Bioassay determination of the distribution of imidacloprid in potato plants: implications to resistance development.

Soil-applied imidacloprid exhibits exceptional efficacy as a systemic insecticide against the Colorado potato beetle, Leptinotarsa decemlineata (Say). An uneven distribution of the chemical within potato plants could result in differential concentrations, which may allow for discrimination between genotypes of varying susceptibility. In this study, susceptible and tolerant larvae were fed leaves from the lower, middle, and upper canopy of treated and untreated plants to characterize within-plant distribution of imidacloprid at 4, 6, 8, 10, 12, and 14 wk after planting. Significant differences in larval mortality and development indicated that the concentration of imidacloprid was unevenly distributed in the potato foliage during 6-14 wk after planting. The concentration of imidacloprid was lowest in the younger tissues of the upper leaves and highest in the older, lower leaves. At 6 wk, a time when the postdiapause beetles are colonizing potato fields, the lower concentration in upper leaves was toxic to susceptible larvae but did not kill a substantial portion of the tolerant larvae. Results suggest that higher concentrations of imidacloprid in the lower canopy leaves may act as a toxic barrier to colonizing susceptible beetles but may allow more tolerant individuals to reach the upper canopy with lower concentrations. Possible scenarios of how different concentrations of the systemic insecticide could influence the rate of resistance development are discussed.