Infective conidia from entomopathogenic fungi are widely used to control insect pests. Many entomopathogenic fungi also produce yeast-like cells called blastospores under specific liquid culture conditions that can directly infect insects. However, little is known about the biological and genetic factors that allow blastospores to infect insects and make them potentially effective for biological control in the field. Here, we show that while the generalist Metarhizium anisopliae produces a higher number of and smaller blastospores, the Lepidoptera specialist M. rileyi produces fewer propagules with a higher cell volume under high-osmolarity conditions. We compared the virulence of blastospores and conidia of these two Metarhizium species towards the economically important caterpillar pest Spodoptera frugiperda. Conidia and blastospores from M. anisopliae were equally infectious, but acted slower, and killed fewer insects than M. rileyi conidia and blastospores did, where M. rielyi conidia had the highest virulence. Using comparative transcriptomics during propagule penetration of insect cuticles, we show that M. rileyi blastospores express more virulence-related genes towards S. frugiperda than do M. anisopliae blastospores. In contrast, conidia of both fungi express more virulence-related oxidative stress factors than blastospores. Our results highlight that blastospores use a different virulence mechanism than conidia use, which may be explored in new biological control strategies.

Metarhizium species can be mutualistic symbionts of plants. They are able to colonize roots, promote plant growth and provide protection against pests. We previously found Metarhizium robertsii and M. brunneum associated with coffee roots in a diversified coffee system. Here, we investigated whether these fungi, when inoculated in coffee seedlings, can associate with roots, improve seedling growth and indirectly protect against the coffee leaf miner (CLM) Leucoptera coffeella (Lepidoptera: Lyonetiidae). We performed a greenhouse experiment with coffee seedlings using suspensions of each Metarhizium species applied as soil drenches to potted seedlings. We also challenged these plants with CLM infestation (two adult couples per plant). We recovered Metarhizium spp. from most of the seedling roots 43 days after fungal inoculation. Plants inoculated with M. robertsii showed a 30% leaf area increase compared to the control. Both isolates promoted protection against CLM in coffee seedlings, reducing the percentual of leaf area mined and prolonging CLM development time by two days versus controls. Besides this protection provided by Metarhizium, M. robertsii also improves seedling growth. Therefore, these Metarhizium species could be considered for the development of inoculants for coffee seedlings.