Factors involved in the origin of plagues of Schistocerca cancellata (Serville) in Argentina were examined using a simple model for locust development. Plagues were present in 48 of the 58 years between 1897–1954 with plague sequences occurring before 1900, 1903–11, 1913–28, 1931–38 and 1944–54. Plagues originated when winter rain fell in the outbreak areas in La Rioja and Catamarca resulting in three generations in a season. Plagues continued for 8–15 years. Their decline was usually gradual resulting from a combination of several seasons when it was dry in the outbreak areas, and only one generation per year was possible. Before 1954, locust swarms were common in 15 of Argentina's 22 provinces but it was not obvious where the plagues originated. Since then most swarms have been reported from two provinces in the northwest: La Rioja and Catamarca. Within these provinces, bands and swarms have been most common in the semi-arid outbreak area. Populations have been highest when the outbreak area received winter/spring/summer rain which allowed three generations in a season. However, infestations have never approached plague proportions in recent years because regular control of bands and swarms in the outbreak area has kept populations in check.