A multidisciplinary study was undertaken on the bionomics of the African armyworm 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 armyworm outbreaks. It is concluded that the onset of the first outbreaks of an armyworm epidemic is caused by oviposition at high density by moths concentrated by wind convergence at storm outflows. The sources of these moths 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 moths in flight and by sunshine during caterpillar development. Persistent wet weather reduces the spread of outbreaks. This study demonstrates the importance and value of developing and implementing a long-term monitoring system over a large region, in this case in Africa, not only for the generation of data used by the forecasters for the prediction and location of possible outbreaks, but also for obtaining a clearer understanding of the epidemiology of a highly mobile pest.

1. About 166,OOO African armyworm moths, Spodopteru exemptu (Walk.), were marked at an emergence site near Nairobi when they fed at night on trees baited with dyed molasses.2. Six marked moths were captured in pheromone traps, including one at 90 km after flying for only one night, and another at 147 km.3. Moth flight trajectories deduced from radar and from marking showed that migration was downwind.4. During migration, moths become dispersed; hence the high densities that lead to outbreaks must be produced by concentration.5. Some moths were ready to mate on the same night they completed their long-distance flight.

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