Seasonal and geographical variation in the migratory potential of outbreak populations of the African armyworm moth, Spodoptera exempta

Implicit in a genetic model to explain variation in the migratory potential of outbreak populations of Spodoptera exempta (Lepidoptera: Noctuidae) are predictions regarding the temporal and spatial incidence of migratory phenotypes based on the pattern of rainfall through the season. Migration in female S. exempta moths occurs predominantly in immature adults, therefore migratory potential is assumed to correlate with the duration of the pre-reproductive period (PRP). PRP variation in moths was quantified from 14 outbreak sites in E Africa during the 1989-90 and 1990-91 seasons and correlating this with rainfall prevalences. Extensive intra- and inter-population variation in PRP was observed. Mean PRP was inversely correlated with prevalence of rainfall at the outbreak site during the month that the moths initiating the outbreaks were migrating: longest PRPs were exhibited by moths originating from early season outbreaks in regions of low, variable rainfall, shortest PRPs by moths from areas of high, consistent rainfall. Also as predicted by the model, the mean PRP of moths from outbreaks in Kenya and Tanzania tended to decline through the long rainy season, significantly so for females. Males matured consistently earlier than females. PRP variation cannot be accounted for by any major environmental factor, suggesting that the differences between samples are genetic in origin. -from Author