Novel report on soil infection with Metarhizium rileyi against soil-dwelling life stages of insect pests

Entomopathogenic fungi are the most effective control remedy against a wide range of medical and agricultural important pests. The present study aimed to isolate, identify, and assess the virulence of Metarhizium rileyi against Spodoptera litura and Spodoptera frugiperda pupae under soil conditions. The biotechnological methods were used to identify the isolate as M. rileyi. The fungal conidial pathogenicity (2.0 × 10⁷, 2.0 × 10⁸, 2.0 × 10⁹, 2.0 × 10¹⁰, and 2.0 × 10¹¹ conidia/mL⁻¹) was tested against prepupae of S. litura and S. frugiperda at 3, 6, 9, and 12 days after treatments. Additionally, the artificial soil-conidial assay was performed on a nontarget species earthworm Eudrilus eugeniae, using M. rileyi conidia. The present results showed that the M. rileyi caused significant mortality rates in S. litura pupae (61-90%), and S. litura pupae were more susceptible than S. frugiperda pupae (46%–73%) at 12 day posttreatment. The LC₅₀ and LC₉₀ of M. rileyi against S. litura, were 3.4 × 10¹⁴–9.9 × 10¹⁷ conidia/mL⁻¹ and 6.6 × 10⁵–4.6 × 10¹⁴ conidia/mL⁻¹ in S. frugiperda, respectively. The conidia of M. rileyi did not exhibit any sublethal effect on the adult stage of E. eugeniae, and Artemia salina following a 12-day treatment period. Moreover, in the histopathological evaluation no discernible harm was observed in the gut tissues of E. eugeniae, including the lumen and epithelial cells, as well as the muscles, setae, nucleus, mitochondria, and coelom. The present findings provide clear evidence that M. rileyi fungal conidia can be used as the foundation for the development of effective bio-insecticides to combat the pupae of S. litura and S. frugiperda agricultural pests.