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Title: RECOVERY OF EASTERN HEMLOCK FROM ADELGID ATTACK

Author
item Webb, Ralph
item FRANK, J. - IR-4 PROGRAM
item RAUPP, M. - UNIV. MARYLAND

Submitted to: Journal Of Arboriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2003
Publication Date: N/A
Citation: N/A

Interpretive Summary: Interpretive Summary: Eastern hemlock and Carolina hemlock are valued members of native and urban forest communities in the eastern United States. The hemlock woolly adelgid, an invasive species, is the most important pest of these two species in both natural and managed settings. Since its introduction in the 1920¿s it has spread into at least eleven eastern states. This pest is among the most deadly of any attacking a native tree. Our objectives were twofold. First, we wanted to document the presence or absence of a recuperative response of hemlocks infested by adelgids following the application of imidacloprid insecticide through the soil. Second, we wanted to determine if the initial condition of the hemlock affected its ability to respond to treatment. The importance of these findings is twofold. First, we have confirmed a high level of long lasting control of hemlock woolly adelgid afforded by the systemic insecticide imidacloprid. Second, we have demonstrated that after adelgid populations are suppressed hemlocks will recover by producing new growth on most branches. Even trees that have experienced a cessation of new growth with attendant needle loss and dieback will experience dramatic recovery following the application of imidacloprid. Results should be useful to homeowners, arborists, and all managers of lands possessing hemlocks.

Technical Abstract: Technical Abstract: (Same requirements as Interpretive Summary) Hemlock woolly adelgid, Adelges tsugae, is a very serious pest of hemlocks, Tsuga spp., in the eastern United States. Although a variety of insecticides are capable of controlling hemlock woolly adelgid, the 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 trees in a residential landscape, we found that hemlocks recovered dramatically once the pressure of the adelgids was removed following an application of imidacloprid. More importantly, the response of trees to imidacloprid therapy differed in relation to their condition at the onset of the experiment. Trees with the healthiest canopy improved the least following the reduction in adelgid populations. Trees with little new growth but no dieback recovered the quickest and most dramatically. Trees in the poorest condition at the onset recovered impressively but more slowly. Trees left untreated remained ugly and in poor health. These results confirm the value of imidacloprid in improving the quality of hemlocks under attack by the hemlock woolly adelgid in urban forests.