Topical Infection of Cordyceps militaris in Silkworm Larvae Through the Cuticle has Lower Infectivity Compared to Beauveria bassiana and Metarhizium anisopliae

Abstract

Topical infection of entomopathogenic fungi in insects occurs when the fungal conidia attach to the insect's surface (cuticle), germinate, and then form appressoria that penetrate the cuticle and enter their bodies. In this study, we inoculated silkworm larvae with three entomopathogenic fungi, Cordyceps militaris, Beauveria bassiana, and Metarhizium anisopliae, and investigated their mechanisms of infection. Attachment of the conidia of the three entomopathogenic fungi to the surface of silkworm larvae was observed under a microscope. We counted the number of conidia attached to the surface of the silkworm larvae and the number of conidia detached from the surface was counted. The number of C. militaris conidia that attached to the surface was less than that of B. bassiana and M. anisopliae; however, it germinated and formed appressoria on hydrophobic surfaces, similar to the other two strains. Mycelial growth of C. militaris was inhibited compared to that of B. bassiana in PDA medium containing 0.1% linoleic and linolenic acids. The germination of C. militaris conidia was also inhibited in PD medium containing 0.1% linoleic or linolenic acids. These results suggest that the attachment of low numbers of C. militaris conidia on the surface of silkworm larvae and presence of inhibitory linoleic or linolenic acids in the silkworm cuticles may cause low topical infectivity by C. militaris. This study improves the efficacy of topically infecting silkworms with C. militaris to produce fungal fruiting bodies for use in traditional Chinese medicine and dietary supplement production.