Adult activity and reproductive condition of the green cloverworm (GCW) were monitored in soybeans during 2 outbreak years (1978 and 1979) and 2 endemic years (1980 and 1981). Although four adult flights may occur in any year, adult phenology was substantially different between the two types of years. Furthermore, flights 1 and 4 may be too small to be detected in endemic and outbreak years, respectively. Regardless of the type of year, no females captured during flights 1 and 2 were unmated and preovipositional. It is suggested that both flights are composed of immigrating adults and that flight 1 moths do not directly give rise to flight 2 moths in Iowa. Adult activity also was monitored during 1979 and 1980 in alfalfa, which is the primary alternate host of GCW in central Iowa. The same flights were observed in both crops, but relative abundance and the proportion of unmated females were lower in alfalfa than in soybeans. These results imply that survivorship and adult emergence are lower in alfalfa. The most likely cause of reduced adult emergence in alfalfa probably is the frequent cutting of the crop for hay production. Consequently, alfalfa fields probably serve as population sinks for GCW, with populations in alfalfa contributing relatively little to populations in soybeans. These results are combined into a hypothesis to explain GCW population dynamics in central Iowa.