Stability of Metarhizium anisopliae (Hypocreales: Clavicipitaceae) isolates during repeated in vitro subculture and evaluation of an oil-in-water mycoinsecticide

Abstract

Nine Metarhizium anisopliae (Hypocreales: Clavicipitaceae) isolates were evaluated for efficacy against Melolontha melolontha (Coleoptera: Scarabaeidae) larvae, stability in culture, and the superior isolate used to produce an oil-in-water mycoinsecticide. The phenotypic and genotypic characters of four isolates with high virulence were evaluated for their stability after repeating 12 cycles of in vitro subculture. Repeated subculture did not affect the germination of conidia; however, the morphology of some isolates changed significantly. Three isolates lost their virulence, whereas the KTU-2 isolate remained highly pathogenic. Therefore, KTU-2 was selected as the superior isolate for mycoinsecticide production. After the conidia of KTU-2 were produced by solid-state fermentation using cracked rice as the substrate, the conidia were formulated as an oil-in-water emulsion and its efficacy was assessed. The formulation caused 80% mortality on Me. melolontha larvae even at the lowest application rate (1 x 10(5) conidia/mL) in pot experiments, and complete mortality was obtained with the concentration of 1 x 10(7) conidia/mL. Lethal concentrations that kill 50% and 95% of Me. melolontha larvae present were estimated as 9.29 x 10(3) and 2.1 x 10(6) conidia/mL, respectively. Oil-in-water mycoinsecticide could be a potential candidate for the commercial control of Me. melolontha and other white grubs.