The fungus Metarhizium acridum (Driver & Milner) (Hypocreales, Clavicipitaceae) is the most effective alternative to conventional insecticides for controlling the desert locust, Schistocerca gregaria (Forskål, 1775) (Orthoptera; Acrididae). Demonstrating its storability and efficacy under variable field conditions is essential for promoting the systematic implementation of biological control in desert locust management, but little information is available. In 2014, the Mauritanian National Desert Locust Centre received a fungus powder formulation (Green Muscle) from FAO/CLCPRO. Spore viability was monitored for 10 years while the product was kept in a refrigerator at 4°C. For the field efficacy assessment, seven 20-ha plots were treated with the formulation. The formulation was transported, used, and stored in the field without storage facilities. Applications were conducted using a hand-held sprayer with a dose rate of 50 g in 2 L gasoil per hectare. Two of the seven plots were selected to investigate the influence of host plant (Schouwia thebaica) size and cover on efficacy. Plot A had large, widely spaced plants, and Plot B had small, closely spaced plants. Spore viability and virulence remained unaffected after 10 years of storage. Field application of the biopesticide reduced the locust population by more than 80% within two weeks. Plant size and cover had no significant effect on biopesticide efficacy. Mortality of caged nymphs from Plot A was 82.8% and from Plot B was 83.9% compared to 4.3% in the untreated control. These results demonstrate the remarkable storability and field efficacy of M. acridum and support its integration into desert locust management programs.