Comparison of an organophosphate insecticide with a mycoinsecticide for the control of Oedaleus senegalensis (Orthoptera: Acrididae) and other Sahelian grasshoppers at an operational scale.
Operational scale field trials were conducted in 1996 and 1997, in the east of the Niger Republic, on 50 and 800 hectare plots, to compare the efficacy of an oil-based formulation of the entomopathogenic fungus, Metarhizium anisopliae var. acridum (Deuteromycotina: Hyphomycetes) with fenitrothion for the control of Sahelian grasshoppers. Oedaleus senegalensis was the most abundant species in the trials. M. anisopliae was applied at 5×1012 spores ha-1 at volume application rates of 2 and 0.5 l ha-1 in successive years. Fenitrothion was applied at 220 g ha-1 at 1.25 and 0.22 l ha-1 volume application rates. Ultra-low volume equipment mounted on a vehicle (1996) or a fixed wing aircraft (1997) was used for application. The M. anisopliae treatment reduced the grasshopper population significantly after 7 days and by 93% within 16 days. Fenitrothion caused a population reduction of more than 90% shortly after application, but due to immigration, the grasshopper population recovered to the initial level within 16 days. Grasshoppers treated with the fungus and given the opportunity to thermoregulate in the sun died more slowly than grasshoppers incubated in the shade. The survival of spores in the spray residue of the M. anisopliae plots assessed by exposing grasshoppers to the sprayed vegetation at intervals and monitoring disease levels during subsequent laboratory incubation, showed the spray residue to remain highly infective, for 3 weeks after spraying. At the end of the 1997 season, egg pod density and viability in the plot treated with the fungus were reduced compared with both untreated and the fenitrothion plots. Compared with the existing practice of large-scale treatment of grasshopper infestations with fenitrothion, use of M. anisopliae would not only be safer to mammals and less damaging to non-target organisms, but also be more effective in the long-term control of grasshoppers.