## 1 Introduction  
The western corn root worm _Diabrotica virgifera virgifera_ LeConte (Coleoptera, Chrysomelidae), an accidentally introduced, but now firmly established maize pest, causes major damage in maize growing areas in Austria, particularly in regions of Southeast Styria with continuous maize cultivation. The application of the biocontrol agent _Metrahizium brunnem_ (Petch) against _D. v. virgifera_ larvae has been investigated in a few studies [1, 2, 3], but very limited data is available on long-term field trials using only the entomopathogenic fungus (EPF) as insecticide against the maize pest _Diabrotica_. The inundative use of _Metrahizium_ aims at controlling the pest within a short period of time and the application has to be repeated if the pest population increases again, because reproduction and/or a permanent establishment of the fungus is not expected. This strategy is mostly used for short-term crops where high population densities of the pest need to be controlled to prevent damage [4]. Large amounts of the biocontrol agent are necessary to achieve a control effect, as soil dwelling insects can only be infected by direct contact with spores. However, annual field processing such as mechanical cultivating or ploughing bears the risk of substantially diminishing the applied microbial agent [5; 6]. It was suggested by [PERSON] et al. [1], that the fungus should be applied preventively, before the pest has established a large population, and pest pressure is still low (i.e., number of beetles should not exceed the economic threshold value of approximately one beetle per ten plants for continuous maize cultivation). Soil is an extremely complex milieu, an environment with a high number of diverse microorganisms [7; 8]. The presence of a viable soil microbiota have an impact on the persistence and/or efficacy of entomopathogenic fungi and vice versa. Therefore, studies on diversity and distribution of soil inhabitants, especially _Metarhizium_, are requested and further knowledge is needed [9; 10; 11]. The _Metarhizium_ community is influenced by changes in agricultural practice, e.g., abundance changes depending on the crop [10] as well as on type of land-use [12]. After application in high doses, biocontrol agents are exposed to resource restrictions and compete with the well-established indigenous microorganisms [5] including native _Metarhizium_ strains. [PERSON] et al. [9; 13] investigated effects of _M. brunneum_-based control agents on microbial communities in pot and field experiments. They found small effects in some treatments, but these were attributed to the product formulation and not to the activity of the fungus itself. However, knowledge on microbial interactions is still limited. Further studies are necessary to assess on the one hand possible effects of mass application of specific strains on microbial communities [10; 13], and on the other hand how microbial communities may affect the establishment and development of applied EPF strains. In this study, we investigated whether long-term inundative mass application of the biocontrol agent _M. brunneum_ BIPESCO 5 improves the efficacy of this EPF to control _D. v. virigifera_. In addition, the persistence of the application strain was evaluated with and without the presence of target pest and in co-occurrence with the indigenous _Metarhizium_ species and genotypes. In Styria, the pest pressure was high at the beginning of our investigations in 2012, the fungus was applied annually in a six-year long efficacy study to investigate the long-term control effect of the fungus against the larvae of _Diabrotica_. In addition, a preventive application strategy was tested in Tyrol, by annually applying the product in a maize field in a noninfested area over a period of three years.