Effect of Insecticide Treatments on Root Lodging and Yields of Maize in Controlled Infestations of Western Corn Rootworms (Coleoptera: Chrysomelidae)

Granular soil insecticides were applied at planting time to plots of maize (Zea Mays L.) infested with known populations of eggs of western corn rootworm, Diabrotica vtrgifera vtrgifera LeConte, to determine how treatments protected plants from root lodging and yield loss caused by larval feeding. The percentage of lodged plants increased significantly with increases in egg density. Percentage lodging in untreated plots also differed significantly between years, and there was a significant year-by-egg density interaction. Insecticides significantly reduced root lodging, but lodging in carbofuran-treated plots was greater than in all other treatments. Percentage yield loss caused by larval feeding was consistent each year for each egg density, and yield loss was significantly greater in plots infested with higher egg densities. Terbufos and isofenphos were not consistent in preventing yield loss; these insecticides caused a significant year-by-treatment interaction. As main effects, yield protection by insecticides was consistent each year, and the insecticides did not differ in their ability to protect yield. Yields in treated plots infested with 300 and 600 eggs per 0.3 m of row did not differ significantly from untreated plots; however, yields in treated plots infested with 1,200 and 2,400 eggs per 0.3 m of row were significantly higher when insecticides were used. Correlations between root damage ratings and yields of untreated plants were highly significant. For insecticide-treated plots, root damage ratings were not significantly correlated with yield, which suggests that root damage ratings are poor criteria for evaluating insecticide efficacy.