16 Doccola et al.: Trunk-Injected Imidacloprid for Hemlock Woolly Adelgid should be noted that deadHWAremain on the twigs. IMA-jet treatment efficacy was evaluated on the basis of four mea- surements, including percent HWA mortality, live and total HWA/linear centimeter shoot growth, and current-year linear shoot growth. Percent HWA mortality is derived using the equation, [(dead HWA/total HWA) × 100], as determined by microscopic inspection. Current-year shoot growth is the measurement of the length of the new growth increment of each branchlet for that year. Total and live HWA/linear cen- timeter shoot growth is the number of total or living HWA found on the current growth increment divided by the length (cm) of that growth increment. Total HWA/linear centimeter shoot growth is reported as this correlates to McClure’s ob- servations (1991). Climate Data Costa et al. (2004) report −25°C (−13°F) as the point of initiation of significant HWA mortality and −30°C (−22°F) and −35°C (−31°F) as the lower lethal threshold temperatures of no HWA survival. The lowest temperatures recorded for the period of HWA evaluations in this study was −14°C (6.8°F) on 20 and 21 December 2004, well above the reported transitional point and lethal thresholds. The mean ambient temperature for the December months was 1.2°C (34.2°F). Low temperatures would not have had a significant impact on evaluation of HWA mortality. Figure 1 presents the mean monthly temperatures for the study and evaluation periods. These data were summarized from the National Oceano- graphic and Atmospheric Administration web site (www. weather.gov/climate). RESULTS Four parameters were tested each year (2003, 2004, and 2005) with a Student’s t-test at P < 0.05 using Minitab 14 statistical analysis software (Minitab Inc., State College, PA). The 95% confidence intervals (CIs) were also calculated with Minitab 14. Mean values were used to compare IMA-jet- treated hemlock and control hemlock sample populations. The four parameters tested were percent HWA mortality, total and live HWA per linear centimeter shoot growth, and current year shoot growth. The percent HWA mortality data were transformed to arcsine values to ensure the data are normally distributed. The arcsine values were assessed for significance using a Student’s t-test, and the mean arcsine values were then converted back to percent HWA mortality and reported in Table 2. Percent Hemlock Woolly Adelgid Mortality Each year of evaluation, the number of dead HWA on new growth was determined. Total sample populations and means for each year are presented in Table 2. The mean percent HWA mortality for IMA-jet treated hemlock increased from 91.4 to 98.5 from 2003 to 2005. The percent HWA mortality for controls varied from year to year with a high of 53.3 to a low of 20.5. Student’s t-test indicated significant differences between IMA-jet and control samples each year after treat- ment in 2003. These results suggest that we can expect 10.8% to 41.2%, 52.0% to 60.3%, and 34.1% to 45.4% greaterHWA mortality in treatments than in controls for 2003, 2004, and 2005, respectively, at the 95% CI. The HWA mortality ob- served affects the subsequent year’s growth response (e.g., 91.4% mortality in 2003 resulted in 5.79 cm growth in 2004). This needs to be kept in mind when assessing treatment re- sponse. Hemlock Woolly Adelgid per Linear Shoot Growth Each year, the number of total and live HWA per linear centimeter shoot growth was determined. Sample population numbers and mean total and live HWA/linear centimeter shoot growth are presented in Table 2. The mean total HWA/ linear centimeter shoot growth for IMA-jet treated hemlock were 2.57, 0.59, and 0.20 (6.43, 1.48, and 0.50 total HWA/ linear inch) for 2003, 2004, and 2005, respectively. The mean total HWA/linear cm shoot growth for controls were 3.46, 3.04, and 4.91 (8.65, 7.60, and 12.28 total HWA/linear inch) for 2003, 2004, and 2005, respectively. Significant differ- ences (P 0.000) were found between IMA-jet treatments and controls in the years 2004 and 2005. The mean numbers of live HWA per linear centimeter for IMA-jet treated hemlock were 0.59, 0.06, and 0.02 (1.48, Table 2. Mean results (2003, 2004, and 2005) for percent hemlock woolly adelgid (HWA) mortality, total and live and HWA/linear centimeter shoot growth, and new linear shoot growthz . 2003 Percent HWA mortality (transformed) 78 91.4% ay Total HWA/cm Live HWA/cm New linear shoot growth (cm) 2004 2005 (n) IMA-jet (n) Control (n) IMA-jet (n) Control (n) IMA-jet (n) Control 17 53.3% b 154 98.5% a 238 20.5% b 33 98.5% a 129 46.8% b 108 2.57 a 19 3.46 a 549 0.59 a 281 3.04 b 185 0.20 a 134 4.91 b 108 0.59 a 19 1.92 a 549 0.06 a 281 2.16 b 185 0.02 a 134 2.76 b 108 6.03 a 19 5.71 a 549 5.79 a 281 4.78 b 185 6.26 a 134 5.48 b zBranchlets with no HWA were discarded for percent HWA mortality. yMean values in the same year and parameter followed by a different letter are significantly different according to a Student’s t-test. n number of data points use for statistical analysis in the specified treatment and year. ©2007 International Society of Arboriculture
January 2007
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