Ultrastructure of the infection of spruce budworm larvae by the fungus Entomophaga aulicae

Abstract

The infection of spruce budworm larvae by the fungus Entomophaga aulicae was studied by light, scanning, and transmission electron microscopy. Exposure of the larvae to conidia resulted in the death of the host and the production of a crop of infective conidia within 5 to 6 days. Germinating conidia produced appressoria prior to penetration of the larval cuticle. Nuclei in appressoria were larger and contained less heavily condensed chromatin than nuclei in other stages of fungal development. Attachment and infection were characterized by distinct changes in the wall layers of the fungus. An infection hypha grew through the insect tissues directly into the hemocoel where discontinuities were observed in the wall layers at the apex of the infection hypha. These discontinuities are probably associated with protoplast formation in vivo. Protoplasts with polysaccharide and lipid storage material were present in the hemolymph of larvae 3 days after exposure to conidia. Thick-walled hyphal bodies with abundant lipid reserve filled the larval body cavity by the 5th day. Hyphal bodies germinated to produce unbranched, aseptate conidiophores which emerged through the host cuticle prior to conidiogenesis.