Relationship of Root Ratings, Root Size, and Root Regrowth to Yield of Corn Injured by Western Corn Rootworm (Coleoptera: Chrysomelidae)

Abstract

Root damage ratings are a major method of assessing larval injury to corn roots by corn rootworms. This 2-yr study was conducted to examine the relationships between root ratings, root size, root regrowth, and grain yield. Treatments included several nitrogen, plant density, and western corn rootworm infestation levels. Root ratings were not consistent predictors of yield over both years of this study. Yields of plants having root injury were highly variable when agronomic and environmental conditions were favorable for plant growth. Root biomass was a more consistent predictor of grain yield with R2 of 0.52 and 0.61 in 1984 and 1985, respectively. Root growth (regrowth in infested plants) was quantified in 1985 by subtracting root weights in early July from average weights sampled after regrowth had occurred. During this time period, root growth of noninfested plants was not statistically different than regrowth of infested plants. The interaction between infestation and nitrogen was significant, which indicated that regrowth of damaged roots increased greatly with the application of nitrogen. Regrowth of injured roots also was enhanced in the moderate plant density (63,000 plants per ha) treatment. Grain yields were related to regrowth of injured plants (R2 of 0.65 and 0.60 at the moderate and high infestation levels, respectively). These results suggest that an index for regrowth should be used when relating root damage to grain yield.