| Spodoptera exigua (Lepidoptera: Noctuidae) is an important vegetable pest in the world, and has strong resistance to many synthetic insecticides such as the chrysanthemum insecticides and the organic phosphate; But Spodoptera exigua isn't sensitive to Bacillus thuringiensis like other sensitive pest, and more, among them some strains have resistance to Bacillus thuringiensis. Therefore, lacking of highly toxic stub of Bacillus thuringiensis is one of the best serious restrictive factors which restrict Bacillus thuringiensis to be utilized in the control of Spodoptera exigua, and it is very necessary to continuously search highly toxic Bacillus thuringiensis to Spodotera exigua. Apart from it, screening of highly toxic Bacillus thuringiensis and studying the toxic features has very important practical significance to reasonably alternate different sorts of Bacillus thuringiensi so as to lower resistance and detain development of resistance.In this paper, the first job is to screen 5 more highly toxic strains to Spodoptera exigua neonaf e from 23 initial strains of Bacillus thuringiensis through many times screening of ferment, the second job is to decide that the strain YL-17 among the 5 strains is the best highly toxic to Spodoptera exigua, which the LCso's of 48 hours sporulated liquid cultures to the first instar Spodoptera exigua is 1.6ul/ml.The continuous job is to separate crystals and spores from sporulated liquid cultures of YL-17 by the method of water-oil; then to determine the concentration and the molecule size of crystals from sporulated liquid cultures and separated crystals by SDS-PAGE so as to study the crystals' features of YL-17. The crystals' concentration is 5.2ug/ul, and the molecular masses is 68.5kDa, 76. 3kDa, 91.9kDa and 130. 8kDa. The result show that the crystals play an important role in poisoning neonates of Spodoptera exigua between crystals and spores through the study of toxicity of crystals, spores and combination of them with different ratios, and the spores have partly toxicity. When studying the toxicity of the combination of spores and crystals with different ratios, we can find that the combination of spores: crystals =1: 1 has very good toxicity, and the spores are well helpful to the toxicity of crystals. The LC50's of 48 hours of separated and unseparated spores and crystals is 230. 18ug/ml, 22.06ug/ml, 20.23ug/ml.Through studying effects of YL-1 7 on growth and development of Spodoptera exigua, we can find that the toxin of YL-17 has not significantly influence on eggs of Spodoptera exigua, but influence the survival of the larvae which hatch from the eggs by the toxin of YL-17; And the further study also show that the survival rate of larvae which are raised by the diet polluted by the toxin of YL-17 for certain time falls following with increment of the Bacillus thuringiensis toxin concentration; that the growth rate of the 3th instarSpodopera exjgna rises with the increase of the concentration of toxin in the diet; that the life span of larvae increases with the increment of the toxin concentration in the treated group: 30ug/ml, I5ug/ml, 7.5ug/ml, 3.75ug/ml besides the treated group: 60ug/ml, I20ug/ml, in which there is no larvae to finish the whole growth development. The study also show that there is no difference in the pupating rate, the emergence rate of pupa and the hatching eggs of the treated group(3.75ug/ml) between with the control group, but there is significant different in the same study content between the treated group(30ug/ml, 15ug/ml, 7.5ug/ml) and the control group(p<0.05), and the pupating rate and the emergence rate of the pupa lower with the increment of the toxin concentration in the diet. When the larvae is treated with the 3.75ug/ml in the first stage of larvae, the even eggs hatched has no difference with the control. The result show that sublethal doses of Bacillus thuringiensis has no influence on the even eggs hatched by the single female adult, but can detain the peak time of the hatching eggs of female larvae. The study a... |
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