Sensitivity Variance In Different Instar Larvae Of Diamondback Moth, Plutella Xylostella, To Insecticides During Qctober 2008 And Qctober 2010

In this thesis, the toxicity of 11 insecticides, i.e alpha-cypermethrin, indoxacarb, abamectin, Bacillus thuringiensis, spinosad, cartap, fipronil, chlorfenapyr, diafenthiuron, chlorfluazuron, tebufenozide, to 3rd instar larvae of diamondback moth, Plutella xylostella (Linnaeus) collected from the vegetable fields in Pudang, Fuzhou city was tested during October, 2008 and October 2010. Also, the toxicities of 5 insecticides, i.e. chlorpyrifos, fenvalerate, abamectin, cartap, fipronil, to larvae of P. xylostella at different instars either collected from the fields or raised in the laboratory were compared. The main results were as follows:(1) Variences in the toxicity of the 11 insecticides at different time The toxicity of the 11 inscticides to the 3rd instar larvae of P. xylostella was kept comparatively stable during October, 2008 and October, 2010. Mean while the toxicity of indoxacarb, Bacillus thuringiensis, chlorfluazuron and tebufenozide increased with time. For example, the LC₅₀s of indoxacarb to the tested insects were 110.63mg/L, 25.02mg/L, 7.39mg/L, 8.21mg/L and 5.16mg/L, respectively in October, 2008, August, 2009, October, 2009, August, 2010 and October, 2010, respectively. It showed that the value of LC₅₀ decreased which indicated that its toxicity increased with time.Among the 11 insecticides, the toxicities of alpha-cypermethrin, abamectin, chlorfenapyr, diafenthiuron, and Bt were positively related.(2) Effect of life-stage of the tested insects on the toxicities of the insecticides The toxicities of the insecticides were found to decrease with the life-stage of the tested insects either collected from the fields or raised in the laboratory. For instance, the LC₅₀s of chlorpyrifos to the 2nd, 3rd and 4th instar larvae raised in the laboratory were 120.36mg/L, 372.34mg/L and 434.92mg/L respectivly. According to the value of LC₅₀, its toxicity to the 4th instar larvae was nearly four times lower than than that to the 2nd instar ones. To those collected from the fields, the LC₅₀s were 290.20mg/L, 692.96mg/L and 1268.24mg/L which suggested that the toxicity decreased, too.The effect of the life-stage of the tested insects varied with insecticides. Take cypermethrin and abamectin as examples. The LC₅₀s of cypermethrin to the 2nd, 3rd and 4th instar larvae of P. xylostella raised in the laboratory were 180.12mg/L, 294.18mg/L and 37.55mg/L, respectively. Compared with that to the 2nd instar larvae, its toxicity to 3rd or 4th instar larvae decreased by 0.63 or 0.87 time. But for abamectin, thar was 1.00 or 4.05 times. The differences in the effects of the life stage on the toxicities of the insecticides also existed in the different P. xylostella populations. For example, to the insects raised in the laboratory, the toxicity of cypermethrin was 0.63 and 0.87 times lower in the 3rd and 4th instar larvae, but that was 1.42 and 3.44 times in those collectd from the fields.Just because of the effects as well as their compexity of the life stage of the tested insects on the toxicities of the insecticides, the toxicity ratio between different insecticides were found to vary with insecticide types, life stage and source of the tested insects. Similarly, the realtively resistance ratios were different in different life stage, too. For example, for cypermethrin, the relative resistance ratios of P. xylostella in the vegetable fields in Pudang, Fuzhou city were 1.72, 2.56 and 4.09 times in the 2nd, 3rd and 4th instar larvae, respectively.