboxylase (DDC) activity were significantly reduced compared to that of control at the same tebufenozide-treated times, and the inhibition rates of high tebufenozide concentrations were stronger than that of the low concentrations. For example, After the 5^th instar larvae were continuously treated with 46 mg·L^-1, the inhibition rates of TH 24 h was 8.65%, 48 h was44.62%, 72 h was 66.49%; the inhibition rates of DDC 24 h was 14.89%, 48 h was27.33%, 72 h was 41.07%. The inhibition rates of TH and DDC in hemolymph, epidermis, heads and midgut of 5^th instar larvae reduced with tebufenozide-treated times increasing. The inhibition rates of TH at the same tebufenozide-treated times were highest in hemolymph, lower in epidermis and lowest in midgut. The inhibition rates of DDC at the same tebufenozide-treated times were highest in hemolymph and lowest in midgut. Continuous use of sublethal doses of tebufenozide feeding S. litura larvae, the larvae decreased its active capacity, probably due to the decrease of tyrosine hydroxylase and dopa decarboxylase activity. (3) After the 5^th instar larvae were continuously treated with five sublethal concentrations (94.82 mg/L, 143.52 mg/L, 192.22 mg/L, 240.92 mg/L and 289.62 mg/L) of hexaflumuron for different times, the tyrosine hydroxylase and dopa decarboxylase activity were significantly reduced compared to that of control at the same hexaflumuron-treated times, and the inhibition rates of high hexaflumuron concentrations were stronger than that of the low concentrations. For example, After the 5^th instar larvae were continuously treated with 46 mg·L^-1, the inhibition rates of TH 24 h was 9.20%, 48 h was29.87%, 72 h was 42.47%; the inhibition rates of DDC 24 h was 5.41%, 48 h was 27.60%, 72 h was 30.39%. The inhibition rates of TH and DDC in hemolymph, epidermis, heads and midgut of 5^th instar larvae reduced with hexaflumuron-treated times increasing. The inhibition rates of TH at the same hexaflumuron-treated times were highest in epidermis, the second in heads, the third in midgut and lowest in hemolymph. The inhibition rates of DDC at the same hexaflumuron-treated times were highest in hemolymph, lower in epidermis and lowest in midgut.