Arboriculture & Urban Forestry 43(6): November 2017 found higher levels of spotted tentiform leafmin- ers (Phyllonorycter blancardella) and apple blotch leafminers (P. crataegella) in the lower to middle canopy of apple trees while parasitoid activ- ity was more evenly distributed throughout the canopy with a trend for higher parasitism in the middle to upper canopy regions. While no infested foliage was held for parasitoid emer- gence, examination of infested tips and ALM larvae during field and laboratory IBT assess- ments did not reveal any obvious and/or discern- able evidence of parasitism, suggesting natural enemies were not a significant factor in con- tributing to differences in IBTs across cultivars. The remaining 16 cultivars exhibited a grada- tion of damage from moderate (10%–20% branch tip damage) to severe (>20% branch tip damage). Observations in the current study are somewhat consistent with at least one other report not- ing pyramidal, globose, and golden arborvitae seem to be preferred, in that order (NCSU 2015). However, in this study, it is unclear why ‘Globosa’ and ‘Umbraculifera’—both of which are short, globose, and have dense canopies—had high levels of damage (22% and 31%, respectively). Apparently, other factors are contributing to ovi- positional preference with these two cultivars. In addition to plant height, height above ground also appears to influence ovipositional preference. Non-random distribution of leafminers is com- mon among plants and among leaves within a given plant (Cornelissen and Stiling 2008). Find- ings in the current study suggest that adult female ALMs prefer to oviposit in a narrow range within the tree canopy, judging from the higher-level feed- ing damage observed at three to five meters above ground, which corresponds to the lower and mid- dle canopy zones of tall trees and the upper can- opy zone of short trees. Shorter trees tended to be noticeably less infested, with the highest levels of injury near the top of the canopy, and taller trees had generally higher levels of infestation particu- larly in the lower and middle sections of the canopy. These findings are consistent with a study by Miller and Ware (2014) in which percent leaf area consumed by elm leafminer larvae (Fenusa ulmi) was greater in the lower exterior zones of medium-sized (5–7 m) elm trees. Barrett (1994) found the mean number of spotted tentiform 211 leafminers and apple blotch mines per leaf were generally greater in the lower-middle canopy regions of apple trees compared to upper canopy regions. Auerbach et al. (1995) observed ovi- positing P. tremuloidella female leafminers pre- fer small quaking aspen trees for oviposition. A possible explanation for higher ALM tip dam- age in the middle and upper zones of arborvitae trees may be partially explained by Brown et al.’s (1997) hypothesis on lack of movement and egg depletion. In studying a leaf-mining moth (Cam- eraria hamadryadella) on oak (Quercus spp.), Brown et al. (1997) suggested higher leafminer densities on foliage in the lower oak tree crown may be due to the lack of other potential causal factors and due to the fact that the insect overwin- ters in the leaf litter. Females begin ovipositing on foliage closest to overwintering, emergence, and resting sites. While density of C. hamadryadella, in the lower crown of oaks conflicts with the current study’s findings, Brown et al. (1997) further stated this same hypothesis could also account for verti- cal stratification for insects more abundant in the upper tree crown. This hypothesis is supported by studies by Stark (1952; 1958), of a leaf-mining weevil (Recurvaria starki), and Harris (1958), studying Ocnerostoma piniariella, a needle miner of white pine, in which adults emerge from nee- dles high in the tree crown, and these same needle miners are more dense in the upper tree crown. Field observations by the study authors and oth- ers have revealed adult arborvitae leafminers tend to fly briefly in a cloud no more than two to four meters off the ground, and then settle within a minute or so on the host tree or nearby vegetation (Busck 1907; Maier et al. 2011). It is possible that adult female ALMs, being weak fliers, may tend to oviposit at plant canopy levels as high or higher, from which they emerge, congregate, and rest. In addition, ALM adults emerging from mines higher on the plant may find sunny portions and congregation of adults more attractive for mating and oviposition. Additional studies are needed to support and confirm Brown et al.’s (1997) hypoth- esis on lack of movement and egg depletion. In this study, it appears that canopy aspect was not a major factor in plant susceptibility; no signif- icant differences in damage level were associated with cardinal direction. However, the findings are ©2017 International Society of Arboriculture
November 2017
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