The relationship between outbreaks, rainfall and low density populations of the African armyworm, Spodoptera exempta, in Kenya

This study has demonstrated that low numbers of African armyworms (Spodopte ra exempta) (Walker) (Lepidoptera: Noctuidae) can be found throughout the y ear in parts of the coastal and highland regions of Kenya where there is fr equent rainfall. During the study period the numbers of moths caught in phe romone traps in these regions built up during the short and long rains, and decreased dramatically during the intervening dry seasons. There was a lag of one to two months between the peaks for rainfall and moth numbers. This contrasts with the situation at sites of the seasonal outbreaks of armywor m larvae, where a sudden preceding influx of moths coincides with the rainf all. A positive correlation was found, for the long rains seasons only, bet ween the number of armyworm outbreaks throughout the country and the peak n umbers of moths in coastal and highland regions. It is suggested therefore that the peak numbers of moths trapped in the eas tern highlands and coastal regions during the rainy seasons arise principal ly from outbreaks in the extensive seasonal grasslands. These grassland are as diminish considerably during the prolonged dry periods between the two r ainy seasons, and it is only the eastern highland and coastal regions that are likely to provide suitable habitats for breeding during the dry seasons . The significance of these populations for initiating the first outbreaks of the following season is discussed.