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

Abstract

The survival of the entomopathogenic fungusBeauveria brongniartii and its efficacyagainst the European cockchafer, Melolontha melolontha, was examined over 16months after application as a biologicalcontrol agent in different soil types inSwitzerland. In the absence of M. melolontha,the reduction of the number of the CFU in thesoil was nearly 90%. In soils with highorganic content and increased catalase activitythe decline in the CFU density of B.brongniartii was more pronounced. At siteswhere M. melolontha was present, thesurvival was significantly longer. Theapplication of B. brongniartii resultedin a maximal infection rate of 75% in the M. melolontha population. The number of CFU ofB. brongniartii in the soil started todecrease only when the M. melolonthapopulation was reduced during the epizootic(reduction of 46% on average). The rapiddecrease of the fungus in the absence of thehost gives further evidence of the highspecificity of the fungus and that asaprophytic multiplication without the host isunlikely. The fungus was isolated on aselective medium as well as by baiting withGalleria mellonella. The latter methoddid not reveal the significant differencesmentioned above, which leads to the assumptionthat the two methods may select for differentfungal strains.