Ultrastructural study of the penetration by Metarhizium anisopliae through Dimilin-affected cuticle of Manduca sexta

Abstract

Fourth instar larvae of the tobacco hornworm, Manduca sexta, were inoculated with conidia of Metarhizium anisopliae then fed a diet containing the insect chitin synthesis inhibitor Dimilin. Cuticle of Dimilin-treated insects provided reduced resistance to penetration by hyphae of M. anisopliae. Widespread histolysis of postecdysial (Dimilin-affected) cuticle occurred. In addition, although lamellate preecdysial cuticle was not affected by Dimilin, the majority of the cuticle in the vertical cuticular columns was laid down at the same time as the postecdysial cuticle. Therefore, the vertical cuticular columns were areas of weakness in the preecdysial cuticle which as a consequence failed to provide a mechanical barrier to the penetrating fungus.