African armyworm outbreaks : why do they occur after drought?

The African armyworm, <em>Spodoptera exempta</em> (Walker) (Lepidoptera; Noctuidae), feeds almost exclusively on plants of the families Gramineae and Cyperaceae. It is a severe pest of crops, including maize, sorghum and millet, especially on the eastern side of the African continent where a marked seasonal occurrence of outbreaks coincides with the rains. First outbreaks of a season can arise soon after the start of the short rains (October-November) in the so-called primary outbreak areas, which are located in the zone separating the lower desert area to the east from the highlands to the west in Kenya and Tanzania. Adults from these primary outbreaks migrate downwind on the prevailing easterly winds to give rise to subsequent generations further west in secondary outbreak areas. The outbreak season subsequently ends during the long dry season. No further outbreaks are reported, and densities remain very low, until the start of the following short rainy season. Current control aims to destroy, with insecticides, any outbreaks that are critical in terms of their potential to generate upsurges in following generations. Many primary outbreaks are critical in this sense.Outbreak development of the African armyworm is known to vary greatly between years in eastern Africa: in some years hardly any outbreaks are reported while in others severe infestations occur over very large areas. Drought (below average precipitation) in particular, has often been associated with the most severe outbreak seasons. As outbreaks usually seem to develop on plants which are free of drought stress but that are growing on soils that have previously been subjected to severe drought for a considerable time, the existence of a delayed indirect effect of drought on outbreak development of the African armyworm is postulated. It is hypothesized that:<BLOCKQUOTE><em>Periods of more severe drought encourage the occurrence of more severe outbreaks by stimulating the mineralization process in the soil when it is remoistened by rainfall as the drought breaks. This results in higher soil nitrate levels and, consequently, higher nitrogen levels in host plants of the African armyworm, thus increasing larval developmental rate and survival (especially of very young larvae) as well as the fecundity of subsequent adults.</em></BLOCKQUOTE>This hypothesis is based on the knowledge that the decomposition of organic material by bacteria in a soil after wetting is correlated with the duration and temperature of the drying period to which the soil has been exposed (the socalled 'Birch-effect'), and that higher nitrogen contents in host plants have often been observed to increase herbivore fitness. The aim of the present study has been to test this hypothesis.