Control Effect Of Beauveria Bassiana On Spodoptera Frugiperda And Its Joint Control With Microplitis Prodeniae

Spodoptera frugiperda is one of the world’s important migratory pests,which poses a significant threat to corn plants due to its strong ability to reproduce and migrate.Since its invasion into China in 2019,chemical control has been the primary method of emergency response,with biological control,physiochemical control,and other measures serving as supplementary.The reason why the S.frugiperda did not cause significant harm in its place of origin is mainly because natural ecological regulation played an important role,with pathogenic microorganismsand and natural enemy insects as the main natural enemies significantly suppressing its population growth.The use of multiple natural enemies to control pests has become a trend in modern biological control,with highly entomopathogenic fungi or efficient natural enemy insects both able to significantly improve pest control.However,whether the combined use of different biocontrol measures will enhance or weaken their efficacy and its compatibility needs to be further studied.Therefore,this study isolated and cultured highly pathogenic strains of S.frugiperda and compared the toxicity of different strains using different methods.It also explored the combined use of entomopathogenic fungi and local efficient parasite natural enemy insects to control S.frugiperda larvae,providing a theoretical basis for biological control and sustainable management of S.frugiperda.The main experimental results are as follows:1.Isolation and identification of pathogenic fungi for S.frugiperda.During the process of large-scale laboratory breeding of resource insects,a large number of resource insects were found to die from infection with entomopathogenic fungi.When the entomopathogenic fungi were inoculated onto S.frugiperda larvae from the surface of the dead insects,it was found that S.frugiperda can be infected by this entomopathogenic fungi.After isolation and purification,four strains of entomopathogenic fungi with identical morphology and a single colony were obtained.Morphological observation revealed that all four strains of pathogenic fungi were white on the front and light yellow on the back when grown on PDA medium.The spores were circular or nearly circular,similar to the morphology of the Beauveria bassiana strain.Further molecular biology identification was performed by amplifying the ITS sequences of the four strains of entomopathogenic fungi,and BLAST online comparison found that they were highly homologous to B.bassiana,with a similarity of over 98%.The four strains of entomopathogenic fungi were determined to be B.bassiana by both identification methods and were named B.bassiana B01,B02,B03,and B04,respectively.2.Determination of the pathogenicity of different strains of B.bassiana on S.frugiperda larvaeThe pathogenicity of four strains of B.bassiana on S.frugiperda larvae was determined using immersion and spray methods.The immersion method experiment showed that at a spore concentrations of 1×10⁸ spores/m L,the LT₅₀ values（time to death of 50%of the infected larvae）of B.bassiana B01,B02,B03,and B04 were between 2.2 d-3.2 d,and the cumulative corrected mortality rate at day six was between 73.3%-85.1%.At a spore concentrations of 1×10⁷ spores/m L,the LT₅₀ values of the four strains were between 3.3 d-5.0 d,and the cumulative corrected mortality rate at day six was between 49.3%-85.3%.The spray method experiment showed that at a spore concentrations of 1×10⁷ spores/m L,the LT₅₀ values of the four strains were between 4.1 d-8.2 d,and the cumulative corrected mortality rate at day six was between 35.5%-76.1%.Comparative analysis revealed that B.bassiana B02 exhibited the highest toxicity effect on S.frugiperda larvae.3.The synergistic effect of high-toxicity B.bassiana B02 and Microplitis prodeniae on the control of S.frugiperda larvae was determined.The control effects of the two methods were compared under different application conditions through indoor bioassays and pot experiments.The results showed that using the two biocontrol measures together,regardless of whether B.bassiana B02 was inoculated first or M.prodeniae was released first,significantly increased the mortality rate of S.frugiperda larvae compared to using a single control method.The cumulative corrected mortality rate of S.frugiperda larvae that were M.prodeniae was first released and then inoculated with B.bassiana or M.prodeniae was released after inoculation with B.bassiana were 81.8%and 88.4%（indoor bioassays）and 47.5%and 54.9%（pot experiments）,respectively,significantly higher than the mortality rate caused by a single application of either inoculated with B.bassiana B02 or release M.prodeniae（indoor bioassays:48.1%and 40.3%;pot experiments:21.8%and 16.7%）.However,the combined use of the two biocontrol measures reduced the emergence rate of parasitic wasps and significantly decreased the offspring quantity under indoor bioassay conditions,which did not reach a significant level in the pot experiments.Therefore,the experimental use of the two measures significantly enhanced the control effect on the target pest,but various factors affecting the establishment of stable populations of M.prodeniae should also be considered.In summary,this study isolated and identified four highly pathogenic strains of B.bassiana and compared and screened for a highly toxic strain,B.bassiana B02.The combined use of B.bassiana B02 with the M.prodeniae showed significantly better control effect on S.frugiperda than using a single control method.The research results provide a theoretical basis for the biological control and sustainable management of S.frugiperda.