14 Doccola et al.: Trunk-Injected Imidacloprid for Hemlock Woolly Adelgid (Brodribb and Felid 2000) from needle stomata. Guard cells close in response to atmospheric vapor pressure deficit to maintain needle turgor (Oren et al. 1999; Baker et al. 2001). Injection rate is likewise commensurate with xylem sap flow and dependent on conditions favoring evapotranspiration (Schulze et al. 1985). Temperature plays a significant role, affecting atmospheric vapor deficit and soil moisture avail- ability. Systemic movement of stem-injected dyes in trees occurs when soil temperatures reach a minimum 5°C (41°F) (Tattar and Tattar 1999). These temperatures are more likely to be met in April through November in the northeastern United States (see Figure 1). The environmental conditions that favor injection uptake in hemlock are moderate soil and ambient temperatures, high relative humidity, and available soil moisture. Fall treatments also coordinate with HWA life cycle and susceptibility. HWA is most susceptible to exposure to an injected systemic insecticide when it is actively feeding. HWA breaks its summer aestivation in October in the north- eastern United States and resumes feeding activity. This sis- tens generation will continue to develop through April (Ward et al. 2004). Spring treatments are also possible because the progrediens generation, which occurs May through June, may be targeted (Ward et al. 2004). Fall assessments of HWA were conducted on the current year’s growth and before the onset of very low temperatures to minimize the effects of winter mortality (Costa et al. 2004). MATERIALS AND METHODS The Tree–Device Interface The Tree I.V. and the VIPER method (Arborjet, Inc.) were the devices selected to deliver the treatments (Figure 2). The Tree I.V. uses low pressure (35 PSI) to aid in the delivery of formulation. Low pressure ensures that the contents of the 1-L bottle empty completely and that flow is commensurate with hydraulic movement in the xylem. The VIPER method refers to the injection interface, that is, the Tree I.V. needle and Arborplug (Arborjet, Inc.) (Figure 3). The Arborplug is a plastic device constructed with a guide cap, three anchoring barbs, an internal self-sealing septum, and infusion legs. It is designed to deliver formulation directly into the most current (12 to 15mm[0.48 to 0.6 in]) xylem and to prevent backflow. In this study, #4 Arborplugs (15 mm l × 9 mm w [0.6 in l × 0.36 in w]) were used. A clean, sharp 9 mm (0.36 in) brad point bit was used to drill into the sapwood. Without applying pressure, the drill bit cuts easily through the lower-density phloem tissues; it will not cut into the denser xylem until pressure is applied. Once at the xylem–phloem interface, the bit cuts to the 12 to 15 mm (0.48 to 0.6 in) depth with a quick push in and pull out. The Arborplug is inserted using a set tool and mallet. The Tree I.V. needle, when inserted, pierces the septum to deliver the formulation. The injected liquid moves around the infusion legs and into the vascular tissue. The Method and Rate Determination Eastern hemlock (Tsuga canadensis) selected in this study had high initial HWA pressures on both the treated and con- trol trees. Sixteen HWA-infested hemlocks in Winchester, Massachusetts, were selected at random for two consecutive fall treatments (i.e., 2002 and 2003). IMA-jet insecticide (5% imidacloprid w/w) was applied at the label rates based on tree size class with the Arborjet Tree I.V. and VIPER method. Tree diameter at breast height (dbh, 135 cm [54 in] from soil level) was used as a basis to determine dosage. The tree size classes were 5 to 29, 30 to 59, and 60 to 89 cm (2 to 11.6, 12 to 23.6, and 24 to 35.6 in), and the HWA label rates for each class were 0.8, 1.6, and 2.4 mL per centimeter dbh (2, 4, and Figure 1. Mean monthly temperature in degrees Celsius 2002 through 2005 in Boston, Massachusetts. ©2007 International Society of Arboriculture
January 2007
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