The Red locust Nomadacris septemfasciata (Serville, 1838) Orthoptera: Acrididae is found in southern Africa and on islands in the Indian Ocean. Although responsible for plagues in southern Africa, only a few local outbreaks have been reported in Madagascar. Yet, in 2002, locusts became gregarious in the Sofia basin, i.e., swarms and larval bands threatened the two main rice-producing basins of the country. The overall scope of this study is to explain this new gregarisation of the Red locust in Madagascar. Factors controlling the locust population dynamics were determined during a four year field monitoring program (2003 to 2007). The ecological requirements of this locust were assessed during the different periods of its life cycle. The evolution of the locust habitat was mapped using remotely sensed data (Spot satellite). Our results show a close connection between Red locust ecological requirements and environmental modification in the Sofia basin. First, the locust potential of each habitat at the eco-regional and macro-regional scale was quantified. Second, these habitats were mapped in 1986 and 2004. Satellite images revealed extensive deforestation thus providing new favourable breeding habitats for locusts. This environmental alteration also opened migration pathways between complementary ecological areas. The combination of both these factors allowed the Red locust gregarization to be possible. A Red locust forecasting system is outlined within the scope of a preventive control strategy. (Texte intégral)