The bionomics of the African armyworm Spodoptera exempta in relation to its status as a migrant pest.
A multidisciplinary study was undertaken on the bionomics of Spodoptera exempta in eastern Africa as a basis for the development of control strategies for this important pest of cereal crops and pastures. Relevant findings from studies on seasonal distribution, migration, field infestations and population cycles are reviewed in relation to understanding the causes for the onset and spread of outbreaks. It is concluded that the onset of the first outbreaks of an epidemic is caused by oviposition at high density by S. exempta concentrated by wind convergence at storm outflows. The sources of these insects seem to be low-density populations which survive from one season to the next at sites receiving unseasonable rainfall. Some areas in Tanzania and Kenya are particularly prone to early outbreaks which are potentially critical for the initiation of a subsequent spread of outbreaks downwind throughout eastern Africa. These areas have low and erratic rainfall, and are near the first rising land inland from the coast. Below average rainfall prior to the development of outbreaks increases the probability of their occurrence. Their subsequent spread is enhanced by storms downwind which concentrate insects in flight and by sunshine during larval development. Persistent wet weather reduces the spread of outbreaks.