Biological control agents are slowly and steadily replacing traditional chemical pesticides. Among them, fungal entomopathogens have assumed a key role in controlling a growing number of pests. Research on fungal entomopathogens has expanded constantly during the last decades, confirming real interest in their biocontrol potential. The production of fungal entomopathogens by solid-state fermentation has arisen as a sustainable production method, which is slowly becoming preferred over submerged fermentation considering the advantage of using of agro-industrial wastes as substrates. This review discusses relevant studies on fungal entomopathogens, with a special interest in their production as biocontrol agents by solid-state fermentation processes, including reactors used and operational conditions, focusing mainly on the two most-used fungal entomopathogens worldwide, Beauveria spp. and Trichoderma spp.

This work presents the scale-up of the conidia production of the entomopathogenic fungus Beauveria bassiana using two different wastes, coupled with concentration and virulence tests of the produced conidia against the pest Tenebrio molitor. Beauveria bassiana CECT 20374 was used in solid state fermentation (SSF) operating under batch strategy. Two substrates with different biodegradability (rice husk and beer draff) were tested, successfully scaling from 1.5 L to 22 L bioreactors. Higher conidia production was reached using beer draff as substrate (2.5 × 10⁹ and 6.0 × 10⁸ conidia g^-1 dry matter in 1.5 and 22 L reactors respectively) highlighting air free porosity relevance as scale-up parameter. Concentration and dose-response tests against larvae and adult Tenebrio molitor were performed to compare strain CECT 20374 with control strain KVL 13-39 (a B. bassiana strain previously tested against T. molitor). Virulence effect of the 22 L fermentation product of strain CECT using rice husk or beer draff was tested against T. molitor adult stage. However, quality loses between conidia produced in agar plates and fermented products were observed (from 75 to 80% mortality in plates to 40% in rice husk and 50-60% in beer draff fermented products respectively). The differences between plate and fermented samples also indicated fermentation process, extraction and conservation steps as possible causes for quality losses, highlighting the need to optimize them to maximize virulence maintenance.

The packed-bed bioreactor is among the most promising reactor configurations for solid-state fermentation. However, the bed thickness poses several limitations involving mass and energy transfer, heat generation, and the homogeneity of the material, hampering its development at the industrial scale. Fungi are among the most promising microorganisms used in this configuration; however, only polypropylene bags and trays are used at the industrial scale. In this work, Beauveria bassiana is used to demonstrate the potential of solid-state fermentation for conidia production. A scale-up from 0.5 L to 22 L is presented, starting with substrate selection, optimization via design of experiments and 22 L batches. The optimized parameters were 70% moisture, 6.5 × 10⁶ conidia mL⁻¹ inoculum concentration, 20 mL min⁻¹ airflow, 25 °C; temperature, and 40 C/N ratio. After optimization, beer draff was chosen as the preferred substrate for scale-up. Air-filled porosity was found to be the key parameter in fungal solid-state fermentation scale-up, establishing values of around 80% as necessary for fungal conidia production when working in a 22 L packed-bed bioreactor. When compared with the tray bioreactor, the packed bed obtained higher conidia production due to its better use of the total reactor volume. Our study harnesses the potential of the packed-bed bioreactor and serves as a base for further scale-up to industrial scale.