Arboriculture & Urban Forestry 33(6): November 2007 393 and at the time of insecticide application in 2004, they were uniformly 3 m (9.9 ft) tall and 7 cm (2.8 in) diameter breast height (DBH). At the end of the study in 2006, trees were approximately 4m(13.2 ft) tall and 9 cm (3.6 in) DBH. Trees were surrounded by several acres of turfgrass that served as a source for Japanese beetles that readily colonized and con- sumed leaves of the study trees. Each tree was randomly assigned to one of two treatments to establish ten replicates of each treatment. Treatments in- cluded the application of imidacloprid (Merit 75 WP; Bayer Corp., Kansas City, MO) and remaining trees left untreated. Trees in the imidacloprid treatment received 2 g (0.07 oz) of imidacloprid per 2.54 cm (1.02 in) DBH in 3.8 L (1 gal) of water. Imidacloprid was mixed in a plastic pail and applied as a soil drench around the circumference of the tree at the root collar within 1 m (3.3 ft) of the trunk. Application took place on 7 May 2004, approximately 1 month before the appear- ance of Japanese beetles. Feeding Deterrence It was the goal of this study to determine if leaves from trees treated with imidacloprid were consumed less than leaves from untreated plants. To assess this, we removed single leaves of the same phonological age, specifically the first fully expanded leaf on a branch with an active meristem, from each treated and each untreated tree. The area of each leaf was measured with a Li-Cor LI-3100C area meter (LI- COR Biosciences, Lincoln, NE). The leaf was placed in a large Petri plate lined with moistened filter paper. Japanese beetles were collected from untreated linden trees at the study site. A single beetle was randomly assigned to a Petri plate containing either a treated or untreated leaf and allowed to feed for 24 hr. At the end of the feeding interval, beetles were removed and the leaf area was once again measured. The amount of leaf area consumed by each beetle was estimated by subtracting the leaf area remaining from the leaf area at the start of the assay. Defoliation We assessed the ability of imidacloprid to mitigate defolia- tion severe enough to reduce the aesthetic quality of lindens growing in a landscape as perceived by a homeowner or urban forester. In a previous study Raupp et al. (1988) dem- onstrated a direct relationship between levels of defoliation and perception of injury to arborvitaes attacked by the ever- green bagworm, Thyridopteryx ephemeraeformis. When de- foliation by bagworms reached 50%, more than 95% of people agreed that the plants were damaged and that control was warranted. This and other studies such as those by Sadof and Raupp (1987), Coffelt and Shultz (1990), and Sadof and Raupp (1997) confirmed the finding that 50% defoliation was regarded as damaged by than 90% of people surveyed. To provide an unambiguous definition of defoliation, we chose 50% or more defoliation of an individual leaf to be an indi- cator of severe damage. To estimate the efficacy of imidacloprid in preventing se- vere damage, we examined 80 leaves on each tree treated with imidacloprid and those untreated. At each of four car- dinal points, north, east, south, and west, we examined one shoot in the upper half of the tree and one shoot in the lower half of the tree. Ten leaves were examined on each shoot and the number of leaves recorded with 50% or more of their leaf area missing. Defoliation was quantified in the second week of August in 2004, 2005, and 2006 when feeding by Japanese beetles was nearly complete. The response variable used for our statistical analysis was the proportion of leaves severely damaged. Severe damage was expressed as the proportion of leaves in a sample of ten that sustained 50% or more defo- liation. Statistical Analyses Data from the feeding deterrence assays were not normal and could not be normalized through transformation. Variances were not homogeneous. Therefore, amounts of leaf area con- sumed by beetles eating leaves from treated and untreated trees were compared with a Kruskal-Wallis nonparametric analysis of variance (Zar 1999; Statistix Analytical Soft- ware, Tallahassee, FL). We studied not only the effect of imidacloprid on Japanese beetle feeding, but also if the application of imidacloprid affected the pattern of damage caused by Japanese beetles as they fed. These data were analyzed as a split, split plot with cardinal direction and leaf position (height) in the canopy as subplots. Proportional data were transformed with the arcsine square root to satisfy the assumptions for analysis of variance as suggested by Zar (1999). The proportions of severely dam- aged leaves per tree were analyzed with the general linear models procedure (Statistix Analytical Software) in 2004 and 2005. A Tukey test was used to separate treatment means following the analysis of variance (Statistix Analytical Soft- ware). In 2006, Japanese beetle populations were very low at our study site and imidacloprid treatments could not be ex- amined for interactive effects with tree height and cardinal direction. We could examine only the direct effect of imida- cloprid on severe damage caused by Japanese beetle. Data collected in 2006 did not meet assumptions for analysis of variance and transformations of these data failed correction for normality. Therefore, differences in amounts of severe damage on treated and untreated trees were compared with a Kruskal-Wallis nonparametric analysis of variance (Zar 1999; Statistix Analytical Software). RESULTS AND DISCUSSION Feeding Deterrence Japanese beetles consumed significantly less leaf tissue on leaves from trees treated with imidacloprid (P < 0.0001, ©2007 International Society of Arboriculture
November 2007
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