| P-alanine plays an important role in the process of insect cuticle sclerotization. Recent research demonstrated that in some insects melanism was caused by changed level of P-alanine.The melanic mutant strain of Spodoptera exigua (SEM), with black color of pupae instead of yellow color was derived from wild strain (SEW).The effect of P-alanine on development and enzyme activites of S. exigua was conducted. The main results were as follows:As a precursor substance in the process of pigment synthesis, P-alanine participates in the change of cuticle color, meanwhile, it influences the growth and development of insects.After added 6%β-alanine to diet, for SEM, the periods of larval stage,prepupal stage and pupal stage were prolonged by 7.71,2,38 and 1.18 d compared to that of the untreated control group, respectively. For SEW, the periods of the three stages extended by 7.61,2.09 and 0.57 d, respectively. P-alanine prolonged the periods of growth and development.P-alanine had lethal effect on beet armyworm. The mortality of each stage was increased, especially for pupal stage.For SEW, the death rate of pupa increased by 0.49, 0.82 and 1.73-fold, respectively; For SEM, the death rate of pupa increased by 1,1.34 and 1.6-fold, respectively.Oviposition amount and hatching rate were reduced after treatment withβ-alanine, and this trend was obvious with the increase of concentration. Oviposition amount reduced from 863.33 to 196.67 eggs per female adult of SEM. Compared with SEW, the change range of oviposition amount and hatching rate of SEM were larger.The color of SEM was turned to yellow when the concentration ofβ-alanine reached 3%,and P-alanine also prolonged the speed of pupa coloring, meanwhile, pupa weight and cold-hardiness were reduced by the addedβ-alanine. For SEM, the supercooling point was increased from-20.77℃to-13.37℃.For SEW, the supercooling point was increased from -19.3℃to-11.67℃. Determination of susceptibility of S. exigua to insecticides showed that the sensitivity of SEW and SEM to phoxim and beta-cypermethrin both increased with the increase of P-alanine concentration. LC50S of SEM and SEW to phoxim were decreased from 166.12mg/L and 87.28mg/L to 38.11mg/L and 31.01mg/L, respectively. And LC50s of SEM and SEW to beta-cypermethrin were decreased from 5.52mg/L and 5.19mg/L to 2.77mg/L and 3.16mg/L, respectively. SEM showed higher insecticide resistence than SEW,β-alanine made the sensitivity of beet armyworm to. the above insecticides increased.P-alanine can also effect the activities of enzymes.Determination of the activities of carboxylesterase (CarE), glutathione s-transferase (GST),acetylcholinesterases (AChE) showed that, for SEW,6%β-alanine inhibited the activity of CarE from 2.54 to 1.69μmol-(mgPro)-1·min-1,and the activity of GST was induced from 0.33 to 0.41μmol-(mgPro)-1·min-1.The activity of AChE was induced when treated with low concentration ofβ-alanine, and inhibited when treated with high concentration ofβ-alanine.For SEM,β-alanine almost had no effect on the activities of CarE and GST.The activity of AChE was induced when treated with low concentration ofβ-alanine. The activities of CarE and GST were higher than those of SEW for both treatment group and control group.Determination of the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) showed that, the activity of SOD for SEM was 1.105 times higher than that of SEW for control group and the activity was induced after treated with P-alanine. SOD activities of SEM and SEW were increased from 48.61 OD560·(mg Pro)-1·min-1 and 23.08 OD560·(mg Pro)-1·min-1 to 69.77 OD560·(mg Proy)-1·min-1 and 43.16 OD560·(mg Pro)-1·min-1,respectively.The activity of POD for SEM was lower than that for SEW in control group,β-alanine had little effect on the activity.The POD activity was induced only at low concentration ofβ-alanine. The activities of CAT were similar for SEM and SEW in control group, and both were induced by P-alanine. POD activites of SEM and SEW were increased from 22.53 OD240·(mg Proy)-1·min-1 and 23.67 OD240·(mg Pro)-1·min-1 to 25.78 OD240·(mg Pro)-1·min-1 and 27.85 OD240·(mg Pro)-1·min-1,respectively. In summary, the results ofβ-alanine feeding experiment indicated that SEM showed more vitality than SEW, it had advantages in terms of survival ability, insecticide resistence and cold resistence ability over SEW.
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