From 2002 to 2004, three field trials were conducted in Senegal to compare the efficacy of a new biopesticide for grasshopper control, based on the entomopathogenic fungus Metarhizium anisopliae var. acridum Driver & Milner, with mixtures of this biopesticide and cyhalothrin, a widely used pyrethroid insecticide to control the Senegalese grasshopper Oedaleus senegalensis Krauss. The aim of these trials was to obtain a knockdown effect without compromising the environmental credentials of the biopesticide too much. During the first two trials, 50 ha (2002) and 25 ha (2003) plots were treated with Metarhizium at a rate of 50 g/ha, cyhalothrin at a rate of 40 g/ha, a mixture of Metarhizium at a rate of 25 g/ha and cyhalothrin at a rate of 20 g/ha. For the third trial in 2004, several mixtures of cyhalothrin at a rate of 10 g/ha and Metarhizium at dose rates of 6.25, 12.5 and 25 g/ha were prepared. These mixtures were compared with 50 g/ha of Metarhizium, 40 g/ha of cyhalothrin and 10 g/ha of cyhalothrin on 25 ha experimental plots. The volume application rate for all trials was 1 l/ha. The Metarhizium applications resulted in a slow but long-lasting reduction of grasshopper numbers. By contrast, cyhalothrin produced a rapid reduction, but grasshopper numbers re-increased a few days after application, except in 2003, when grasshopper densities in the surrounding areas were low. The mixtures caused a similarly fast reduction in grasshopper numbers, but this lasted throughout the 3 week post-application assessment period. These trials demonstrated that adding a low dose of a pyrethroid insecticide to a slow-acting biopesticide can increase the speed of population reduction, while retaining the latter's long-lasting effect. A bonus is that the price of the combination is lower than that of each product at the recommended dose rates. Since the insecticide is added at a low dose, its environmental impact is reduced compared with the full-dose insecticide only treatment.