Screening And Pathogenic Mechanism Of Beauveria Bassiana With High Virulence To Spodoptera Frugiperda

The fall armyworm（Spodoptera frugiperda）,one of the most important plant pests in the world.It invaded China at the end of 2018,posing a major challenge to China’s agricultural production and ecological environment.With the extensive use of chemical pesticides,the resistance of S.frugiperda to chemical insecticide increased rapidly,and the ecological environment has been destroyed,which is also harmful to natural enemies of pests and humans.Beauveria bassiana as a broad-spectrum insectpathogenic fungus,which is environment-friendly and the host is not easy to develop resistance.It is harmless to warm-blooded animals and is an ideal biocontrol agent.In this study,76 Beauveria bassiana strains were used as materials,and 25 dominant strains were selected according to the difference of spore production.The lethal rate of the spore suspension of the dominant strains to the 3rd instar larvae of Spodoptera litura was determined,and the high virulence strains bb455 and BB378 were selected.On this basis,we further studied the changes of antioxidant enzyme activity in the 3rd instar larvae infected with the highly virulent B.bassiana.The differentially expressed genes of S.frugiperda larvae after 36h and 48h infection were analyzed and screened.In addition,the effects of B.bassiana application on the growth of corn and the feeding selectivity of S.frugiperda larvae on corn after applying B.bassiana were also studied.The main results are as follows:1.Screening of high virulence strains of B.bassiana:The biological characteristics and pathogenicity of 25 strains of B.bassiana were determined,and the highly virulent strains Bb455 and Bb378 were screened,and their half-lethal concentrations were 1.84×10⁶conidia·ml^-1and 1.11×10⁸conidia·ml^-1.After 14days of treatment,the maximum cumulative corrected mortality rate was 71.74%and48.61%.2.Changes in antioxidant response of S.frugiperda infected with B.bassiana:The spore suspension of B.bassiana Bb378 and Bb455 at aconcentration of 1×10⁸conidia/m L was used to treat S.frugiperda larvae,and the changes of SOD,POD and CAT activities in the larvae were determined after 6 hours of treatment.The results show:after the infection of two strains of B.bassiana,the change trend of the three defensive enzymes of the insect body was the same.The activities of SOD and CAT in the larva were significantly higher than the control in the early stage（12h）,and then gradually decreased.POD activity of Bb378 was higher than the control at the initial stage infection（6-12h）,but Bb455 with higher virulence was significantly lower than the control.It indicates that B.bassiana may affect the antioxidant enzyme activity of the insect,then affect its metabolism.The reason for the difference in virulence between the two strains may be related to the POD activity.2.Changes of gene expression in S.frugiperda after B.bassiana infection:The RNA-seq technology was used to analyze the differentially expressed genes of S.frugiperda infected by B.bassiana.After 36 hours of treatment,1629 differentially significant genes were obtained,including 1089 up-regulated genes and 540 down regulated genes.After 48 hours of treatment,45 differentially significant genes（up-regulated 37 and down-regulated 5）.There were 3505 significantly different genes（1902 up-regulated and 1603 down-regulated）between the 36h treatment and the control,and 875 significantly different genes（359 up-regulated and 516 down-regulated）between the 48h treatment and the control.According to the analysis of Gene Ontology,the four comparison groups all annotated with functions such as metabolic process,cellular process,catalytic activity,structural molecular activity,transport activity,antioxidant activity,etc.Pathways are enriched in transport and catabolism,cell growth and death,signaling molecules and interaction,infectious diseases:Bacterial,the immune system,etc.3.Effect of applying B.bassiana on corn at the seedling stage:The spore suspension of Bb378 strain at aconcentration of 1×10⁸conidia/m L,1×10⁷conidia/m L,and 1×10⁶conidia/m L was used in three methods:seed soaking,root irrigation and foliar spraying at the seedling stage of corn.Plant height,leaf area and fresh weight were measured at 14 and 21 days after treatment.The results showed that root irrigation and seed soaking had significant effects on plant height,and foliar spraying had significant effects on the growth of leaf area and fresh weight of corn.The spore suspension of 1×10⁷conidia/m L and 1×10⁸conidia/m L had significant growth promoting effect.The seedling-stage corn treated with the above 3 methods of B.bassiana spore suspensions was used to investigate the effect of selective and non-selective feeding of the newly hatch and 3rd instar larvae of S.frugiperda.The results showed that for the newly hatched larvae,the concentration of 10⁸and 10⁷conidia/m L spore suspension treatment with seed soaking and root irrigation methods had obvious repellent effects within 1-60hours.There is no obvious difference between the foliar spray treatment and the control.For the 3rd instar larvae,the treatment of 10⁸and 10⁷conidia/m L of seed soaking method showed the avoidance effect,but gradually weakened with time.