Optimization of soil insecticide rates for managing corn rootworm (Coleoptera: Chrysomelidae) larvae in the north central United States

Optimization of soil-applied granular insecticides for control of northern and western corn rootworms, Diabrotica barberi Smith & Lawrence and D. virgifera virgifera LeConte, respectively, was investigated in 8 states in the north central United States. Differences in success ratios (Yes:No, whether root ratings were maintained below the economic injury level) of corn rootworm insecticide rates were analyzed at state and regional levels. The following insecticide rate comparisons were carried out: 0.5 versus 1X rate and 0.75 versus 1X for chlorpyrifos, fonofos, phorate, tefluthrin, terbufos; and 0.6 versus 1X and 0.8 versus 1X for chlorethoxyfos. Regional data also were analyzed using 3.0 and 3.5 (based on 1 to 6 root rating scale) economic injury levels under low, medium, and high rootworm feeding damage. Individual state data were analyzed under combined feeding pressures (low to high) at both 3.0 and 3.5 economic injury levels. Under high rootworm pressure and using an economic injury level of 3.0, there were no significant success ratio (lowest versus 1X rate) differences at the regional level, regardless of insecticide. Findings within individual states may be more important than regional analyses when interpreting optimum insecticide rate performances. Significant within-state differences between success ratios of low and 1X rates (chlorethoxyfos in South Dakota, chlorpyrifos in Minnesota, fonofos in Iowa, phorate in Iowa and Wisconsin, and terbufos in Iowa), although infrequent, were observed using the more conservative 3.0 economic injury level. No failures were seen using tefluthrin (0.5 versus 1X that was evaluated in only half of the participating states). With the exception of the 0.5 versus 1X fonofos rate comparison in Iowa, all incidences of significant failure were eliminated at the 3.5 economic injury level (lowest versus 1X comparisons). Thus, less-than-labeled rates of these insecticides applied in an 18-cm band can be used to manage rootworm larvae effectively with as much consistency as full (1X) rates.