Arboriculture & Urban Forestry 47(1): January 2021 apparent in the second year following treatment, whereas for the soil drench treatment the populations were reduced gradually so that by the third year after treatment the densities were equivalent to those resulting in tree injection treatments (Table 2). Although we report that the untreated control (UTC) trees had HWA mortality in year 4 similar to that of the imidacloprid treatments (Table 4), the per- centage of growing tips degraded in the UTC. That is, the trees continued in poor health. The deteriorated resource most likely resulted in the high mortality observed in the UTC. In contrast, the imidacloprid treatments resulted in high HWA mortality, and the trees recovered. This is reflected in the 3-year mean percent new growth of imidacloprid treatments, which varied from 65.6% to 71.7%, differing signifi- cantly from only 10.5% in the UTC (Table 3). The extended activity of trunk-injected imidaclo- prid has been linked to the production and persistence of imidacloprid and its olefin metabolite in the plant (Coots et al. 2013), to storage in the symplast (xylem ray parenchyma)(Turcotte et al. 2017) and to peren- nial needle retention. Needle retention plays a role in imidacloprid persistence as it (and its metabolites) would be shed with needle drop (Doccola et al. 2012). However, in contrast to Dilling et al. (2010), we found trunk-injected IMA-jet to be as, if not more effective than soil drench. Furthermore, there were differences in the results reported for tree injection in these studies. In each study, imidacloprid residues from the mid-canopy at 24 months post-treatment at 0.15 mL AI per 2.5 cm DBH were reported by Dilling et al. at ~23.6 to 56.2 parts per billion (ppb), and by Doccola et al. at 2.04 parts per million (ppm) or 205 ppb. In these studies, we observed an order of difference in the amount of imidacloprid from twig and sap tissues. Three factors may help to explain the difference in findings, which are (1) the formulation used, (2) the method of application, and (3) the tree vascular sys- tem. In each study, trees were injected using either Imicide (10% wt./wt. imidacloprid)(JJ Mauget, Co., Arcadia, CA) or (in one treatment) IMA-jet (5% wt./ wt. imidacloprid) at the rate of 0.15 mL (AI) per 2.5 cm DBH. The lower solute load of the IMA-jet for- mulation may help to increase the uptake through the restrictive vascular tissues by protecting it from pre- cipitation at the injection site, which we have observed with increasing imidacloprid concentrations in post- injection tree autopsies (unpublished). Uptake of 31 product applied by tree injection relies on transpiration from the canopy. However, we have observed differ- ences around the hemlock bole: each injection point does not take up liquid at the same rate. We believe this is related to the variability in transpiration rates within the canopy. Tree capsule injectors (as used in the Dilling study) apply product at low pressure [~14 to 34 kilopascal (kpa)], into an injection point of 0.15 cc (0.4 cm diameter drill bit × 1.2 cm deep) and therefore are sensitive to points in the bole where movement of sap is limited. The air hydraulic applies product at higher pressure (~689 kpa), into an injection point of 2.4 cc (0.9 cm diameter drill bit × 3.75 cm deep) to deliver product and therefore is less sensitive to points in the bole where movement of sap is limited. The imidacloprid soil drench was slower to act systemically, but has potential for long-term activity. Uptake of imidacloprid from soil drench is dependent upon available soil moisture, which is highly vari- able. Tree injection is not dependent upon soil mois- ture per se for movement within the vascular system. It is, however, dependent upon the environmental conditions that favor evapo-transpiration from leaf stomata. These conditions in general require a nega- tive vapor deficit at the leaf surface. Tree injection has utility in riparian areas, near aquatic habitats, and in soils with low cation exchange capacity (CEC); soil applications on the other hand, are more appro- priate in areas away from surface waters, and in soils with high cation exchange capacity (CEC). Imidacloprid, a systemic neonicotinoid insecti- cide, is an effective tool to manage HWA. Previous work demonstrates that imidacloprid 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 east- ern hemlock. Tree injection is an option to consider when treating hemlock in environmentally sensitive areas (e.g., near streams and ponds) and is exempt from per-acre use restrictions. This study demon- strates that tree injection with IMA-jet is effective against HWA and comparable with soil drench to pro- tect trees in the long term (≥ 4 years) until other long- term approaches, such as the introductions of predators, become more widely established. LITERATURE CITED Benton EP, Grant JF, Webster RJ, Nichols RJ, Cowles RS, Laga- lante AF, Coots CI. 2015. Assessment of imidacloprid and its ©2021 International Society of Arboriculture
January 2021
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