Laboratory bioassay to assess the effects of the fungus Lecanicillium lecanii (Zimm.) Gams and Zare on the aphid Schizaphis graminum (Rondani)

The interactions between the entomopathogenic fungus Lecanicillium lecanii ITEM 3757 (syn. Verticillium lecanii), isolated from aphids, and winged and wingless adult morphs of the species Schizaphis graminum were investigated under laboratory conditions. The fungus was applied in the form of finely ground dried culture grown on autoclaved rice kernels. The growth on cereal grains is one of the most utilized method for the production of fungal biomass of bio-pesticides and is also fully compatible with organic farming and other forms of environmental friendly agriculture. The aphids were treated by dipping in a suspension containing two different dosage of fungal formulation (25 mg ml-1 or 25 g ml-1). The Colony Forming Unit/g value of the fungal formulation was 2.18 108. The strain affects the survival and the reproduction of treated aphids, but it interacts differently with the two aphid morphs. Scanning electron microscope observations point out a different distribution of the fungal formulation over the integument of the two treated morphs, with a higher amount of adhering formulation and faster rates of fungal germination on the winged cuticle. We therefore hypothesize a crucial role played by the cuticle in the different initial spore adhesion, retention and germination, considered as a most effective reason of the survival difference between the two morphs, after treatment by using fungal formulation. Histological analysis revealed that hyphae invaded the host hemocoel of a limited number of winged and wingless specimens, suggesting that the lethal action of formulation might be expressed independently from penetration. In terms of route of penetration, the analysis revealed that the strain entered only through the spiracles. The fungus was also transferred from sporulating colonies on cadavers to uninfected individuals. Our results suggested that this strain is a good candidate for a programme of bio-control against S. graminum and other aphid species.