Quantification of the insecticide imidacloprid in pecan trees using a competitive ELISA technique

Soil applications of imidacloprid are widely used in western pecan growing regions for controlling black-margined pecan aphid (BMA), Monellia cayella (Fitch). This study seeks to investigate the behavior of imidacloprid in pecans with respect to: optimal soil application method; temporal and spatial distribution within the tree; and possible correlation between imidacloprid titers in xylem fluid and aphid mortality. In August 2007 imidacloprid was applied to a commercial pecan orchard using three soil application methods. A commercially available ELISA kit was selected for quantifying imidacloprid in pecan xylem fluid. It was determined that a 10-fold water dilution of pecan xylem fluid was required to minimize interference between xylem fluid constituents and ELISA analysis. A pressure bomb was used to extract xylem fluid from experimental trees. Following one irrigation imidacloprid concentrations in trees treated with a four shank application method were 15.2 ppb, with single shank shallow and single shank deep application methods resulting in concentrations of 6.5 and 8.3 ppb, respectively. After the second irrigation and through October 26, 2007 all application methods resulted in imidacloprid concentrations ranging between 7.6 and 19.5 ppb. There were no significant differences in imidacloprid concentrations between leaves collected from the upper and lower canopies, irrespective of application method. No significant difference in BMA density was detected between untreated and treated trees. In the following year, initial spring samples resulted in imidacloprid concentrations varying from 4.5 to 6.5 ppb.