Arboriculture & Urban Forestry 47(1): January 2021 Benton et al. (2015) did not find significant differences in strata, but treatments in that study were applied by soil drench alone. Xylem cellulose is 44% carbon (Heukelekian and Waksman 1925), constituting an extensive adsorptive surface. The carbon adsorption coefficient of imidacloprid is ~350 (Cox et al. 1997), which suggests moderate adhesion to xylem. Pressure bomb extractions such as were conducted by Dilling et al. (2010) extract sap from the xylem tissues; whereas needle and twig tissue milling (Dilling et al. 2010; Benton et al. 2015) extract from the entire vas- cular tissues (xylem and parenchyma). However, Dil- ling et al. report that sap from needles and twigs (milled) met or exceeded the LC50 for HWA, whereas pressure bomb extraction (sap from xylem) did not. This differential analysis supports symplastic absorp- tion of imidacloprid. The application of imidacloprid in an evergreen is retained over the leaf’s life span and protected from photolytic degradation (Scholz and Reinhard 1999). In hemlock, needles are retained perennially (i.e., from 3 to 6 years)(Ford and Vose 2006), and may play a role in the extended activity reported. METHODS This 4-year study was conducted from June 24, 2010, through May 20, 2014, in the Greenbrier area of the Great Smoky Mountains National Park, Gatlinburg, 27 TN (UTM E: 281559, N: 3957949). Eastern hemlock (Tsuga canadensis) infested with hemlock woolly adel- gid (Adelges tsugae)(HWA) were selected for study. Trees were randomized using 10 polygon locations within the Greenbrier area that: (a) were free of previ- ous HWA chemical or biological treatments, (b) had at least 7 eastern hemlocks within the diameter range for the study (i.e., 20 to 30 cm DBH), and (c) had overall tree health within the normal parameters of HWA treatments at GRSM (a minimum 10% live crown ratio and 70% or less canopy transparency) (Schomaker et al. 2007). For each of the 10 sites, 4 treatment types (3 plus untreated control) were ran- domly assigned to the 7 trees within the polygon. The scarcity of untreated trees healthy enough within the region limited our ability to use a more random study design. The trees ranged in size from 20.6 to 30.5 cm DBH with a mean of 24.7 cm. Tree transparency rat- ing was an average of 58% with live crown canopy rating of 50% (Figure 1). Four treatments were selected to evaluate the effi- cacy of imidacloprid applied to the soil or by tree injection. These included Merit® soil drench (75% imidacloprid WP, Bayer Environmental Science, Clayton, NC) or trunk injection using IMA-jet (5% wt./wt. imidacloprid SL, Arborjet, Inc., Woburn, MA). Tree injections were applied using the Ecoject capsule injector (BioForest Technologies, Inc., Sault Figure 1. Example of canopy transparency in hemlock reflects the poor growth and thinned foliage in 2011 (left), a result of HWA infes- tation. Overall, trees in the study were in such poor condition that no foliage samples were taken until 2012, two years after treatment. Photo (right) was taken of the same hemlock in 2013 showing increased canopy density and improved growth following tree injection with imidacloprid. ©2021 International Society of Arboriculture
January 2021
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