42 T. canadensis, the LC95 respectively. Doccola et al.: Imidacloprid Treatments for HWA Interestingly occurred in four weeks and 12 weeks, in P. strobus, soil applications reached lethal concentrations in 12 weeks, but tree injected imidacloprid did not reach the LC95 for five months. Dilling al. 2006). Trunk injected imidacloprid translocated relatively quickly into the foliage of Ulmus spp. and reduced defoliation by elm leaf beetle, Xanthogalleruca luteola (ELB) (Lawson and Dahlten 2003). The authors suggested that tree injection treatments after sampling egg density offered an advantage over soil-applied treatments that required application in the winter months (presumably when moisture is more available in the area of Sacramento, California, U.S.) for absorption and translocation. Bioassays showed imidacloprid toxicity to ELB larvae at 33 days but not at one year after treatment. Poland et al. (2006) reported rapid translocation of imidacloprid in three host species (Populus nigra, Salix matsudana, and Ul- mus pumila) to Asian longhorned beetle, Anoplophora gla- bripennis, with significant mortality up to nine months after treatment. Leaf and twig residues of imidacloprid ranged from 0.27 to 0.46 ppm. Studies in green ash (Fraxinus pennsylva- nica Marsh) and white ash (F. americana L.) have reported that imidacloprid accumulates in the canopy from the point of injection and is lost in leaf fall, but suggest that stem injec- tion could provide a reservoir for continued systemic activity (Cregg et al. 2005; Tanis et al. 2006; Tanis et al. 2007; Tanis et al. 2009). Imidacloprid residual activity in hemlock has been investigated and reported (Cowles et al. 2006; Doccola et al. 2007). Cowles et al. (2006) reports that a single soil applica- tion of imidacloprid suppresses HWA for >2 years. Doccola et al. (2007) reported HWA control following imidacloprid tree injection for two years; however, residues were not conducted in that study. The differences in the time injected hardwoods and conifers accumulate imidacloprid (e.g., Tattar et al. 1998) is consistent with Hagen-Poiseville law which describes the rate of flow as a function of the xylem radius to the fourth pow- er (Kramer et al. 1996). Therefore, hardwoods that have wide vessels (e.g., oaks) move liquid at a faster rate than conifers, which move liquids solely by comparatively narrow tracheids (e.g., hemlocks, pines) (Esau 1977). Even so, there must be an alternative explanation for the slow movement of imidaclo- prid observed in conifers. The physical chemistry of imida- cloprid (e.g., water solubility, affinity to organic carbon) may play a role in its differential movement upward in trees. How- ever, such an investigation is beyond the scope of this paper. This study focused on imidacloprid treatment efficacy in the southern part of the hemlock range, where HWA has been dev- astating. Imidacloprid, a neonicotinoid insecticide, is labeled for use and studies have established its efficacy in managing HWA (Steward and Horner 1994; Steward et al. 1998; Webb et al. 2003; Cowles et al. 2006; Doccola et al. 2007). Imidacloprid is avail- able in water soluble packets (WSP), in soluble tablets, or in liq- uid (SL) formulations, and may be applied to the foliage, soil, or injected directly into the tree. The latter two methods rely on systemic movement of insecticide from the point of application upward into the canopy. Soil and tree-injected formulations are efficiently applied to trees in forested and woodland settings. While spray applications are more commonly used, there may reported imidacloprid peaking in T. canadensis 9–12 months following soil applications or tree injections, although none achieved residues of 0.300 µg/g, the LC50 for HWA (Cowles et. ©2012 International Society of Arboriculture be advantages to combining soil and tree injections for short- term therapeutic and sustained tree protection in some instances. The feeding habits of adelgids favor systemic applica- tions. For systemic activity, the insecticide must move upward in the vascular tissues, to concentrate at the growing points at which insects feed. Soil-applied imidacloprid needs to be in solution to be absorbed by tree roots before systemic activ- ity can occur. Trunk-injected imidacloprid is directly intro- duced into the sapwood for movement upward into the canopy. Methods of soil application include basal drenches, soil injec- tion, or the use of soluble tablets. The availability of soil-applied imidacloprid (to the tree) depends upon soil moisture, which may be inconsistent in woodland environments. A possible dis- advantage to these techniques is their slow action. Imidacloprid applied to soil binds to fine soil particles and to organic matter (organic carbon coefficient, ~350) and is slowly dissolved by moisture in the soil (water solubility of imidacloprid is 0.51 g/ Liter) (Cox et al. 1977; EXTOXNET-PIP/Imidacloprid). Soil- applied treatments maintain residual activity over several years (Cowles et al. 2006) and are simple and quick to apply. Eastern hemlock trees in this study were in poor condition and declining; the trees had thin, sparse foliage and twig die- back. Tree recovery depended on the immediate and sustained adelgid control. Webb et al. (2003) reported that recovery and dramatic new growth of hemlock in poor condition occurred following imidacloprid treatment, although at a slow rate. Woodland trees depend upon natural rain events for mois- ture, which could be highly variable. Cruziat et al. (2002) dis- cuss the increase in hydraulic resistance at the soil–root inter- face during periods of drought and among its consequences: reduced water absorption, conductance, and stomatal closure. Drought was of particular concern because shoot growth, which is critical to tree recovery, depends on adequate soil moisture (Onken 1994), and root uptake of soil-applied imidacloprid de- pends on it being dissolved (i.e., available) in the soil solution. The study objectives were to evaluate tree responses to dif- ferent methods and rates of imidacloprid application and to evaluate the extent of residual activity of the treatments. To evaluate the effectiveness of the techniques, assessments were conducted on HWA density and twig growth, and imida- cloprid was extracted from hemlock needles for three years. MATERIALS AND METHODS Study Site Eastern hemlock (Tsuga canadensis) trees were selected for treatment: large diameter, mature trees ~30 meters in height lo- cated on the Biltmore Estate in Asheville, North Carolina, U.S. (latitude: 35.567°N, longitude: -82.5448°W). Treatments were assigned at random in two blocks. In poor condition and declin- ing, the trees had thin, sparse foliage with tips dying back. The trees had ~25% live crown ratio, and ranged from 26.5 to 86.8 cm diameter at breast height (DBH) with a mean DBH of 49 cm. Application Techniques Forty-eight hemlocks were treated with insecticide formula- tions containing the active ingredient imidacloprid (1-[(6-chlo- ro-3-pyridinyl) methyl]-N-nitro-2-imidazolidinimine) for HWA
March 2012
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