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Baseline carbaryl susceptibility and potential resistance mechanisms were identified in U.S. populations of western corn rootworm, Diabrotica virgifera virgifera LeConte, using lethal concentration, diagnostic concentration, and synergism bioassays. Twenty-two adult rootworm populations were examined from the states of Indiana, Illinois, Iowa, Kansas, Nebraska, Ohio, Pennsylvania, South Dakota, Texas, and Virginia. Lethal concentration bioassays, conducted within carbaryl-coated glass vials on 12 populations, identified resistance in only 2 Nebraska populations. A diagnostic concentration of carbaryl was subsequently identified which caused 100 and 40% mortality in standard susceptible and known resistant populations, respectively. In diagnostic concentration bioassays on all 22 populations, substantial decreases in mortality were observed only in 3 populations from outside of Nebraska. Synergism bioassays on standard susceptible and resistant Nebraska populations indicated significant reductions in resistance levels with inhibitors of both cytochrome P450 monooxygenases and esterases, suggesting an involvement in resistance by both these classes of detoxification enzymes. Results are discussed with respect to the validation of diagnostic concentration bioassays as a resistance monitoring tool for existing corn rootworm areawide management programs, and the potential roles of cytochromes P450 and esterases in carbaryl resistance.