Use of Metarhizium flavoviride for control of Zonocerus variegatus: A model, linking dispersal and secondary infection from the spray residue with mortality in caged field samples

A field trial was conducted using an oil formulation of the entomopathogenic fungus, Metarhizium flavoviride Gams and Rozsypal (Deuteromycotina: Hyphomycetes), to control the variegated grasshopper, Zonocerus variegatus L., (Orthoptera: Pyrgomorphidae). The influence of dispersal and secondary pick-up from the spray residue on mortality was assessed by sampling insects from the treated plots at intervals and monitoring disease levels during subsequent incubation in the laboratory. The cumulative mortality curves showed the pathogen to be highly effective, even in samples taken 8 days after spraying. A model was developed to link the influence of mortality, incubation, secondary pick-up of spores and insect dispersal, on the shape of these cumulative mortality curves. The model proved useful for data received from an experiment using small plot sizes, where insect dispersal is a big problem. The spray residue had an important influence on overall insect mortality. The model also indicated a difference in speed of kill between field and laboratory samples, suggesting an incubation delay in the field.