Background: Imidacloprid is the primary insecticide for controlling the tobacco-adapted form of the green peach aphid (TGPA), Myzus persicae (Sulzer), a major pest of tobacco worldwide. This study used leaf-dip bioassays to assess TGPA resistance to imidacloprid in the eastern United States from 2004 through 2007.
Results: When combined over the 4 year study, 18, 14 and 3% of the TGPA had imidacloprid resistance ratios (RRs) of 10-20-fold, 20-30-fold and 30-90-fold, respectively, compared with the most susceptible colony tested. This indicates that some colonies have developed moderate levels of resistance to imidacloprid. A colony collected near Clayton, North Carolina, had the highest RR of 91 (LC(50) value = 31 mg L(-1)). This resistance declined for six tests over a 3 year period in the laboratory culture from >130-fold RR (LC(50) = 48 mg L(-1)) to 40-fold RR (LC(50) = 15 mg L(-1)). Over the same period, the most susceptible colony and a standard colony not exposed to imidacloprid for over 7 years had consistently low LC(50) values.
Conclusion: Moderate levels of resistance to imidacloprid are noticed among TGPA colonies from the eastern United States. The variation in resistance indicates that the factors responsible are present in the populations at low frequencies and are just not enough to cause field failures yet.