Arboriculture & Urban Forestry 35(3): May 2009 mine whether a reduction in REBW numbers could be effected in a situation where injected trees were next to uninjected trees. Injection Treatment The Pest Management Regulatory Agency (PMRA, Health Can- ada) granted authorization to utilize a 97% dry flowable (DF) formulation (Arysta LifeScience) for this test, under the national “Risk Reduction Initiative” in support of pest management exper- imentation and refinement. The target injection application rate was determined according to circumference at breast height (cbh), 0.3 g ai / cm cbh (1.5 g ai / injector / 6 in cbh). Trees were mea- sured and assigned a number of injectors based on tree size: one 8 mL (0.27 fl oz) injector canister per 15 cm (6 in) trunk circumfer- ence. The number of injectors selected, determined at the time of treatment by dividing the tree circumference in cm by 15 (cbh in inches divided by six), ranged from 12 to 22 injectors canisters per tree. Injector canisters were pressurized with the Ecoject Sys- tem and then inserted into holes (one per canister) drilled near the base of the trees, 1.3–1.9 cm (0.5–0.75 in) into the cambium. The injector pressure for this method is approximately 0.379 MPa (55 psi) at the start of the loading stroke, and 0.448 MPa (65 psi) at the end of the stroke. The nozzle size was 5.6 mm (7/32 in). Injectors typically delivered all of the formulated product into the tree with- in 10 s. The injection procedure was completed on June 6, 2006. Sampling Foliage Two branches of at least 8 leaves were collected from each tree on two sampling dates: immediately following the day of injec- tion, and 8 weeks later (August 2, 2006) for chlorophyll anal- ysis. Chlorophyll content has been used in previous studies as an indicator of chemical challenge and resulting tissue damage (Knudson et al. 1977), or pest feeding damage (Johnson 1983). In the current REBW study, elm leaves were removed, frozen, and returned to the grassland ecosystem laboratory at the Uni- versity of Lethbridge (Alberta, Canada). Leaves were kept frozen until being dried and powdered. A sample of 1.0 g from each branch sample was extracted with 80% acetone and water in a liquid grinder/mixer. Absorbance was measured using a Bio- chrom Ultrospec 3300 pro UV/Visible spectrophotometer, and recorded between 300 and 800 nm for every sample; data at 664, 652, 645, 456, and 432 nm were saved for statistical analysis. On August 23, 2006, the trees in the experiment were photo- graphed to give additional general information on leaf condition. Insects Five yellow “Tanglefoot” sticky traps [10.2 x 15.2 cm (4 x 6 in)] were placed in each tree canopy. A total of 250 traps were placed at the beginning of each sampling period, and then re- moved and returned to the lab for counting of adult REBW un- der stereomicroscopes, to estimate changes in abundance. Sam- pling periods were 0, 2, 4, 6, and 8 weeks following treatment. Relative humidity and ambient temperatures within the canopy and branch tissue were monitored during the experi- ment with HOBO data recorders (Onset Computer Corp., Bourne, MA) and Oakton temperature recorders in the canopy of one of the trees. General weather data for the site was pro- vided by a permanent weather station within 1 km (0.62 mi), at the Agriculture and Agri-Food Canada Research Centre. RESULTS AND DISCUSSION Tree Measurements Circumference at breast height of the 50 trees used in the experi- ment ranged between 135 and 361 cm (53–142 in), mean = 96.8 cm (38.1 in), SEM = 2.73 cm (6.9 in) (good fit to a normal distri- bution; G.O.F. Shapiro-Wilk Test, W = 0.9808, P > 0.5, SAS In- stitute, 2007). The 25 trees assigned to the control did not differ in average size from the 25 trees assigned to the treatment (t = 0.304, P > 0.5; this confirmed there was no initial bias in trunk size). Chlorophyll Content Chlorophyll content (and related variables affecting leaf col- or), as represented by spectrophotometric absorbance at 664, 652, 645, 456, or 432 nm, did not differ between treatments (injected versus noninjected) either at the time of treatment application, or 8 weeks after injection (Figure 1, ANOVA, P > 0.3), for all five measured wavelengths. An example plot of the absorbance spectrum for one sample is shown in Figure 2. 149 Figure 1. Relative absorbance at 664 nm, for leaves from control versus treated trees, at the time of injection (date 1) or 8 weeks later (date 2). Insect Sampling and Population Estimates Of the total 500 sticky traps that were attached in the canopies of the 50 trees for the first two sampling periods, 18 and 55 (respectively) were lost because of wind, but losses were ap- proximately equal for treated and untreated trees. The resulting counts ranged typically between 0 and 5 adult beetles per sticky trap card. The counts were log-transformed before statistical analysis: ln (REBW+0.5). Tree size was not a significant co- variate in the analysis of covariance, and did not alter the ef- fectiveness of injection as indicated by REBW counts. REBW populations on the 25 treated and 25 untreated trees did not dif- fer significantly at the time of injections (Table 1; ANOVA on log-transformed REBW counts, P = 0.41). By two weeks after injection, the average REBW densities on the untreated trees had increased, whereas the densities on the treated trees had decreased (Table 1). At that time, the average count on traps from the treated trees was significantly lower than in the control group (ANOVA on log-transformed REBW counts, P = 0.0245). ©2009 International Society of Arboriculture
May 2009
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