Seed treatment of cool-season turfgrasses with entomopathogenic fungi stimulated plant growth but had limited effects as a biocontrol agent

Entomopathogenic fungi (EPF) such as Metarhizium anisopliae and Beauveria bassiana are well-known biological control agents of soil-dwelling insects worldwide. Moreover, there is growing interest in applying EPF as seed treatments to serve as biostimulants for improving plant growth and stress tolerance. In this study, we examined the effects of M. anisopliae (ESF-1) and B. bassiana (GHA) seed treatments on the growth and development of turfgrasses (Lolium arundinaceum (syn. Festuca arundinacea) and Poa pratensis) in growth chamber, greenhouse, and field experiments, and also tested whether the seed treatments were a viable method of introducing EPF into the soil to increase insect suppression following soil contact using Galleria mellonella as a model. We found that EPF applied as dry powder seed treatments had overall positive or neutral effects on turfgrass growth and establishment in the greenhouse and field. In the greenhouse, we observed low to moderate levels of insect infection in G. mellonella bioassays, with infection levels varying based on the fungal species. In the field, insect infections from Beauveria spp. were much lower than in the greenhouse, while infections from Metarhizium spp. were higher than in the greenhouse, though infection levels of both fungi varied seasonally. Our findings suggest that treating turfgrass seed with EPF may aid in turfgrass establishment and improve stand density early on; however, the potential secondary benefits for biological control of soil-dwelling insects were limited.