The contribution of surface waxes to pre-penetration growth of an entomopathogenic fungus on host cuticle

A locust wing bioassay, that allowed an entomopathogenic fungus to be removed from host cuticle before penetration, was used to investigate the role of surface lipids and waxes in pre-penetration growth of the specific locust pathogen Metarhizium anisopliae var. acridum. SEM and atomic force electron microscopy showed the impact of the fungus on the architecture of the cuticle surface. Although the fungus can germinate on authentic alkanes as the sole carbon source, only low levels of germination occurred on crude, non-polar wing cuticle extracts, containing a mixture of long-chain n-alkanes and other waxes (identified in particular by gas chromatography and mass spectroscopy). The fungus removed a large proportion of non-polar and polar components during pre-penetration growth on the wing. Polar crude extracts from Schistocerca gregaria hindwings, which contained fatty acids, fatty acid esters, glucose, amino acids and peptides, were strong promoters of germination, and poor germination was observed on a locust hindwing from which the extract had been taken. Thus simple polar compounds, also present on the surface, may be required to stimulate germination before the fungus can make use of a complex mixture of non-polar lipids.