SEASONAL AND GEOGRAPHICAL VARIATION IN THE MIGRATORY POTENTIAL OF OUTBREAK POPULATIONS OF THE AFRICAN ARMYWORM MOTH, SPODOPTERA-EXEMPTA

1. Parker & Gatehouse (1985; Gatehouse 1986) proposed a genetic model to explain variation in the migratory potential of outbreak populations of the African armyworm moth, Spodoptera exempta (Walker) (Lepidoptera: Noctuidae). Implicit in this model are predictions regarding the temporal and spatial incidence of migratory phenotypes based on the pattern of rainfall through the season. 2. Migration in female S. exempta moths occurs predominantly in immature adults, therefore migratory potential is assumed to correlate with the duration of the prereproductive period (PRP). The aim of the present study was to test the predictions of the Parker & Gatehouse model by quantifying PRP variation in moths from 14 outbreak sites in eastern Africa during the 1989-90 and 1990-91 seasons and correlating this with rainfall prevalences. 3. Extensive intra- and inter-population variation in PRP was observed. As predicted by the model, mean PRP was inversely correlated with the prevalence of rainfall at the outbreak site during the month that the moths initiating the outbreaks were migrating: the longest PRPs were exhibited by moths originating from early season outbreaks in regions of low, variable rainfall and the 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. 4. Males matured consistently earlier than females. 5. PR.P variation cannot be accounted for by any major environmental factor, suggesting that the differences between samples are genetic in origin. This suggestion is strengthened by results from selection and sib-analysis experiments (Wilson & Gatehouse 1992), which demonstrate a substantial additive genetic component to PRP regulation in females, as assumed by the model.