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 nort hern and western corn rootworms, Diabrotica barberi Smith & Lawrence a nd D, virgifera virgifera LeConte, respectively, was investigated in 8 states in the north central United States. Differences in success rat ios (Yes:No, whether root ratings were maintained below the economic i njury 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, f onofos, phorate, tefluthrin, terbufos; and 0.6 versus 1X and 0.8 versu s 1X for chlorethoxyfos. Regional data also were analyzed using 3.0 an d 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 us ing an economic injury level of 3.0, there were no significant success ratio (lowest versus 1X rate) differences at the regional level, rega rdless of insecticide. Findings within individual states may be more i mportant than regional analyses when interpreting optimum insecticide rate performances. Significant within-state differences between succes s ratios of low and 1X rates (chlorethoxyfos in South Dakota, chlorpyr ifos 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 te fluthrin (0.5 versus 1X that was evaluated in only half of the partici pating states). With the exception of the 0.5 versus 1X fonofos rate c omparison in Iowa, all incidences of significant failure were eliminat ed at the 3.5 economic injury level (lowest versus 1X comparisons). Th us, less-than-labeled rates of these insecticides applied in an 18-cm band can be used to manage rootworm larvae effectively with as much co nsistency as full (1X) rates.