Population dynamics and migration of the red locust Nomadacris septemfasciata in the south-west of Madagascar

The Red Locust is a recurring problem for crop protection in Madagascar. The past few years have seen the problem worsen, probably due to intensive deforestation. In Madagascar this species remain little known, unlike in Africa. In order to establish the cause of recent outbreaks and to implement a locust control strategy, several studies have been carried out in the South-West of the Island, where locust damage is regularly reported: rural population investigations, in the field studies of locust population dynamics, and the analysis of over 25 years of archive data at the National Locust Control Centre. A study of this information shows several zones between which the Red Locust, a single generation and imaginal diapause species, make seasonal migration, from the North-East to the South-West and back, depending on rains and wind alternance. We note: 1) refuge zones where only immature imagos are present during the dry season, 2) breeding zones during the rainy season with massive arrival of maturing imagos at the beginning of the rainy season, abundant hopper populations and emigration of new generation imagos in early dry season, 3) intermediary passage zones for maturing imaginal populations in the early rainy season and immature populations in early dry season, 4) a limit zone to the extreme south, populated late in the rainy season by old imagos which do not reproduce. The dynamics of their movements and inter-annual particularities linked to the irregularity of rainfall are certainly one cause of recurring outbreaks in this species. (Texte intégral