Survival of Beauveria brongniartii in the soil after application as a biocontrol agent against the European cockchafer Melolontha melolontha

The survival of the entomopathogenic fungus Beauveria brongniartii and its efficacy against the European cockchafer, Melolontha melolontha, was examined over 16 months after application as a biological control agent in different soil types in Switzerland. In the absence of M. melolontha, the reduction of the number of the CFU in the soil was nearly 90%. In soils with high organic content and increased catalase activity the decline in the CFU density of B. brongniartii was more pronounced. At sites where M. melolontha was present, the survival was significantly longer. The application of B. brongniartii resulted in a maximal infection rate of 75% in the M. melolontha population. The number of CFU of B. brongniartii in the soil started to decrease only when the M. melolontha population was reduced during the epizootic ( reduction of 46% on average). The rapid decrease of the fungus in the absence of the host gives further evidence of the high specificity of the fungus and that a saprophytic multiplication without the host is unlikely. The fungus was isolated on a selective medium as well as by baiting with Galleria mellonella. The latter method did not reveal the significant differences mentioned above, which leads to the assumption that the two methods may select for different fungal strains.