Insecticidal activity of Beauveri bassiana ERL1170-egfp against Tribolium castaneum

Background Beauveria bassiana is one of the commercially available
entomopathogenic fungi (EPF), and a number of isolates with high virulence
and broad host spectrum have been used to control agricultural and forest
pests. Although the functional importance of genes in EPFs’ pathogenesis
have been extensively studied, the precise ultrastructural mechanism of
the fungal infection, particularly penetration of the host insect
cuticles, is not well understood. Results In this study, we investigated
the morphology and ultrastructure of the larval cuticle of the red flour
beetle, Tribolium castaneum, after treatment with B. bassiana ERL1170
expressing an enhanced green fluorescent protein (Bb-eGFP). The Bb-eGFP
showed high virulence against the larvae, with approximately 90% mortality
at 48 h after treatment (HAT) and 100% at 72 HAT under our infection
conditions. In these larvae, the regions of the body wall with flexible
cuticles, such as the ventral and ventrolateral thorax and abdomen, became
darkly melanized, but there was little to none melanization in the rigid
dorsal cuticular structures. Confocal microscopy and transmission electron
microscopy (TEM) indicated that germinated conidia on the surface of the
larval cuticle were evident at 6 HAT, which formed penetration pegs and
began to penetrate the several cuticle layers/laminae by 12 HAT. The
penetration pegs then developed invading hyphae, some of which passed
through the cuticle and reached the epidermal cells by 24 HAT. The larval
cuticle was aggressively and extensively disrupted by 48 HAT, and a number
of outgrowing hyphae were observed at 72 HAT.