46 Doccola et al.: Imidacloprid Treatments for HWA presented in Table 1 and in Figure 4 in µg of compound per gram of dried needles (i.e., ppm). Trunk injected imi- dacloprid accumulated more rapidly (1 year) into the can- opy compared to soil-applied only imidacloprid. Although soil-applied imidacloprid accumulated more slowly (two years) in the canopy, it may persist longer. Forecast plots were conducted to predict imidacloprid residues beyond three years following treatments. Trend plot analysis for tree injection low dose (quadratic equation, Yt = -2.4 + 3.174*t – 0.56*t**2) forecasted residues >0.300 µg/g (LC50 for HWA) to four years following treatment, while tree in- jection + soil injection (Figure 5) and soil injection alone (quadratic equation, Yt = -2.39 + 2.73*t – 0.300*t**2, and Yt = -1.83 + 1.83*t – 0.155*t**2, respectively) forecasts residues >0.300 µg/g to six years following treatment. Figure 4. Concentrations of imidacloprid in µg/g in hemlock nee- dles 70, 435, 800, and 1165 days after treatment. Tree injected imi- dacloprid accumulated in the canopy at a faster rate compared to the soil injection treatment. Imidacloprid residues in the trunk injected (+/-soil injection) treated trees were consistently above the LC50 value (of 0.30 µg/g) for HWA in the three years studied. Monthly Precipitation The monthly mean precipitation for Asheville, NC, is 9.81 cm. The 2006 mean monthly precipitation for Asheville (10.06 cm) was not significantly different from the normal value, and ad- equate for hemlock growth in the subsequent year. However, poor growth was observed in 2007. The 2007 mean monthly precipita- tion was significantly below normal (7.16 cm, p = 0.015), which could have played a role in the limited refoliation observed in 2008. The monthly precipitation in 2008 was below normal, but was not statistically significant (7.42 cm, p = 0.068). Below normal rainfall in 2008 was probably not a factor in limiting tree growth in 2009. Precipitation in 2009 was not significantly different from normal, and did not appear to limit growth in hemlock in 2010. Analysis of Imidacloprid Residues Imidacloprid residues were extracted from hemlock needles 70, 435, 800, and 1165 days after treatment. These results are CONCLUSIONS AND DISCUSSION The study authors examined HWA-infested trees in the southern part of the hemlock range and collected data on foliage growth, HWA density, and imidacloprid residues for three years. At the time of treatment in 2007, the hemlocks had thinned canopies and significant dieback: 51.6% of the trees were in decline. Only 17.1% of the branch samples collected had enough foli- age to conduct an analysis of adelgid activity, from which the adelgid numbers were low. Terminal dieback and low adelgid density indicated a protracted infestation, leaving the stand in poor condition. The trees were treated in late summer to co- ordinate with the resumption of HWA feeding. Low levels of imidacloprid were recovered from needles of trees treated at 70 days. However, significant growth responses to treatment would not be observed for two years. Translocation upward was likely slowed by sparse canopies and reduced transpiration potential. As trees refoliated, adelgid density was observed to in- Figure 5. IMA-jet low + imidacloprid soil injection actual µg/g imi- dacloprid with forecast trend to 2013. This model plots residues above the LC50 HWA of 0.30 µg/g through 2013, six years following treatment. ©2012 International Society of Arboriculture analysis conducted after 70 days (Figure 4). The treatments with the highest residues (tree injection +/- soil treatments) at 435 days and 800 days were associated with statistically sig- nificant growth in subsequent years (2009 and 2010, respec- tively). In addition to insecticidal activity, imidacloprid may have played a direct role in aiding refoliation (Chiriboga 2009). Tree responses were positive, but slow, consistent with that reported by Webb et al. (2003), the greatest growth occurred in the second year following treatment (Table 1; Figure 2). The study authors observed a comparatively slower initial rate of accumulation of residues from the soil alone treatment. Relatively dry soil conditions (in 2007, 2008) may have ac- counted for the low imidacloprid residues obtained in the first two years studied. Imidacloprid is absorbed by roots when it is dissolved in the soil solution. The sparse trees showed signs of recovery when imidacloprid systemic activity occurred. Though mean residues were ≥LC50 for HWA in 2008, HWA/ crease. Three of the treatments (though not statistically signifi- cant) had mean adelgid densities that were approximately great enough (~2.0 HWA /cm) to impact hemlock growth, pointing to a need for continued protection. While adelgid density re- mained high across the years 2008–2010 in the untreated trees, their numbers dropped to negligible levels in the imidacloprid treatments at 800 days and remained so at 1165 days (Figure 3), supporting insecticidal efficacy and residual activity. Imi- dacloprid residues exceeded the LC50 value for HWA in each
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