Environmental forcing and high amplitude fluctuations in the population dynamics of the tropical butterfly Acraea acerata (Lepidoptera: Nymphalidae)

1. The population dynamics of the sweet potato butterfly, A craea acerata, were studied for 3 years in sweet potato fields in southern Ethiopia in an area where farmers practice successive plantings throughout the year and do not use pesticides. Densities of eggs, young and old larvae, pupae and adults were estimated for 18 or more generations. 2. The insect breeds during all seasons with about six generation cycles per year. Superimposed on the generation cycles are longer-term intergeneration fluctuations with amplitudes of three to four orders of magnitude. Population decline and build-up are gradual processes taking place over several generations. 3. We used time-series techniques to analyse the patterns of density-dependent stage-specific survival and reproduction, and the influence of enemies and weather fluctuations on A. acerata intergeneration dynamics. These techniques allow statistically valid inference about insect demography. 4. The analyses revealed strong density-dependent effects on recruitment, due possibly to the combined effects of reductions in adult female size and density-dependent emigration. 5. Weather conditions affected dynamics in two opposing ways. Rainy conditions increased recruitment. In contrast, heavy rains decreased the survival of young larvae and pupae, while very dry conditions also decreased their survival, leading to a non-linear effect of rainfall on survival of immature stages. 6. Enemies appeared to have little effect on the population dynamics of A. acerata. 7. The net consequences of the sensitivity of A. acerata to seasonal fluctuations in rainfall are high-amplitude multigeneration fluctuations in abundance. This case study demonstrates that high-amplitude fluctuations in abundance may be driven by weather, rather than by predator-prey interactions.