Defoliation by the Armyworm (Lepidoptera: Noctuidae) on Field Corn in Iowa1

Abstract

To determine the importance of herbivory and subsequent effect on grain yield from armyworm (AW), Pseudaletia unipuncta (Haworth), infestations on conventionally tilled field corn, field plots in a split plot design were manually infested with larvae of various ages (instars 4, 5, and 6) and densities (one, two, and three larvae per plant). In 1982, significant differences in defoliation were observed on plants infested with AW larvae compared with un infested (nondefoliated) corn. In 1982, differences were also noted between defoliation produced by AW larvae of different instars and defoliation produced by various densities of AW larvae. In 1983, however, high temperatures (31°C) had a depressing effect on AW survival, larval feeding behavior, or both. Therefore, all AW instars and densities consumed significantly less corn foliage in 1983 than in 1982. In 1982, yields of plants infested with AW larvae and un infested corn were similar. In 1983, however, significant differences in yield were obtained on plants infested with AW fourth instars compared with plants infested with fifth or sixth instars. Differences in leaf defoliation and plant yield that occurred within and between years were attributable to temperature and precipitation. These results indicate that larval AW attack has little influence on yield of conventionally tilled corn in midwestern cornfields.