Eastern Hemlock Recovery from Hemlock Wooly Adelgid Damage following Imidacloprid Therapy

Abstract

Hemlock woolly adelgid (Adelges tsugae) is a serious pest of hemlocks (Tsuga spp.) in the eastern United States. A variety of insecticides are capable of controlling hemlock woolly adelgid. The use of a systemic insecticide, imidacloprid, has gained widespread acceptance and use in the plant care industry. While several studies demonstrate the efficacy of imidacloprid in reducing adelgid populations, none have examined how hemlocks recover following imidacloprid therapy. Using specimen Tsuga canadensis trees in a residential landscape, we found that hemlocks recovered dramatically with new growth once the pressure of the adelgids was reduced following an application of imidacloprid. Most important, the response of trees to imidacloprid therapy differed in relation to their condition at the onset of the experiment. Trees with the healthiest, most foliated canopy improved the least following the reduction in adelgid populations. Trees with little new growth but no dieback recovered the quickest and most densely. Trees in the poorest condition at the onset recovered impressively but more slowly. Trees left untreated remained sparsely foliated, with dieback. These results confirm the value of imidacloprid therapy in improving the quality of hemlocks under attack by the hemlock woolly adelgid in urban forests.