| Summary: | Fungal entomopathogens are largely facultative parasites and play an important role in controlling the density of insect populations in nature. A few species of these fungi have been used for biocontrol of insect pests. The pattern of the entomopathogen competition for insect individuals is still elusive. Here, we report the empirical competition for hosts or niches between the inter- and intra-species of the entomopathogens <i>Metarhizium robertsii</i> and <i>Beauveria bassiana</i>. It was found that the synergistic effect of coinfection on virulence increase was not evident, and the insects were largely killed and mycosed by <i>M. robertsii</i> independent of its initial co-inoculation dosage and infection order. For example, >90% dead insects were mycosed by <i>M. robertsii</i> even after immersion in a spore suspension with a mixture ratio of 9:1 for <i>B. bassiana</i> versus <i>M. robertsii</i>. The results thus support the pattern of competitive exclusion between insect pathogenic fungi that occurred from outside to inside the insect hosts. Even being inferior to compete for insects, <i>B. bassiana</i> could outcompete <i>M. robertsii</i> during co-culturing in liquid medium. It was also found that the one-sided mycosis of insects occurred during coinfection with different genotypic strains of either fungi. However, parasexual recombination was evident to take place between the compatible strains after coinfection. The data of this study can help explain the phenomena of the exclusive mycosis of insect individuals, but co-occurrence of entomopathogens in the fields, and suggest that the synergistic effect is questionable regarding the mixed use of fungal parasites for insect pest control.
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