The effect of natural enemies on populations of western yellow-striped armyworm (Spodoptera praefica [Grote]) was assessed in hay alfalfa in the Sacramento Valley of California. Analysis of partial age-specific life tables (i.e., egg to pupa) revealed that most of the generation or real mortality of S. praefica occurred during the egg-small-larval age interval. Experimentation revealed that such mortality was largely due to a complex of polyphagous predators. Predators included larvae of Chrysopa carnea Stephens; adults and nymphs of Geocoris pallens Sål, G. punctipes (Say), G. atricolor Montandon, Nabis americoferus Carayon, N. alternatus Parshley, and Orius tristicolor (White); adults of Collops vittatus (Say); and the notorious “pest” species, Lygus hesperus Knight. Parasites and disease had a relatively minor impact on populations of S. praefica. The results provide further support for biological control of insect pests in temporary agroecosystems and illustrate the importance of polyphagous predators in such biological control.