Liquid Fermentation, Formulation, and Bioefficacy of Metarhizium rileyi Microsclerotia for Control of Spodoptera frugiperda

Abstract Microsclerotia (MS) are melanized hyphal aggregates produced by submerged liquid fermentation by entomopathogenic fungi such as Metarhizium rileyi, an important pathogen of lepidopteran pests including the fall armyworm, Spodoptera frugiperda. In this study, we evaluated MS production under different carbon-to-nitrogen (C:N) ratios and assessed the bioefficacy of MS against S. frugiperda. Lower C:N ratios (10:1 and 12:1) yielded 1.4 × 104 and 3.4 × 104 MS/mL within five days of fermentation, respectively. MS also germinated and sporulated more rapidly on maize and soybean leaves than on water agar, reaching 94.9% and 92.7% germination, respectively, after 24 h, compared with 29.3% on the artificial substrate. A granular formulation containing 1 × 105 MS/g + diatomaceous earth caused 95% mortality of 3rd instar S. frugiperda (but only 2.5% sporulation) on leaves and 85.5% (with 38.8% sporulation) of prepupal mortality in soil. In addition, MS granules maintained high viability after six months of storage under refrigerated conditions. These results demonstrate that M. rileyi MS granules are stable during cold storage and support high germination and sporulation upon high humidity. Hence, M. rileyi MS + diatomaceous earth granules could potentially offer an additional strategy based on inoculative control to the toolbox management of S. frugiperda.