The effects of African Army Worms (AAW) in southern Africa can be devastating. When there is an outbreak the pest can destroy 1000s of hectares of grass type crops (including Maize and pastures). Most governments in Africa will purchase and distribute pesticides to try to control the pest, but when conditions are favorable for an outbreak they are very difficult to control. Most yearsworms cause no problems, but when it is a particularly wet rainy season following a drought year the conditions are right for a major infestation.
The AAW moths are just the adult stage of the insect. The moths will migrate to new areas to lay their eggs during large infestations. The damage to the crops comes from the hatched larva that damage the crops.
This is a good reference on AAWs: http://www.infonet-biovision.org/default/ct/95/pests