Residual activity of imidacloprid and its metabolites in eastern hemlock multiple years after treatment

Hemlock woolly adelgid (HWA), Adelges tsugae (Annand), has devastated eastern hemlock populations in the eastern United States.  In the last ten years the front of adelgid infestation has progressed through the Great Smoky Mountains National Park (GRSM).  To combat the threat to the Park’s hemlock resources, park personnel have applied systemic imidacloprid treatments.   The effectiveness of low dosage treatments of imidacloprid on HWA populations was examined four, five, and six years after application in four different size classes of hemlock trees.  Branchlet samples were taken in three strata of the canopy for each size class.  Concentrations of imidacloprid and three metabolites (5-hydroxy, 2-di-hydroxy, and olefin) were determined using liquid chromatography-mass spectrometry.  Olefin is of particular interest because it is ten times more toxic to insects than imidacloprid.  A significant negative relationship between the concentrations of imidacloprid and olefin and the HWA population on branchlet samples was documented.  Trends in the data indicate olefin present in the foliage at concentrations exceeding lethal doses for HWA by over two-fold with an abundance of imidacloprid still within foliage yet to be metabolized into olefin.  This trend was observed in all sizes and within each canopy stratum.   These results indicate that low dosage imidacloprid treatments are effective to suppress HWA populations for extended periods of time, enabling GRSM to maintain effective HWA control while treating hemlocks with imidacloprid less often.  Knowledge of the longevity of imidacloprid treatments is important for enhancing the HWA control program in GRSM.