Baseline susceptibility of western corn rootworm (Diabrotica virgifera virgifera) adults to pyrethroid insecticide bifenthrin

The Western Corn Rootworm (WCR), (Coleoptera: Chrysomelidae) is one of the most important corn pests in the US corn belt. The larvae feed on roots causing pruning and lodging of the corn plants. Control techniques have become dependent on the transgenic corn hybrids expressing Cry3Bb1 and Cry34/35 Bt toxins. However, resistance to the former trait was reported in Iowa, in 2011, resulting in increasing use of insecticides. The objective of this research was to establish baseline susceptibility among eight WCR laboratory populations to byfenthrin, in order to compare with WCR field populations from areas where bifenthrin has been used. Adults from this colony were exposed to six bifenthrin concentrations plus control to estimate the lethal concentration that kills 50% of the sample. The insecticide was diluted in acetone, 500 µl of the solution was applied to 20ml glass vials, and vials were rolled on a commercial hotdog roller until dry. Ten WCR adults were transferred to each vial in three replicates, totalizing 30 adults per treatment. Only a two-fold difference among the LC50s from the eight populations was observed. The highest LC50 was 0.195 µg/vial (0.100-0.572, 95% C.I.), compared with the lowest LC50 of 0.101 µg/vial (0.046-0.199, 95% C.I.). These lab populations are believed to have received minimal exposure to pyrethroid insecticides, which allows these results to be used to estimate baseline susceptibility. The implications from this research give an understanding of the current situation of WCR susceptibility to the insecticide bifenthrin, and comparison with field populations are discussed for insecticide resistance monitoring.