Regional dynamics of the Red Locust in the south of Madagascar
This area can be described as "a limit area of the extreme-south", where the annual rainfall is < 40 cm/y, which is insufficient for the development of the Red Locust. Type 2 sites occupy a broad crescent-shaped zone, going from the northwest to the southeast.
Lastly, the rate of reproduction is > 1 for 13 sites, indicating that at these locations, Red Locust populations were expanding from one generation to the next.
This paper describes the life cycle, phenology, reproductive diapause, and migration patterns of the Red Locust, Nomadacris septemfasciata (Serville) in southern Madagascar.
Migrations.--Our study of the biological cycle of the Red Locust in south Madagascar shows the importance of the seasonal migrations.
In Africa, the Red Locust is fairly well studied in its outbreak areas, and the movement patterns of gregarious individuals are well documented (Morant 1947, COPR 1982, Bahana 1999).
It is certainly the same for the Red locust, where the migration towards the south at the beginning of the rainy season increases sharply population density in the breeding area, compared to the low densities that exist in the refuge areas during the dry season.
Diapause.--The adult life of the Red Locust is marked by a long period of adult sexual and reproductive diapause during the entire dry season (May-November).
Albrecht (1956) also highlights a population synchrony in laying for the Red Locust; he attributes it to the abrupt change in climatic conditions and to the sudden appearance of the rainy season, which causes the termination of adult diapause simultaneously for all the individuals of the population.
For example, in 2006 and 2007, vegetation greenness in mid-November averaged 30% and 20% respectively, resulting either from the first rains, or from the bush fires of the dry season having stimulated the regrowth of green vegetation and thus provided the nutritive resources necessary to the Red Locust for its breeding activity.
Comparison of the reproduction rates observed in south Madagascar, as well as annual comparison of these same rates on a same site, demonstrates the strong impact of the abundance and distribution of the rains on both egg and nymph survival, eggs and nymphs seeming to constitute critical periods of the biological cycle of the Red Locust. Low or badly distributed rainfall at the beginning of the rainy season can cause partial or complete egg mortality in a given area.
Rainfall > 40 cm/month clearly decreases Red Locust density.