Biocontrol potential of Metarhizium spp. against Myzus persicae and beet mild yellowing virus in the post-neonicotinoid era.

Entomopathogenic fungi (EPF) are promising biocontrol agents that can also function as plant endophytes, mediating interactions within plant-insect-pathogen systems. Here, we evaluated four EPF isolates, two endemic (Metarhizium brunneum strain GxABT-2 and Metarhizium majus strain GxABT-3) and two reference strains (M. brunneum ARSEF4556 and V275), for their efficacy against the green peach aphid (Myzus persicae) and their ability to limit beet mild yellowing virus (BMYV) transmission. Concentration-dependent bioassays assessed aphid mortality, fecundity, and post-mortem fungal development, while seed treatments evaluated the endophytic colonization and impact on BMYV establishment. All isolates caused > 80% aphid mortality at the highest concentration (1 × 108 conidia ml-1), whether applied directly (spray or immersion of aphids) or indirectly (spray or immersion of leaves), with GxABT-2 producing the greatest reduction in fecundity. Three post-mortem developmental stages (melanized, non-sporulating, and sporulating) were primarily observed at lower concentrations (1 × 105 and 1 × 106 conidia ml-1). Overall, melanized aphid cadavers were more frequent with endemic isolates, whereas sporulating cadavers predominated in the V275 and ARSEF4556 treatments. Endophytically colonized plants exhibited significantly lower BMYV loads, with GxABT-2 being the most effective. These results highlight the potential of endemic Metarhizium spp. as sustainable biocontrol agents in a post-neonicotinoid era and underscore the importance of understanding isolate-specific variability in plant-fungus interactions for optimizing viral suppression.