ASSESSING THE RISK OF RESISTANCE EVOLUTION, ADULT AND LARVAL SUSCEPTIBILITY, AND SUBLETHAL EFFECTS AFTER EXPOSURE OF CORN ROOTWORMS TO VACUOLAR ATPase-A AND Snf7 dsRNAS

Pyrethroid insecticides have been used to control larvae or adults of the westerncorn rootworm (WCR), Diabrotica virgifera virgifera LeConte, a key pest of field corn inthe United States. In response to reports of reduced efficacy of pyrethroids in WCRmanagement programs in southwestern areas of Nebraska and Kansas the presentresearch was designed to establish a baseline of susceptibility to the pyrethroidinsecticide, bifenthrin, using susceptible laboratory populations and to compare thisbaseline with susceptibility of field populations. Concentration-response bioassays wereperformed to estimate the baseline susceptibility. From the baseline data, a diagnosticconcentration (LC99) was determined and used to test adults of both laboratory and fieldpopulations. Larval susceptibility was also tested using both laboratory and fieldpopulations. Significant differences were recorded in adult and larval susceptibilityamong WCR field and laboratory populations. The highest LC50 for WCR adults wasobserved in populations from Keith 2 and Chase Counties, NE, with LC50s of 2.2 and1.38 μg/vial, respectively, and Finney County 1, KS, with 1.43 μg/vial, as compared to alaboratory non-diapause population (0.24 μg/vial). For larvae, significant differencesbetween WCR field and laboratory populations were also recorded. Significantdifferences in mortalities at the diagnostic bifenthrin concentration (LC99) were observedamong WCR adult populations with western Corn Belt populations exhibiting lowersusceptibility to bifenthrin, especially in southwestern Nebraska and southwesternKansas. This study provides evidence that resistance to bifenthrin is evolving in fieldpopulations that have been exposed for multiple years to pyrethroid insecticides.Implications to sustainable rootworm management are discussed.