18 Doccola et al.: Trunk-Injected Imidacloprid for Hemlock Woolly Adelgid our observations in reduced shoot growth at 4.0 total HWA/ linear centimeter (10.0 total HWA/linear inch). This was one of the four parameters used to assess the efficacy of the treatments. 2.0 live HWA/linear cm (5.0 live HWA/linear inch) is comparable when HWA mortality in the controls is 50% or less. In 2003, HWA mortality was significantly higher in the IMA-jet trees compared with the controls (91.4% versus 53.3%). We observed a treatment effect of 2.57 total (0.59 live) HWA/linear centimeter compared with 3.46 total (1.92 live) in the controls, but this was not statistically significant. However, it is worth noting that in the treatment samples, HWA development was delayed (i.e., higher ratio of sistens nymphs to adults) compared with the controls. This suppres- sion effect was most likely the result of the initial 2002 treat- ment applied, suggesting imidacloprid activity at a sublethal level. In each, shoot growth was not statistically different (6.03 cm [2.41 in] versus 5.71 cm [2.28 in], respectively), attributable to HWA pressure. In 2004, the treatment and control populations diverged significantly and continued to do so into the Fall of 2005. HWA mortality increased to 98.5% in the IMA-jet treated trees without further treatment in 2004 and 2005. In the con- trol trees, we observed 20.5% and 46.8% HWA mortality in the same 2 years. HWA densities also dropped significantly in the treatments to 0.59 and 0.20 total (0.06 and 0.02 live) HWA/linear centimeter compared with 3.04 and 4.91 (2.16 and 2.76 live) in the controls. The high HWA mortality ob- served (2004 to 2005) in the treatments had biologic signifi- cance, that is, tree health improved. In this 3 year study, we observed a mean of 2.11 cm (0.84 in) more shoot growth in treated trees over the controls. The increase in growth results in increased photosynthesis and carbohydrate accumulation, essential to hemlock recovery. The mean HWA mortality observed in the controls in the 3 years was 40.2% (n384), that is, 59.8% of the population survived. Winter low tem- peratures did not appreciably affect HWA populations. The mean live HWA/linear centimeter calculated for the controls was 2.28, high enough to affect hemlock growth each year. These differences in hemlock response are illustrated in Fig- ure 6. What is interesting is the delay we observed from the time of treatment to the observed effect on HWA mortality. We observed suppressive and lethal effects of imidacloprid on HWA, but did not observe a quick knockdown when trunk- applied into hemlock. In the first analysis, samples were col- lected within 60 days of the second treatment application, yet our most significant results were not recorded until 365 days later. Fall analyses are very practical. HWA development proceeds for at least 90 days before the onset of extreme low winter temperatures. This provides ample time to analyze a high volume of samples. In contrast, the spring window is significantly smaller (30 days) for analyses. HWA enters a ©2007 International Society of Arboriculture Figure 6. Hemlock samples with 3 years of growth (2003, 2004, 2005) illustrating typical patterns of response. All hemlocks were infested with hemlock woolly adelgid (HWA) at the onset of the study. Unchecked, HWA results in reduced twig growth like in the two control branchlets (left). The treatment trees show a marked growth re- sponse when the pressure from HWA infestation is allevi- ated (right). summer aestivation period, defaulting observations to the fall. Furthermore, spring observations would not have provided complete growth increments used in our analyses. We rec- ommend fall monitoring to the arborist because time is a significant consideration. In practice, fall HWA assessment may be used to predict tree growth in the next year and to help decide whether additional treatment is needed. Another implication of slow response is to allow ample time for an observable response. The arborist needs to establish this ex- pectation to his or her clientele. The other possible explanations of our observations may have been the result of reduced translocation of the insecti- cide into suppressed twigs, poor distribution within the tree, or a low dose administered. The first explanation is unlikely, because samples without current-year growth were discarded. A more likely explanation is that the distribution of imida- cloprid within the canopy may not have been uniform. We used a very low injection profile of centimeter dbh/10 to limit wounding. The second application increased the injection profile to centimeter dbh/5, having the effect of better distri- bution, but also increased the level of imidacloprid. Although we cannot strictly separate distribution and dose, we believe the strongest explanation (on the bases of the suppressive effects observed the first year) is that insufficient A.I. was administered. Many microinjection labels allow for repeated treatments in part because of the limited volume in the delivery capsule (typically, 5 to 10 mL [0.15 to 0.30 fl oz]). These are less
January 2007
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