Study On Epidemiology Of Beauveria Bassiana Disease Of Ostrinia Furnacalis Population

The Asian corn borer(Ostrinia furnacalis), a major pest of corn in China, has caused a great loss to maize production. The entomopathogenic fungus Beauveria bassiana has been considered to be one of the most important and promising biocontrol agents of natural pathogens against the Asian corn borer. The use of B. bassiana against Asian corn borer had been promoted by China’s agricultural sectors in the northeast area since the mid 1970 s in the last century. Now the technology has become one of the important means of prevention and control of corn borer in Northeast China. But outside northeast China,few use of B. bassiana against corn borer was reported. Studies on epidemiology of B.bassiana disease of Asian corn borer populations were few. So far,few researches about dynamics of release strains and indigenous strains were conducted.Therefore, in the base of screening of B. bassiana strains with high pathogenicity against Asian corn borer, we studied the use of B. bassiana to control Asian corn borer in the field,and the persistent, spreading and diffusion of B. bassiana in the corn field. Based on the theories and methods of community ecology and molecular ecology, further analyses were carried out to trace of the origin and spreading track of white muscardine of corn borers.Our results were listed as follows:1. Biological characteristics and virulence of 8 B. bassiana strains isolated from the corn borer, bug, Propylaea japonica and soil were determinated. The 8 strains exhibited certain differences in the vegetative growth, sporulation, germination ratio and the pathogenicity against 2nd instar corn borer larvae. Two strains(Bb05, Bb09) were obtained,which showed high sporulation, germination rate and virulence against larva of Ostrinia furnacalis.2. Bioassay conducted with their conidial suspensions against the 2nd instars lavae,screening out the most virulent isolate Bb05 from these 8 strains. A granule preparation,produced after solid fermentation using Bb05, was used to treat corn plant of whorl-stage in late July 2010 and late July 2011, resulting in mortality of 57.5% and 49.6%,respectively against the larvae of the corn borers at 2nd and 3rd generation in 2010, and mortality of 56.3% and 43.7%, respectively against the larvae of the borers at 2nd and 3rd generation in 2011. The efficacy was significantly lower than that by phoxim, a chemical insecticide, against the 2nd generation, but significantly higher than that against the 3rd generation respectively in 2010 and 2011. In addition, the overwintering population of the3 rd generation corn borer was reduced significantly and the production and quality of corn were improved. It suggested that obvious sustainable control was achieved. At the same time, corn borer natural enemy communities of B. bassiana release field and blank control field of maize were investigated. The natural enemy communities of corn borer in B. bassiana release field and bland field were abundant and stable, therefore, B. bassiana release did not pose a threat to the community of natural enemies.3. Dynamics of pathogen populations is one of the most important factors on the epizootiology of insect diseases. The results showed that the inoculum density of B.bassiana surviving in soil and air with treated field was higher than the control field, and irregularly increasing, while the inoculum density of control field was very low. Obviously, the epidemics of B. bassiana occurred in treated field, for the number of corn borer cadavers infected by B. bassiana in treated field was much higher than that in control field. These showed that this artificial ecosystem inoculum density of Beauveria bassiana hardly affected the insect development, and therefore, it is necessary to release B.bassiana isolates against the corn borer. Corn straw stacking is an important way of corn straw processing. During the corn straw stacking, the number of insect cadavers in corn straw stack was increasing, and the highest numbers appeared at next June. The number of insect cadavers with treated field was higher than the control field. Colony forming units(CFU) in the soil of corn straw stack exhibited the same change trend and decreased gradually with the increasing distance from the corn straw stack. B.bassiana spores in the corn straw stack were spread nearby through the air, water, human activities. This illustrates the corn straw stack, as the B. bassiana inoculation library in field, plays a very important role in maintaining the number of B.bassiana populations in field.4. Results of cage experiment showed that the infected corn borers could transfer the disease caused by B. bassiana to healthy ones. When the proportions of infected and healthy corn borer were 3: 15, 6: 12, and 9: 9, the transmission ratio between individuals were 4.4%, 16.7% and 40.7%, respectively. The ratio of the disease spreading followed the increasing of the diseased insect proportion. The diffusion test showed that the corn borer damage ratio increased with the distance increasing from the release field. At the distance of 50 m, the damage ratio of the second generation and the third generation corn borer were 9.50% and 66.33%, respectively, significantly lower than that of the distance of 200 m(14.17% and 84.67%, respectively). So, B. bassiana in the release field could spread nearby, and control the damage of corn borer populations within short distance.5. ISSR markers were used to investigate genetic diversity among 200 isolates of Beauveria bassiana isolated from Xiaoxian, Anhui(B. bassiana granules used in whorl leaves of maize), Baicheng, Jilin(B. bassiana used in corn straw stacking), and Gongzhuling, Jilin(natural epidemic area of B. bassiana). The treatment of B. bassiana in Xiaoxian and Baicheng had obviously increased controlling effects, which caused pathogens epidemic in corn field and straw stack respectively. Meanwhile, the group with the different genetic distance from the released isolations was caused by the treatment. In contrast, the pathogens epidemic in corn straw stack(Xiaoxian) and corn field(Baicheng),where B. bassiana was not released, mainly was caused by the non dominant native groups.However, the dominant groups also effect the epidemic development. The difference has benefits to increase the survival ability of population and subpopulation in different environment.6. Tracing of the origin and spreading track of white muscardine of corn borers, the artificial released B. bassiana was the main source of white muscardine in corn field of Xiao xian and corn straw stack of Baicheng, the Indigenous strains are secondary pathogeny. The artificial released B. bassiana mainly Survived in corn borer cadavers as well as the other insect cadavers and soil, which was the most deposition and natural inoculation library of the released B. bassiana. The B. bassiana spores in the airflow plays an important role in horizontal diffusion of white muscardine. B. bassiana transferred at different ecological niche, corn borer(?)corn(?)other host insect(?)corn straw stack(?)air(?)soil. It will become epidemic, when the environment is suitable. Compared to the diversity of fungus in Xiaoxian and Baicheng, the diversity in Gongzhuling, where B.bassiana was not released, was higher. The low pathogenicity epidemic in corn borer populations was caused by the indigenous strain consistently.7. The inter-subpopulational genetic differentiation showed, the B. bassiana in Xiaoxian, Baicheng and Gongzhuling was clustered in different group. Genetic lineages clearly appeared regional characteristics. The genetic distance was significantly correlated with the geographic distance, meanwhile, it also had some relevance with the B. bassiana host specialization.