Vegetable oils influence the persistence of entomopathogenic fungi isolates Beauveria bassiana and Metarhizium anisopliae

The microbial biocontrol agents Beauveria bassiana (B. bassiana) and Metarhizium anisopliae (M. anisopliae) are highly effective in controlling various insect pests. However, their effectiveness often depends on biopesticide formulation, which could be affected by unfavourable environmental conditions. This study aimed to determine the compatibility of eight vegetable oils derived from raw palm, refined palm, sunflower, cotton, soybean, groundnut, neem, and canola in formulations of B. bassiana and M. anisopliae. This compatibility of oils with fungi was investigated by measuring fungal hyphal growth, conidial yield, fungal biomass, and conidial viability. The results showed a significant inhibition of B. bassiana growth in neem (2.08%) and raw palm oils (0.52%) and M. anisopliae in neem oil (0.28%). Conidia yield was high in the soybean (29.3 x 106 conidia/mL), groundnut (27.2 x 106 conidia/mL) and refined palm (23.83 x 106 conidia/mL) oils as well as vegetative growth for B. bassiana, while conidiogenesis of both fungal species was highest in soybean oil (29.3 x 106 conidia/mL and 44.4 x 106 conidia/mL for B. bassiana and M. anisopliae respectively). Fungal viability varied considerably, with the highest density of viable propagules obtained in Potato Dextrose Agar (PDA) culture media amended by sunflower oil for B. bassiana (43.2 x 104 viable propagules/ml) and in raw palm oil for M. anisopliae (97.4 x 104 viable propagules/ml). Taking these four parameters into account, it was concluded that the compatibility or toxicity of the oils to the two fungal species was mixed. The results of this study were relevant to the formulation of biopesticides based on effective strains of the two fungal species and constituted an essential component for this research program aimed at formulating biopesticides and testing their pathogenicity against insect pests under field conditions. Keywords: Vegetable oil, Cameroonian indigenous isolates, entomopathogens, compatibility, biopesticide.