COMPARISON OF ECONOMIC INJURY LEVELS FOR WESTERN CORN-ROOTWORM (COLEOPTERA, CHRYSOMELIDAE) INFESTING SILAGE AND GRAIN CORN

The effect of known densities of western corn rootworms, Diabrotica virgifera virgifera LeConte, on yield and monetary losses was compared for corn harvested as silage and as grain. In 1991 and 1992, plots were infested artificially with western corn rootworm eggs at densities ranging from 0 to 1,500 viable eggs per 30.5 cm of row. Level of rootworm-feeding did not affect silage quality, as measured by percentage crude protein, acid detergent fiber, neutral detergent fiber, and nonstructural carbohydrates. Western com rootworm caused greater yield reductions and dollar losses in com grown for silage than for grain. Economic injury levels (EILs) were calculated for various combinations of crop use, yield potential, and crop value typical of com production in the Northeast. Assuming an insecticide cost of $39.50 per ha, EILs for silage ranged from 71-127 viable eggs per 30.5 cm of row, corresponding to root ratings ranging from 2.5-2.9. In contrast, EILs were much higher for com harvested for grain, ranging from 242-537 viable eggs per 30.5 cm of row, corresponding to root ratings ranging from 3.5-4.6.