Studies On The Pathogenity Of The Biocontrol Agent Beauveria Bassiana To The Silkworm, Bombyx Mori

The occurrence of silkworm diseases is the important reason for the reduction in cocoons. In recent years, the large outbreak and widely epidemic of the silkworm white muscardine in Guangxi made the silkworm breeding badly damaged and had serious influence on the development of the silk industry in our country. For this, some researches believed that it resulted from the long-term and great using biocontrol agent Beauveria bassiana for controlling Dendrolimus punctatus Walker. To investigate the reasons of the serious occurrence of the silkworm white muscardine in Guangxi provincial silkworm rearing, especially the influence of B. bassina biotic pesticide on silkworm, we carried out the studies on the pathogenity of the biocontrol agent B. bassiana to the silkworm. Meanwhile we hope to provide the basis for the prevention and control of the silkworm white muscardine. In this research, we collected6B. bassiana pesticides from our country and3white muscardine silkworm in Guangxi provincial silkworm rearing. The strains were isolated from the samples on the above and purely cultivated. And the isolates were identified by the colony characters and morphological observation. Then the pathogenicity of B. bassaina isolates and samples to silkworm were determined, and the pathogenicity of the white muscardine silkworm to silkworm was determined to explore its importance on the happenning and spreading of white muscardine. The effects of environment factor and disinfectant on spore germination and pathogenicity of B. bassiana were studied to explore the hazard potential of B. bassiana to silkworm. The biological characteristics of the B. bassiana strains were studied to probe into the association of biological characteristics with pathogenicity of B. bassiana to silkworm. The different pathogenicity of B. bassiana isolates was analysed by esterase molecular marker method combining the biological characteristics of the B. bassiana. The results were as follows:1. We collected6B. bassiana pesticides from our country and3white muscardine silkworm in Guangxi provincial silkworm rearing.9B. bassiana strains isolated from them were identified to be B. bassaina (Bals.) Vuillemin by morpha and ITS sequence analysis. 2. The B. bassiana pesticides had strong pathogenicity to silkworm treated by spraying the B. bassiana pesticides powder to insects and leaves, and the mortality rate reached up88.3%to100%. And the9B. bassiana isolates all could make the silkworm infected the white muscardine and dead by any way that B. bassiana contacted the silkworm. Meanwhile the6B. bassiana pesticides could also infect the Dendrolimus punctatus Walker. The study proved that these strains were not strong host-specific group and they could host more than a kind of insect.3. The Bombyx batryticatus obtained from the infected silkworm by B. bassiana pesticides samples, the bombysbatryticatus of Dendrolimus punctatus Walker conidium infected by B. bassiana pesticides samples, and the conidium got from these two kinds of bombysbatryticatus, all could infect the silkworm and made them got the white muscardine. It indicated that the bombyx batryticatus kept in silkworm rooms and the white bombysbatryticatus around the silkworm area had important influence on the further spread of the silkworm white muscardine.4. The results of the effects of environment factor and disinfectant on B. bassiana spore germination and pathogenicity indicated that the high humitid condition was helpful for B. bassiana to cause silkworm got infected. B. bassina had strong pathogenicity at the temperature which was suitable for the growth of silkworm. Ultraviolet ray had very strong killing effect to bare B. bassiana spores while had weak effect to the B. bassiana pesticides powder. The light of the sun could also make the spores of B. bassiana which adhered to the mulberry leaves lost pathogenicity in short time (24h), but needed longer time (78h above) to make the spores of B. bassiana pesticides powder on the mulberry leaves lost pathogenicity. When B. bassiana pesticides were sprayed to the mulberry leaves and were ingested by silkworm in48h, they would make the silkworm got white muscardine in quantities. And when the B. bassiana pesticides were sprayed to the silkworm they could also cause the occurrence of white muscardine. Spraying mulberry leaves for disinfection with effective chloric which arrived for2%of bleach solution had better disinfection efficacy than spraying the silkworm bodies.5. Spores producing quantity and exocellular protease activity of B. bassiana had significant positive correlation with their pathogenicity to silkworm. It meant the higher the spores B. bassiana produced and the exocellular protease activity of the B. bassiana was, the stronger pathogenic the B. bassiana had to silkworm.6. It’s found that9B. bassiana isolates had genetic difference by esterase isozyme analysis. According to the characteristics of the strains enzyme and cluster analysis,9B. bassiana isolates could be divided into five categories as follow:B. bassiana pesticides powder strain of Henan was one kind; the Bombyx batryticatus strain of Longtou was one kind; the B. bassiana pesticides powder strains of Beijing and Jiangsu were one kind; the B. bassiana pesticides powder strains of Nanning, Guilin and Huanjiang were one kind; and the Bombyx batryticatus strains of Hengxian and Zhongwang were one kind. The strains had different pathogenicity to silkworm, and the pathogenicity order for strength as follow:3bombyx batryticatus strains of Longtou, Hengxian and Zhongwang> B. bassiana strain of Henan> B. bassiana strains of Huanjiang, Nanning and Guilin> B. bassiana strains of Beijing and Jiangsu. Among the interspecific crossing of B. bassiana there were some differences in biological characteristics such as hypha growth, spore yield, and extracellular protease activity. Those all could prove that it was feasible to use the ester enzymes to analyse the classification of B. bassaina, and the genetic differences of B. bassiana could reliably reflect the biological characteristics and pathgenic differences of the strains.7、According to the results, some suggestion were put forward for preventing the silkworm white muscardine.