Isolation of Native Strains of Entomopathogenic Fungi from Agricultural Soils of Northeastern Mexico and their Virulence on Spodoptera exigua (Lepidoptera: Noctuidae)

Spodoptera exigua Hübner (Lepidoptera: Noctuidae) is considered a polyphagous pest distributed worldwide with a wide range of hosts, and is also considered a serious plague for vegetables and ornamentals. The objective of this study was to explore the occurrence of entomopathogenic fungi in agricultural soils from Mexico and to evaluate its virulence and effect on the life cycle of S. exigua. Selected isolates (HEB1 and γ01) were used to perform toxicity bioassays on neonatal larvae of S. exigua; additionally, the GHA (Beauveria bassiana), Pfr-612 (Cordyceps fumosorosea), Ma, and 2 isolates HIB-11 and HIB-12 (Metarhizium anisopliae) were included to obtain the median lethal concentration (LC50) and the median lethal time (LT50). Also, the life cycle of the surviving individuals was followed up after treatment to document the effects of the entomopathogenic fungi application. The isolates belong to 2 different species, which are Beauveria bassiana (Bals.-Criv.) Vuill. (Hypocreales: Cordycipitaceae) and Cordyceps fumosorosea (Wize) Kepler, B. Shrestha & Spatafora (Hypocreales: Cordycipitaceae) (formerly Isaria fumosorosea). In toxicity tests, the isolate HIB-12 of Metarhizium anisopliae (Metschnikoff) Sorokin (Hypocreales: Cordycipitaceae) showed better mortality results against S. exigua than the other fungi evaluated with LC50 of 3.17 × 10⁶conidia per mL. The lethal time of LT50 oscillated between 3 to 5 d and the HIB-12 isolate had the lowest lethal time (3.15 d). During the monitoring of the biological cycle after treatment, the greatest interruption of the metamorphosis was caused by the GHA strain, while the Ma strain was the one that induced the slightest interruption of the biological cycle. These results show the feasibility of using strains of M. anisopliae for control of larval stages of S. exigua and their possible formulation for agricultural use, as well as opening a way for the exploration and research of mechanisms or metabolites involved in the interruption of metamorphosis due to the application of entomopathogenic fungi, specifically B. bassiana.