Planting time application of soil insecticides and western corn rootworm (Coleoptera: Chrysomelidae) emergence: implications for long-term management programs

A 3-yr study assessed the influence of three commonly used planting-time soil insecticides (carbofuran, chlorpyrifos, and terbufos) on emergence of western corn rootworm, Diabrotica virgifera virgifera LeConte, from different tillage systems located near Champaign (1983 and 1984) and Monmouth (1985), Ill. The potential interaction of insecticides and tillage systems was not significant in any of the 3 yr with regard to beetle emergence. In 1983, significantly fewer (P > F = 0.0107) western corn rootworm beetles emerged from control plots than from insecticide-treated plots. Conversely, beetle emergence in 1984 was greater from the control plots than from the insecticide-treated plots (P > F = 0.0001). In 1985, the main effect of insecticide application on beetle emergence was not significant. Total precipitation during the peak larval feeding period in June was distinctly different each year of the study (1983, 23.2 cm; 1984, 3.1 cm; 1985, 8.0 cm) and may have influenced the dynamics of soil insecticide and beetle emergence interactions. In 1983 and 1984, beetle emergence among the insecticide-treated plots was greatest in plots treated with carbofuran, the most water-soluble of the compounds examined. The effect of tillage on root damage was not significant; whereas, the insecticides provided satisfactory root protection compared with the control. Results suggest that producers who use planting-time soil insecticides to protect corn roots from damage are not actually managing rootworm populations (i.e., reducing the population level) in some seasons. Although applying an insecticide in a band is intended only to protect the root system, greater beetle emergence from insecticide-treated areas in some years implies that the long-term severity of western corn rootworm infestations may be exacerbated.