CORN-ROOTWORM (COLEOPTERA, CHRYSOMELIDAE) LARVAL INJURY AND ROOT COMPENSATION OF 12 MAIZE HYBRIDS - AN ASSESSMENT OF THE ECONOMIC INJURY INDEX

A 4-yr field investigation (1993-1996) to examine the compensatory roo t regrowth of 12 commonly grown maize hybrids after larval injury by c orn rootworms, Diabrotica spp., was conducted at 2 locations in Illino is. Root injury ratings, root volume measurements taken in July and Au gust, and root regrowth parameters were evaluated for their usefulness in predicting yield. Root ratings were as useful as root volumes and root regrowth measurements in predicting yield. Large root systems in July and August generally were positive factors contributing to yield; however, compensatory root regrowth, particularly when soil moisture was adequate, negatively affected yield. Root regrowth after larval in jury typically had a positive effect on yield when soil moisture was i nadequate. Regression equations described the very dynamic nature of r oot injury, root volume, and root regrowth and their impact on yield i n different growing seasons and at different locations. In addition, p rofit margins were estimated using a fixed insecticide cost, actual ro ot injury data, and 4 market prices of maize. Results from different g rowing seasons and locations indicate that root ratings well below 4.0 can contribute to economic losses.