Biocontrol of Melolontha spp. Grubs in Organic Strawberry Plantations by Entomopathogenic Fungi as Affected by Environmental and Metabolic Factors and the Interaction with Soil Microbial Biodiversity

SIMPLE SUMMARY: The study aimed at relating some metabolic characteristics of two entomopathogenic fungi (Beauveria bassiana and B. brongniartii), critical for their virulence and persistence in soil, to the strain’s capacity to reduce the damage of Melolontha sp. grubs in two organic strawberry plantations. Combination of the two species was also tested to achieve a higher efficacy, due to their different living behaviors. This hypothesis was not confirmed in the study, probably due to the B. bassiana metabolic competitive advantage emerged from the phenotypic characterization. The strong in vitro metabolic activity of the B. bassiana strain could be also associated to the higher abundance of this species in the inoculated soils in comparison to B. brongniartii strain. Considering the impact on soil biodiversity, the inoculation with both strains or the co-inoculum did not affect the natural fungal and bacteria communities in the soil, according to terminal restriction fragment length polymorphism (TRFLPs) analysis and qPCR data. The study provides a complex view of the effects of bioinocula to plant protection and soil biodiversity, taking into consideration the mechanisms of fungal virulence and the effect of environmental conditions on them. ABSTRACT: The efficacy of two strains of two Beauveria species (B. bassiana and B. brongniartii), individually or as co-inoculants, to control Melolontha sp. grubs was assessed in two organic strawberry plantations in relation to the environmental conditions, their abundance after soil inoculation, and their in vitro chitinolytic activity, thereby also verifying their impact on soil microbial communities. A reduction of the grubs’ damage to strawberry plants was observed when compared to the untreated control in one plantation, irrespective of the strain used and whether they were applied as single or as co-inoculum. The metabolic pattern expressed by the two fungi in vitro was different: B. bassiana showed a higher metabolic versatility in the use of different carbon sources than B. brongniartii, whose profile was partly overlapped in the co-inoculum. Similar differences in the chitinolytic activity of each of the fungi and the co-inoculum were also pointed out. A higher abundance of B. bassiana in the soils receiving this species in comparison to those receiving B. brongniartii, together with its in vitro metabolic activity, could account for the observed diverse efficacy of pest damage control of the two species. However, environmental and climatic factors also affected the overall efficacy of the two bioinocula. According to the monitoring of the two species in soil, B. bassiana could be considered as a common native species in the studied locations in contrast to B. brongniartii, which seemed to be a non-endemic species. Nevertheless, the inoculation with both species or the co-inoculum did not consistently affect the soil microbial (fungi and bacteria) biodiversity, as expressed by the operational taxonomic unit (OTU) number and Shannon–Wiener diversity index based on terminal restriction fragment length polymorphism (TRFLP) data. A small transient increase of the share of the inoculated species to the total fungal community was noted by the analysis of genes copy numbers only for B. brongniartii at the end of the third growing season.