Effects of temperature and relative humidity on germination, vegetative growth, sporulation, and pathogenicity of native Beauveria bassiana isolates and formulations against adult desert locust (Schistocerca gregaria Forskål).

Entomopathogenic fungi such as Beauveria bassiana are promising biocontrol agents against the desert locust, Schistocerca gregaria, but their performance is strongly influenced by temperature and relative humidity (RH). In this study, native B. bassiana isolates recovered from soils in Isiolo and Laikipia Counties, Kenya, were molecularly identified and evaluated under factorial combinations of temperature (20, 25, 30, and 35 °C) and RH (50, 75, and 90%) to determine their effects on conidial germination, vegetative growth, sporulation, and pathogenicity against adult S. gregaria. Three native isolates and six conidial formulations were assessed, comprising oil-based, whey-based, and diatomaceous earth (DE)-based carriers. Temperature, RH, and their interaction significantly influenced all measured traits (p < 0.0001). Vegetative growth and sporulation were highest at 25-30 °C and 90% RH. Across environmental treatments, isolate 341 consistently showed the highest germination and vegetative growth, followed by isolate 334, whereas differences among isolates were less pronounced under optimal conditions (25-30 °C) than at thermal extremes (20 °C and 35 °C). Among formulations, oil-based products consistently outperformed whey-based and DE-based formulations in germination, vegetative growth, sporulation, and insect mortality (p < 0.05). Adult mortality at 14 days ranged from 57.8% to 100%, with the shortest median lethal time (LT₅₀) recorded at 30 °C and 90% RH. Overall, the findings demonstrate that the performance of native B. bassiana isolates and formulations is strongly modulated by temperature, RH, and carrier type under controlled conditions. These results provide a basis for selecting promising isolate-formulation combinations for further validation under field-relevant conditions.