| Beet armyworm, Spodoptera exigua (Hübner), is a worldwide pest insect that has caused serious loss to agricultural production in China and it is difficult to control. In this article, we made some studies on screening insecticides for controlling S. exigua, the effects of sublethal doses of insecticides on target and insecticide-detoxifying enzymes, as well as the impact of temperature on growth and development of S. exigua. The melanic mutant strain of S. exigua, SEM, with the color of pupae being black instead of yellow, was derived from yellow wild strain, SEW. The effect of dopamine on the melanic mutation of S. exigua was studied. The main research results were as follows:The highest toxicity insecticides were 2.5% Spinosad SC and 4.5% cypermethrin EC, with LC50 of 2.07 mg/L and 2.26 mg/L respectively, followed by 1.8% abamectin EC, 2.5% deltamethrin EC, 10% chlorfenapyr SC, 24% methoxyfenozide SC, and the insecticidal activities of 5% chlorfluazuron EC, 48% chlorpyrifos EC, 40% phoxim EC were relatively low, while toxicity of 90% methomyl SP was the lowest of all tested insecticides, with LC50 of 804.30 mg/L.The mixture of abamectin and cypermethrin (ai 1:10), the mixture of cypermethrin and phoxim (1:9) and the mixture of abamectin and chlorpyrifos(1:187) displayed evident synergism to S. exigua , their Co-toxicity Coefficient (CTC) were 173.4,163.2 and 143, respectively. Other mixture had no evident synergism. In addition, our research results showed that synergist, baishilu, had no synergism effect when mixted with phoxim and deltamethrin.The experiments of the enzyme activities showed that when theμmol·(mgPro) -1·min-1 or nmol·(mgPro)-1·min-1 used as activity unit, the activities of AChE, CarE and GST decreased with larvae development, and when theμmol·(larvae)-1·min-1 or nmol·(larvae)-1·min-1 used as activity unit, the activities of AChE, CarE and GST increased with larvae development. There was a significant decline in CarE and GST activities 24 to 72 hours after treated with sublethal concentration of phoxim and deltamethrin. As for AChE, the activity decreased by 24.87% when compared with the check 24 hours after treated with sublethal concentration of phoxim, while sublethal concentration of deltamethrin had no significant inhibition on AChE activity.The experiments of the temperature showed that the development rate of S. exigua increased with the increasing temperature from 22℃to 32℃. The development rate of SEM was faster than that of SEW, the development durations of full generation of SEM under 22℃, 27℃and 32℃temperature were shorter by 10.24%, 9.54% and 14.52% than those of the SEW, respectively. The growth and development of the two strains were better at 27℃than at other temperature, the survival rates of full generation of SEW and SEM at 27℃were 69.82% and 80.89% respectively and oviposition amount were 1093.37 and 1249.04 aggs per female adult respectively. SEM had more advantage in the viability and reproduction than SEW.Microinjection and feed experiments showed that dopamine could influence the colour of pupae of S. exigua. When SEW pupae were injected 1.2mg of dopamine, the resulting phenotype resembled the SEM rather than the SEW. An intermediate color was obtained when 0.3 to 1.2mg of dopamine was injected, and no effect was observed when 0.3mg or less of dopamine was injected. The colour was changed when SEW was reared with feed mixed with dopamine.Reversed phase HPLC analysis of dopamine in whole body showed that the concentration of dopamine had no significant difference between the two strains. The role of dopamine in melanic mutation was worthy of further study.
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