Date of this Version
Topical bioassays were conducted in 1995 to estimate the susceptibility of adult western corn rootworm, Diabrotica virgifera virgifera LeConte, populations from Nebraska to technical grade methyl parathion, carbaryl, and bifenthrin. Significant differences in susceptibility occurred among populations for each insecticide. The largest relative differences in LD50 values between the most tolerant and susceptible field populations were 16.4- and 9.4-fold for methyl parathion and carbaryl, respectively. The F1 colonies also exhibited significant differences in susceptibility to methyl parathion and carbaryl indicating that susceptibility traits are heritable. The response to bifenthrin (up to 4-fold difference in LD50 values) was more homogeneous across populations than the response to the other 2 compounds. Populations with the largest LD50 values were located in 2 areas where adult management programs using carbamate and organophosphate insecticides have been extensively applied and control failures have been commonly reported. Results strongly suggest that corn rootworm management practices have selected for significant levels of resistance to methyl parathion, carbaryl, or both in certain areas of Nebraska.