26 Doccola: Stem-Injected Hemlock Against Hemlock Woolly Adelgid in the GRSM al. 2011). Previous work demonstrates that imidaclo- prid is an effective product for managing HWA when used as a soil drench (Cowles et al. 2006; Cowles 2009), but there are per-acre limits to its use for the protection of eastern hemlock. Tree injection is an option to consider when treating hemlock in environ- mentally sensitive areas (e.g., near streams and ponds) and is exempt from per-acre use restrictions. Trees in the Greenbrier area of the Great Smoky Mountains National Park (GRSM), Gatlinburg, TN (UTM E: 281559, N: 3957949), USA were selected for the study. The Greenbrier site is typical of the riparian zone. In an independently evaluated study at the Cornell Botanic Gardens (CBG), IMA-jet (Arborjet, Inc., Woburn, MA, USA) was successfully injected and effective in control of HWA-infested hemlock in Cascadilla Gorge, a riparian zone (Graziano 2010). However, in contrast to the CBG study [located in USDA hardiness zone 5b (−26 to −23 °C)], the Green- brier site is in USDA hardiness zone 7a (−18 to −15 °C). Winter low temperatures and rapid temperature change have effects on the survival of HWA (Costa et al. 2004). We’d expect lower winter sistens mortality in the Southeast, and therefore see more direct effects of imidacloprid applications. This study was designed to consider the utility of tree injection in riparian areas where soil drench cannot be used and to evaluate the comparative effectiveness of two methods of tree injection over a 4-year period. Imidacloprid and its two primary metabolites, 5-hydroxy and olefin, were detected in sap and tissue 15 and 36 months post-treatment, respectively (Coots et al. 2013). In another study, concentrations of imi- dacloprid and olefin were evaluated following a basal drench of imidacloprid in the GRSM, where the con- centrations decreased below the LC50 for HWA in year 5 (Benton et al. 2015). Long-term activity there- fore is attributed to imidacloprid and olefin in hem- lock. In forested settings, HWA suppression has been observed at 120 ppb (0.12 µg/g) 2 years post-treatment (Cowles et al. 2006). In a 3-year study conducted at the Biltmore Estate (Asheville, NC), imidacloprid residues in tree-injected hemlock were detected at 0.2, 1.8, 2.0, and 1.4 µg/g, at 70, 435, 800, and 1165 days, respectively, compared to soil-injected imida- cloprid where residues were 0.14, 0.33, 3.1, and 2.4 for the same periods (Doccola et al. 2012). In that study, trees were in similarly poor condition as in the present study. Normal resumption of growth was not ©2021 International Society of Arboriculture observed in the Biltmore study until 2 years post- treatment, where 72.3% and 41.7% of tip growth was observed in the tree and soil injections, respectively. Although the LC50 was met or exceeded at 70 days, trees recovered slowly. This observation was consis- tent with those reported by Webb et al. (2003), where they found that trees with significant dieback recov- ered only slowly, compared to hemlock that were infested, but in better condition. Untreated hemlock remained sparsely foliated and exhibited dieback. HWA feeds in the xylem ray parenchyma cells (McClure 1991). Parenchyma is vascular tissue that grows as axial and radial tissues as a reticulated net- work within the xylem. It is a component of the sym- plast (or living tissue) and plays a role in growth, metabolism, reproduction, storage, and defense (Shigo 1991).The xylem tissue is apoplastic (nonliving) and functions to move liquid and dissolved solutes; move- ment of liquids is bi-directional and acropetal, with net movement upward. Long-term storage of solutes is associated with the symplast. The initial movement upward into the canopy occurs within the xylem. In deciduous trees, much of the imidacloprid is shed at leaf fall. In a study by Mota-Sanchez et al. (2009) using 14 C-labeled imidacloprid, the researchers found that the imidacloprid accumulated in the foliage during the first season following injection into ash. However, in the second year they found imidacloprid primarily in the trunk tissues. They concluded that the movement of imidacloprid in ash trees was through the xylem. Green ash (Fraxinus pennsylvanica Marsh) and white ash (F. americana L.) shed their leaves annually, and as a consequence, much of the imida- cloprid is lost from the tree. Conifers are comprised of uniseriate parenchyma characterized by a single layer (von Arx et al. 2015). Both parenchyma and foliage are the storage tissues in evergreen conifers, including hemlock. Imidacloprid is likely stored within the xylem axial and radial parenchyma. Dilling et al. (2010) found a concentration gradient within the can- opy that decreased with tree height. This observation may be in part due to tree condition (tree transpira- tion) and to formulation used and method of applica- tion. For example, transpiration from the tree canopy is negatively affected by dieback, a sparse canopy, or in trees in a suppressed condition. These conditions may also result in imidacloprid binding to cellulose, particularly if formulations are already saturated, such as a 10% imidacloprid used in that study. However,
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