RESPONSE OF WESTERN CORN-ROOTWORM INFESTED CORN TO NITROGEN-FERTILIZATION AND PLANT-DENSITY

Growth response of corn (Zea mays L.) to corn rootworm (Diabrotica spp.) infestation is poorly understood and may be influenced by management practices and environmental conditions. The objectives of this 2-yr experiment were to determine the effect of three N rates (0, 168, and 336 kg ha-1) and three plant density treatments (39 000 [1984 only], 63 000, and 87 000 plants ha-1 on dry-matter accumulation and partitioning of corn plants infested with three levels of western corn rootworm (WCR, Diabrotica virgifera virgifera LeConte) [0, 1967, and 3934 eggs m-1 row]. Plant dry weight, taken periodically and subdivided into vegetative and reproductive fractions, was used to calculate the harvest index and mean relative growth rates (R(t)). In 1985, leaf area was measured and leaf area index (LAI), mean net assimilation rate (NAR), and leaf area ratio (LAR) were calculated. In 1985, a dry year, rootworm-injured plants had significantly reduced dry weight, leaf area, harvest index, R(t), and NAR. Dry-matter accumulation of injured plants was greater in low plant-density and applied-N treatments. In comparison with parameters of healthy plants, dry weight, R(t), and NAR of injured plants were reduced in high-N treatments at the time of rootworm feeding. This significant rootworm x N interaction did not occur after feeding ceased. Rootworm infestation significantly reduced the harvest index only in 1985, suggesting that dry conditions increase the impact of root injury on ear development and yields.