The effects on pathogenicity of Metarhizium anisopliae (Metsch.) Sor. following one or more passages through the original and the heterologous insect host species were investigated by using two pathotypes specific for Cetonia aurata L. and Oryctes rhinoceros L., respectively. A significant increase in virulence occurred after a single host passage. Thus, the infection potential of host-passed inocula could be increased by a factor of 10 to 100, and its host-induced adaptation was almost complete after the first in vivo passage. Bacteria located on the integument of the host cadaver were not implicated in this phenomenon. When the fungus was again grown on artificial medium, the loss in virulence was also very fast. Consequently, changes in virulence of the two tested strains seemed to be phenotypic responses implicating inducible enzymatic mechanisms. Nevertheless, the parasitic behaviour of the two pathotypes toward the larvae of the nonsusceptible species was not modified by host passaging through their respective insect hosts. It is concluded that changes in entomopathogenic hyphomycete virulence, resulting from host passage, concern only the naturally susceptible host species.
