Effect of Insecticide Treatments on Root Damage Ratings of Maize in Controlled Infestations of Western Corn Rootworms (Coleoptera: Chrysomelidae)

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Abstract
During a 5-yr study, recommended rates of soil insecticides were applied at planting time to plots of maize, Zea mays L., located on uniform soil. Plots were infested with known populations of eggs of western corn rootworm, Diabrotica virgifera virgifera LeConte, to determine the degree to which insecticides protected roots from larval attack. Moisture in the upper 10 cm of soil and precipitation were recorded each year. In the last two years of the study, we used laboratory soil bioassays to establish larvicidal activity for each insecticide application. Our results indicated that larval feeding damage for each egg density was consistent in the untreated plots every year except 1984, when excessive rainfall that coincided with egg hatch apparently prevented larval establishment. Root protection provided by insecticide treatments was highly variable. Significant interactions that occurred between root worm infestation rate, year, and insecticide treatments suggested that the level of root protection provided by insecticides was dependent on year (edaphic and environmental factors) and pest population density. The laboratory bioassays indicated dramatic differences in persistence of some insecticide treatments applied when maize was planted.