Study of the persistence and viability of Metarhizium acridum in Mexico's agricultural area

Locust is one of the most predominant agricultural pests in Mexico, followed by non-migratory grasshoppers. Their devastating effects cover numerous Mexican states, generating important economic losses. Metarhizium acridum (Hypocreales: Clavicipitaceae) has been employed in Mexico as a microbial agent for biological control for some time. However, questions related to the long-term persistence and viability of the fungus after it has been sprayed over crop fields remain to be elucidated. To resolve these questions, we performed a study in a crop field where an abundant population of the grasshopper Sphenarium purpurascens (Orthoptera: Pyrgomorphidae) is present. The fungus M. acridum was sprayed during August and October 2007. Its presence in the air, soil and vegetative cover was subsequently monitored using molecular methods and culture techniques. The results showed that the fungus was viable on the ground during the follow-up period (66 weeks). Its population fluctuated markedly, however, reaching four times its initial concentration and later decreasing to nearly half its initial concentration by the end of the study. The initial concentrations of the fungus in the air and in the vegetative cover decayed steadily (with some small, isolated recoveries) until its complete disappearance after 8 months.