EFFECT OF IRRIGATION ON ROOT-GROWTH AND YIELD OF PLANTS DAMAGED BY WESTERN CORN-ROOTWORM LARVAE

Corn rootworm (Diabrotica sp.) larval feeding damage inhibits the ability of root systems to provide water to shoots. Irrigation may reduce these plant water deficits and subsequent yield loss. To investigate this hypothesis, a 2-year field study was conducted to measure root system characteristics and yield under irrigated and dryland conditions in fields artificially infested with western corn rootworm (D. virgifera virgifera Le Conte) eggs. Eggs were applied at 0, 1650, 3300, and 6600 viable eggs per meter of row. Survival of insects to adult stage, root damage ratings, nodal root volume (Nodes 4 and above) at the time of maximum insect damage, and grain yield were measured. In 1988, which was characterized by above normal air temperature and below normal precipitation, plants grown with irrigation had larger nodal root systems and also had higher grain yield than plants grown without irrigation. The reduction in yield as caused by equal rootworm infestation levels was less under irrigated conditions than under dryland during this growing season. In 1989, which had near-normal air temperature but below normal precipitation during the growing season, irrigation had no effect on root system characteristics or yield, and plants suffered no yield loss due to larval feeding. However, yield of plants infested with 1650 eggs per meter was significantly greater than plants that were uninfested or infested with 3300 and 6600 eggs per meter of row. We concluded that irrigation ameliorated yield loss to rootworms when plants were grown under hot and dry conditions. These results are discussed in terms of adjusting economic injury levels for rootworms depending on whether or not the field can be irrigated.