Soil nutrient availability in a primary outbreak area of the African armyworm, Spodoptera exempta (Lepidoptera: Noctuidae), in relation to drought intensity and outbreak development in Kenya

Abstract

AbstractTo explain differences between years in outbreak development of the African armyworm, Spodoptera exernpta (Walker), in eastern Africa, the availability to host-plants of soil nitrogen, phosphorus and potassium was studied in a primary outbreak area during the first month after the start of the short rainy season in relation to the drought intensity during the preceding year. The weather-soil-plant interactions observed during four successive years (1988–1991) indicated that nitrate levels, especially in soils not extremely poor in organic carbon and total nitrogen, can differ greatly between years in re sponse to differences in drought intensity and rainfall pattern. These resulted in large differences (i.e. up to 2.0% dry wt.) in organic nitrogen levels in leaves of host-plants of S. exempta. Phosphorus and potassium levels in soils and host-plants showed no consistent relationship with the weather pattern over the years and in general their differences between years were relatively small. The occurrence of S. exempta during the first two months after the start of the short rainy season was well associated with plant nitrogen levels in the primary outbreak area: outbreaks developed only in years with at some locations organic nitrogen concentrations of almost 5% in young grasses. If fitness of S. exempta is strongly dependent on the organic nitrogen content of its food within the range encountered in this study, weather conditions during the long dry season might indirectly explain the observed differences in subsequent outbreak development via their delayed effect on host-plant nitrogen levels.