Comparative plague dynamics of tropical Locusta (Orthoptera, Acrididae)

Abstract

AbstractThe occurrence and causes of plagues of tropical Locusta spp. are compared for the three major outbreak areas, Middle Niger of Mali (L. migratoria migratorioides (R. & F.)), south-west Madagascar (L. m. capito (Sauss.)), and Mindanao (Philippines) (L. m. manilensis (Meyen)). Multiplication and concentration are the two most important processes involved and may occur concurrently or sequentially depending on the outbreak area. In the Middle Niger and Madagascar heavy rainfall, through its effect of increasing multiplication, is the key density-disturbing factor, whereas on Mindanao the factor is drought leading to concentration of locusts. Variations in multiplication rates are determined by the response of eggs and hoppers to desiccation and by the density-dependent effects of fecundity and natural enemies. Concentration is caused by the restriction of favourable breeding habitats in relation to the seasonal population displacements and, because it occurs at different seasons in the three outbreak areas owing to their differing climates and geographies, profoundly modifies the timing of outbreaks. Man's modification of the environment, particularly in relation to the dry season survival of Locusta, has caused outbreaks to occur in new areas. Long-term solutions to the problem of controlling such outbreaks lie in the field of management rather than in the continued use of insecticides because of the continued reinfestation from surrounding populations. The use of natural enemies does not as yet show any promise as a means of controlling outbreaks of tropical Locusta.