Using remote sensing to improve the evaluation of the Malagasy migratory locust risk Locusta migratoria capito (Saussure, 1884), Orthoptera : Acrididae

In 1974, the definition of the optimal rainfall range (ORR: 50 to 125 mm monthly) for the Malagasy migratory locust formed the basis for the first locust forecasting system. This implied that the risk of gregarization and of the consequent appearance of invasions would become very high if the optimal rainfall range occurred for three consecutive months. After the 1997 - 2000 locust invasion, this tool used to monitor the locust risk had to be updated and improved. As every locust habitat reacts differently to the rainfall, the areas at risk can be defined and organized more accurately by crossing rainfall zones with a qualitative map of the locust habitat. Remote sensing made it possible to characterize the habitats of the Malagasy migratory locust. The selected image, Landsat 7 n°160.077 on August 21, 1999, centred on the Mahafaly Plateau and shows great diversity: it represents the majority of the habitat types within the outbreak area. The habitats were determined by combining a visual interpretation with a supervised analysis (with IDRISI® software) for each of the 5 eco-regions delimited by photo-interpretation. After verification in the field, 22 habitats were described according to plant cover and hygrotrophy. The attractiveness of the habitats for the migratory locust in its solitary phase was arranged in hierarchical order according to 5 rainfall ranges. The spatiotemporal data relative to the habitat types and the rainfall is managed by a GIS that constitutes an operational result while permitting the real-time assessment and localisation of the locust risk. (Texte intégral