Epidemiology of the African armyworm (Spodoptera exempta) : intra- and interseasonal variability in outbreak severity in Eastern Africa in relation to weather and moth migration
The work described in this thesis aims to improve understanding of the environmental factors affecting the epidemiology of the migratory noctuid moth pest the African armyworm, Spodoptera exempta (Walker). The role of weather as a factor causing differences in frequency and location of armyworm outbreaks between seasons is particularly examined. The work is put in context by a review of literature on insect (particulary moth) migration and of the weather and climate of eastern Africa. Armyworm reports and weather records for eastern Africa are analysed for armyworm seasons from 1972-88 and, for a limited dataset, for the beginning of the 1992-93 season. Outbreaks are classified into those derived from low-density populations (primaries) and those from previous outbreaks (secondaries). Moth migrations between outbreaks are estimated from synoptic windfields using trajectory analysis. The association between outbreaks and rainstorms within a season and the relationship between seasonal variations in outbreak frequency and rainfall are investigated. The use of satellite data for identifying rainstorms is described. The results support the hypothesis that outbreaks at the beginning of the season are derived from low-density populations and mostly occur in identifiable 'primary outbreak areas' but that most later outbreaks are secondaries. Severe outbreaks often'follow dry periods and persistent rainfall from October to December is associated with low armyworm seasons. Individual outbreaks are associated with the edge of rainstorms supporting the hypothesis that migrating moths are concentrated by rainstorm outflows before outbreak formation. The frequency of particular long-distance moth migrations is discussed in relation to hypotheses about the change in migratory potential of armyworm during the season and the reason for the apparent lack of return migration.