Partial life tables for corn earworm (Lepidoptera: Noctuidae) on compact- and loose-panicle sorghum hybrids

Population dynamics were assessed and partial life tables constructed for corn earworm, Helicoverpa zea (Boddie), infesting panicles of two sorghum, Sorghum bicolor (L.) Moench, hybrids in field experiments in central Texas in 1985 and 1986. The hybrids were commercially acceptable types, similar agronomically and developmentally but differing in degree of panicle compactness at maturity. Almost all oviposition on panicles occurred just before and during flowering. Subsequent larval stages were associated with progressive stages of panicle development. Significantly more corn earworm eggs and larvae were recovered from panicles of the compact-panicle hybrid than the loose-panicle hybrid. However, life table construction revealed that the distribution and level of egg and larval mortality were similar for corn earworm on either panicle type. Data from all experiments revealed that real mortality (rx) was > 98% by the end of the last larval stage, and was > 90% during the egg and small larval stages. Parasitism by Trichogramma spp. and failure to hatch caused the greatest amount of egg mortality. Few parasitized larvae were found. Disease-induced larval mortality was low but was highest among large larvae. The greater number of larvae recovered from sorghum with compact panicles than those with loose panicles was a function of the greater number of eggs deposited on those panicles. Significant relationships existed between abundance of larvae (> 24 mm) and that of small and medium larvae. Equations were derived to predict densities of large corn earworm larvae on sorghum panicles based upon densities of earlier corn earworm life stages.