Root Protection and Reduced Rates of Soil Insecticides: Results from Illinois On-Farm Studies and University Experiments
Entomologists at several Midwestern universities have evaluated the root protection afforded by using less than labeled application rates of soil insecticides for corn rootworms since the early 1970s (Gray et. al. 1990). Most soil insecticides registered for corn rootworm larval control are labeled to be applied at 1.0 pound of actual insecticide per acre. This rate was established with little regard for the ability of currently grown hybrids to compensate for corn rootworm injury (Steffey et al. 1989). In addition, the 1.0 pound rate was intended to provide root protection at or below a root rating of 3 (several roots eaten off to within 1 112 inches ofplant) on the Iowa 1 to 6 scale (Hills and Peters 1971). Some entomologists currently believe the economic injury level of 3 is too low and that a root rating of 4 (1 node of roots completely destroyed) might be more realistic (Sutter et al. 1990). Achieving a root damage rating of 3 or below may not be worth the environmental and economic costs associated with keeping insecticide application rates at the 1.0 pound level.