212 Miller and Danielson: Relative Susceptibility Among Arborvitae Cultivars to Arborvitae Leafminer not completely consistent with other leafminer studies and/or other ALM field observations. Miller and Ware (2014) found no significant biological difference in mean percent leaf area consumed by canopy aspect for elm trees by the elm leafminer. In contrast, Davidson and Raupp (2010) advised monitoring for ALM by looking on the south side of the plants, where damage is heavier and damage is first seen in full sun. Exposure and plant age may also be a factor affecting severity of damage. Cloyd (2001) stated that ALM is attracted to Thuja planted in shady locations. He suggested locating plants in sun to help alleviate ALM defoliation problems. Hanson and Walker (2015) mentioned ALM is mostly attracted to older, slow-growing trees exposed to sunlight. Further studies are needed to better understand the relationship between canopy aspect in ALM ovipositional prefer- ence and subsequent larval feeding damage. In the current study, no clear association between foliage color and infestation level was observed. While cultivars ‘Aurea’ and ‘Douglasii Aurea’ had yellow foliage and had high amounts of branch tip damage, similar levels of infesta- tion were also seen in green cultivars (‘Smaragd’) and both species. Previous studies on other insect herbivores have been less than consistent regard- ing ovipositional preference, leaf color, and larval suitability. ‘Sunburst’ honeylocust, a yellow-leaf cultivar, has been found more susceptible to feed- ing damage by honeylocust plant bug compared to green-leaf cultivars (Cloyd 2008), but Bastian (1988) found no significant difference in mean larval development times of mimosa webworm (Homadaula anisocentra) feeding on the ‘Sunburst’ and green honeylocust cultivars. In addition, Miller and Hart (1987) found no significant differences in overwintering survival of mimosa webworm pupae when larvae fed on all common variet- ies of honeylocust, including ‘Sunburst.’ Further study with a larger range of yellow cultivars might help determine whether foliage color affects ALM ovipositional preference and larval suitability. While not formally examined in this study, addi- tional factors may influence ovipositional prefer- ence and larval success. This can affect the level of larval feeding damage, including but not lim- ited to intra- and interspecific larval competition, ©2017 International Society of Arboriculture biological control by natural enemies, host plant physical and chemical characteristics, plant age, plant condition (stress, nutrition, etc.), and expo- sure. Discussed here are some field and laboratory observations that may help to partially explain the role of these factors in female ALM oviposi- tional preference and subsequent plant damage. Some studies have examined intra- and inter- specific larval competition with externally and internally feeding insect herbivores, and the results have been inconsistent (Faeth 1991; Bar- rett 1994; Kagata and Ohgushi 2001; Cornelissen and Stiling 2006). In this study, neither intraspe- cific nor interspecific larval competition appears to affect larval development, survival, and sub- sequent feeding damage. Lepidopteran leafmin- ers can detect the presence of conspecific eggs (Simberloff and Stiling 1988; Auerbach and Sim- berloff 1989), and this behavior could be present with adult female ALMs, thus mitigating intra- specific competition. Informal field and labora- tory observations and measurements at the time of IBT assessments revealed mined tips on very dense cultivars were approximately one centi- meter apart, while mine tips on less dense and sparse canopy cultivars were two to three centi- meters apart. In this study, dissection of mines during IBT damage assessments consistently found only one ALM larva per mine, with more than one larva per mine in very rare cases. While not fully investigated in this study, the difference in branch tip spacing between very dense (1 cm) and sparse (2 to 3 cm) canopy species/cultivars might be partially responsible for lower levels of IBTs on very dense plants due to the ability of the ALM female to detect the presence of conspecific eggs, mitigating intraspecific larval competition. The evergreen bagworm (Thyridopteryx ephemeraeformis), Fletcher scale (Parthenole- canium fletcheri), and two-spotted spider mite (Tetranychus urticae) are common leaf-feeding and sap-feeding pests of arborvitae. At least in this study, none of these pests were pres- ent and were not in competition with ALM larvae for plant resources indicating a lack of interspecific competition. However, this may not necessarily be true in other cases. Over the course of this study, biological control by natural enemies did not appear to be an impor-
November 2017
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