To refine estimates of economic thresholds of the armyworm (AW), Pseudaletia unipuncta (Haworth), on conventionally tilled field corn, field plots in a split-plot design were manually infested with armyworm larvae of varying ages (instars 4, 5, and 6) and densities (1, 2, and 3 larvae/plant). During larval infestation, field-collected plants revealed significant differences in defoliation between plants infested with AW larvae and uninfested corn. In 1982, differences were also noted between defoliation produced by AW larvae of different instars and defoliation produced by larvae of different densities. In 1983, however, high temperatures ((GREATERTHEQ) 31(DEGREES)C) had a depressing effect on AW larval feeding behavior; therefore, all AW instars and densities consumed significantly less corn foliage than in 1982. Mean leaf areas of corn leaves on plants defoliated by AW larvae during earlier corn developmental stages varied considerably from nondefoliated plants in 1982. This variability was attributable to random sampling of small plants located in low, poorly drained areas within the field. In 1983, however, most mean leaf areas of plants infested with AW larvae were similar to uninfested mean leaf areas;In 1982, yields of plants infested with AW larvae and uninfested corn were similar. In 1983, however, significant differences in yield occurred between plants infested with fourth instar AW and plants infested with fifth or sixth instar. Differences in leaf defoliation and plant yield that occurred within and between years were attributable to temperature, precipitation, and larval parasitization differences. These results downgrade the importance of larval AW attack on conventionally tilled corn in midwestern corn fields.