Ultrastructure of the entomopathogenic fungus metarhizium anisopliae during cuticle penetration in the locust, locusta migratoria (Insecta: Acrididae)

Abstract

Penetration of the cuticle of Locusta migratoria by the entomopathogenic fungus Metarhizium anisopliae was investigated using scanning and transmission electron microscopy. Scanning electron microscopy revealed that conidia of M. anisopliae attached mostly to the sockets of the setae and to the intersegmental membranes. Fourth instar nymphs of locusts treated with the fungus were maintained at 28°C. At this temperature, the fungal conidia germinated and produced appressoria on the cuticle of the locust within 1 day postinoculation. The penetrant structure entered directly through the epicuticle and exocuticle, with no lateral development. A fusiform penetrant structure then extended laterally between the layers of the endocuticle, with the spaces between the layers being enlarged due to lysis of the layers close to the penetrant. The penetrant structures exhibited various forms; most were fusiform, with many lysosomes and mitochondria, but no vacuoles. The penetrant hyphae ultimately invaded and colonized the haemocoele.