Arboriculture & Urban Forestry 43(6): November 2017 spectively. Short and tall arborvitae had the least tip damage (15% to 21%) in the lower and middle zones, and upper zones, respectively (Table 5). Growth Habit There were significant differences in damage sever- ity related to tree growth habit (Table 6). Globular- shaped trees had significantly (F = 11.0; P < 0.001) less damage (17%) compared to trees with columnar or pyramidal growth habits (24%). Canopy Density Trees with very dense canopies had significantly (F = 12.1; P < 0.001) fewer infested branch tips (8%) compared to those with sparse, medium, and dense canopies (23%–26% IBTs). There was a weak inverse correlation between IBTs and canopy density [R2 = 0.23, P = 0.047, BTD = 36.009 - (5.634 × Canopy Density)] with IBTs de- creasing as canopy density increased (Table 7). Canopy Aspect There was no significant relationship be- tween canopy aspect and infestation severity. In this study, the researchers could not detect a preference in arborvitae leafminer for any particular side of the tree (Table 8). Table 4. Mean percent infested branch tips (IBTs) for all species/cultivars by tree canopy zone, 2000–2001 ± SEM. Canopy zone Low Middle Upper Mean % infested branch tipsz 25b ± 0.66 24ab ± 0.58 21a ± 0.57 z Values within columns followed by the same letter are not significantly dif- ferent (P < 0.05, Student-Newman-Keuls multiple comparison test). Table 5. Mean percent infested branch tips (IBTs) by tree height and canopy zone for all species/cultivars, 2000–2001 ± SEM. Tree height and canopy zone Short – Low Short – Middle Short – Upper Mean Tall – Low Tall – Middle Tall – Upper Mean Mean % infested branch tipsz 15a ± 1.79 16a ± 1.78 24bc ± 6.31 18 30c ± 1.32 26c ± 1.23 21b ± 0.96 26 z Values within columns followed by the same letter are not significantly dif- ferent (P < 0.05, Student-Newman-Keuls multiple comparison test). 209 Table 6. Mean percent infested branch tips (IBTs) for all species/cultivars by growth habit, 2000–2001 ± SEM. Growth habit Globular Columnar Pyramidal Mean % infested branch tipsz 17a ± 0.71 24b ± 1.41 24b ± 1.72 z Values within columns followed by the same letter are not significantly dif- ferent (P < 0.05, Student-Newman-Keuls multiple comparison test). Table 7. Mean percent infested branch tips (IBTs) for all species/cultivars by canopy density, 2000–2001 ± SEM. Canopy density Sparse Medium Dense Very dense Mean % infested branch tipsz 26b ± 1.78 23b ± 0.88 23b ± 1.00 8a ± 1.12 z Values within columns followed by the same letter are not significantly dif- ferent (P < 0.05, Student-Newman-Keuls multiple comparison test). Table 8. Mean percentage infested branch tips for all species/cultivars by canopy aspect, 2000–2001 ± SEM. Canopy aspect North South East West Mean % infested branch tipsz 21a ± 1.11 24a ± 1.23 22a ± 1.20 23a ± 1.28 z Values within columns followed by the same letter are not significantly dif- ferent (P < 0.05, Student-Newman-Keuls multiple comparison test). DISCUSSION Results from this study suggest that plant height, growth habit, canopy zone, and density influ- ence ALM ovipositional preference, judging from significant differences in levels of foliar damage. Host Plant Characteristics The results from this study suggest that canopy density appears to play a role in ovipositional preference by ALM. Short, globe-shaped plants with very dense or dense canopies, such as ‘Hetz Midget’ and ‘Hoopesii’, had very low infestation levels (<7% IBTs) compared to significantly higher infestation levels (10%–42% IBTs) for pyramidal- and columnar-shaped species and cultivars. In addition, a weak but significant inverse relation- ship exists between canopy density and IBTs. While these two cultivars had lower levels of IBTs, it is not clear what factors may influ- ence insect distribution within the tree canopy, including the insect’s mobility, interspecific and intraspecific competition, natural enemies, ©2017 International Society of Arboriculture
November 2017
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