Seasonal patterns of persistence and infectivity of Metarhizium anisopliae var. acridum in grasshopper cadavers in the Sahel

Field-based experiments were conducted to evaluate the fate and infectivity of the entomopathogenic fungus Metarhizium anisopliae var. acridum (Deuteromycotina: Hyphomycetes) in grasshopper cadavers in the Sahel. Unlike uninfected cadavers, which were rapidly scavenged, those infected with the fungus persisted in the environment for a number of weeks. The environmental factor most associated with cadaver disappearance was rainfall. The high environmental humidity associated with rainfall was also required for sporulation of the fungus on host cadavers, although the likelihood of sporulation differed between microsites. Characteristics of the infection profile from infective cadavers were investigated by the sequential exposure of uninfected hosts to sporulating cadavers in field cages. This experiment revealed that cadavers remained infective for > 30 days, with the net infectivity changing through time. The most likely explanation for these changes is climatic influences on both the fungus and host. High humidity was not required for infection. A measurement of the transmission coefficient between healthy hosts and sporulating cadavers in the field was obtained at a realistic density of infectious cadavers. This revealed a figure of 0.45 m² day^-1. Overall, these experiments show that following host death, M. anisopliae var. acridum can be persistent in the environment, sporulate on host cadavers and reinfect new hosts at a realistically low field density, although at least in arid or semi-arid areas, rainfall may be critical to the horizontal transmission of this pathogen.