Improvements in management of corn rootworms (Coleoptera: Chrysomelidae)
Is Version Of
Northern corn rootworms, Diabrotica barberi Smith and Lawrence, and western corn rootworms, D. virgifera virgifera LeConte, are perennial insect pests of corn, Zea mays L., grown in the same field for successive years. The primary control tactics used against corn rootworms in Iowa are soil insecticides and crop rotation. However, in some regions of the Corn Belt, over-reliance on both tactics has resulted in control failures, indicating that alternative strategies need to be incorporated for sustainable management of these pests. The first objective was to determine if row spacing and plant population of corn influenced corn rootworm survival, larval injury, and plant tolerance to the injury. Although beetle survival was greater in 38-cm compared to 76-cm rows, the increase did not translate into greater root injury. There were no differences in tolerance to corn rootworm injury between row spacings. However, tolerance was suppressed at the high plant population. Growers who reduce row spacing from 76 cm to 38 cm to maximize grain yield should not increase the potential for corn rootworm injury. The second objective was to develop and validate models that accurately predicted adult corn rootworm emergence in Iowa. The models predicted adult emergence from the first day a beetle emerged in a field (biofix). Degree-days post-biofix explained 85% of the variability in total corn rootworm emergence over five years, and the model explained 89% of the variability in emergence observed in the validation year. The PheroconRTM CRW Trap (Trece, Inc. Salinas, CA 93907) was highly efficient at determining the biofix. The third objective was to use female ovarian development to determine if scouting could be made more efficient. Based on emergence, population estimates taken on plants, and ovarian development, the period between 120 and 400 degree-days post-biofix was the optimum time for scouting corn rootworm adults in Iowa. This represents approximately one third of the total time beetles could be present in the field. Scouting should be more efficient because grower can focus on a key period, rather than over the entire growing season.