INTER-SEASONAL AND INTRA-SEASONAL VARIATION IN OUTBREAK DISTRIBUTION OF THE ARMYWORM, SPODOPTERA-EXEMPTA (LEPIDOPTERA, NOCTUIDAE), IN EASTERN AFRICA

Reports of the African armyworm, Spodoptera exempta (Walker), in eastern Africa for sixteen seasons (1972-73 to 1987-88) were analysed together with weather records to identify the most frequent locations of outbreaks and their relationships with weather. Outbreaks were classified into 'primary' outbreaks (derived from low density parent populations), 'secondary' outbreaks (derived from previous outbreaks) and 'critical' outbreaks (leading to many subsequent outbreaks'). Differences in seasonal severity of armyworm infestations in relation to the timing and location of primary outbreaks and early season rainfall were also examined. Trajectory analysis was used to estimate migrations of windborne moths, and the distribution of rainstorms was used to help estimate the timing of moth concentration leading to outbreaks of S. exempta. First reported armyworm outbreaks from October to December in Kenya and Tanzania were usually primary. Critical primary outbreaks mostly occurred in east-central Tanzania, north-east Tanzania and the adjacent Taveta district of Kenya, and east-central Kenya. Strategic control (to prevent subsequent, downwind outbreaks) is best carried out in these areas. Migration direction was largely determined by seasonally dominant winds, although windfield disturbances were occasionally important. Three seasons in which very low numbers of S. exempta were reported, had heavy, persistent October to December rainfall leading to high larval mortality. Seasons with many outbreaks often had rainstorms separated by dry periods, during the rainy seasons. n`