Impact of Production Temperature on Metarhizium brunneum F52: Growth, Germination, and Virulence for Biocontrol

The physiological performance and pathogenicity of the entomopathogenic fungus Metarhizium brunneum F52 were evaluated as a function of both inoculum production and incubation temperatures. This study assessed the impact of five production temperatures (10, 15, 20, 25, and 30 °C) on conidial germination, radial growth, spore production, and virulence against Tenebrio molitor larvae. No conidial germination occurred at 10 °C across all treatments. At a suboptimal incubation temperature of 15 °C, germination was initiated exclusively by conidia produced at 15 °C, suggesting a thermal priming effect. Complete germination (GI ≥ 0.5) was achieved at 20 °C and above for all thermal origins. Radial growth peaked at 20 °C, whereas maximal conidial density was achieved at 25 °C, with a value of 9.19 × 10⁶ spores mm⁻². In contrast, heat-stressed inocula produced at 30 °C exhibited significant reductions in vegetative vigour and reproductive output. Virulence assays demonstrated that all inocula, except for those produced at 30 °C, achieved the mortality threshold (FDI ≥ 1.0) within seven days. The most rapid beginning of sporulation (FDI 2.5) occurred at 25 °C. Inocula produced at 15 °C consistently outperformed heat-stressed origins under fluctuating thermal stress, reaching infection milestones significantly faster. These results indicate that the thermal history of the inoculum primes conidial quality and pathogenicity, underscoring the need to match mass-production temperatures to target field conditions to optimise biocontrol efficacy.