210 Miller and Danielson: Relative Susceptibility Among Arborvitae Cultivars to Arborvitae Leafminer micro-climate, nutritional quality of foliage, and leaf production and leaf phenology of the host trees (Brown et al. 1997). The purpose of this study was to document the relative susceptibility of arborvitae species/cultivars to ALM. Identify- ing a specific factor or factors responsible for dif- ferences in feeding damage was determined to be beyond the scope of this study. However, the study authors would be remiss for not offering possible suggestions for observed differences in ALM feed- ing damage between arborvitae species/cultivars. Numerous studies have revealed a variety of fac- tors responsible for differences in ovipositional and feeding preference among insect herbivores, such as leafminers, including across natural light gradi- ents. However, these studies have primarily focused on shaded plants or plants growing in full sun, not canopy density (Moore et al. 1988, and references therein; Bentz 2003; Connor 2006). To the best of the authors’ knowledge, no definitive studies have been conducted on the effect of canopy density on ovipositional preference, particularly on leafminers feeding on evergreen trees. Numerous studies of the effects of light-gradient on oviposition and host susceptibility are conflicting at best. Some herbi- vores are more abundant or damaging on plants in sunny areas, while others are more abundant or cause more damage in shaded environments. Bult- man and Faeth (1988) showed that densities of two leafminer species feeding on Emory oak (Quercus emoryi) were negatively correlated with solar radi- ation, and they further concluded that even subtle differences in shading influences leafminer den- sity and mortality. Further still, in some studies, no difference in feeding or abundance was found between sun and shade environments (Collinge and Louda 1988; Collinge and Louda 1989; Pot- ter 1992; Connor 2006). It is possible, though not confirmed in this study, that light levels may impact ovipositional preference and susceptibil- ity when comparing plants with very dense foliage compared to sparsely foliated plants. Dirr (2009) stated that short, globe-shaped arborvitae culti- vars, such as ‘Hetz Midget’ and ‘Hoopesii’, tend to have shorter internodes and denser canopies com- pared to taller trees. Informal field measurements indicate short cultivars with very dense, thick foli- age (e.g., ‘Hoopesii’) had approximately 50% less light penetration compared to tall cultivars with ©2017 International Society of Arboriculture sparse foliage (e.g., ‘Smaragd’). In addition to diminished light penetration, short, very dense cultivars had thick foliage with branching extend- ing approximately 8 cm into the canopy interior, while sparse cultivars had much thinner foliage and less branching, extending approximately 3 cm from the spray tip into the interior tree canopy. Female leafminer ovipositional behavior may also be a factor. Connor (2006) indicated that daytime fliers discriminate between sun and shade foliage, while nighttime fliers do not. While not confirmed in this study, it might be inferred that female ALMs, being daytime fliers, may discriminate by preferring to oviposit on foli- age receiving more sunlight. This might explain why there was significantly higher tip damage in the upper canopies of shorter cultivars, and middle and upper portions of taller cultivars. Leaf thickness, toughness, water content, sec- ondary compounds, and nutritional compounds have all been implicated at one time or another as possible factors responsible for various feed- ing patterns observed in insect herbivores (Con- nor 2006, and references therein). While none of these factors were directly measured in this study, field and laboratory visual observations at the time of IBT assessments indicated no obvious and/or discernable differences in leaf thickness for the arborvitae cultivars studied. In virtually all of the IBT assessments, only one larva was found per leaf tip, and only in very rare cases were two larvae discovered. This suggests there was adequate food (i.e., parenchyma tissue) for larval development for all species and cultivars in this study, regardless of canopy density. How- ever, more formal laboratory and field studies would be required to support these findings. The potential impact of very dense foliage ver- sus sparse foliage on natural enemy habitat was not included in this study. Predation and parasit- ism are inconsistent at best. Some studies have shown higher rates of arthropod predation and parasitism in shaded areas and by canopy aspect (Trumbule and Denno 1995; Pottinger and LeRoux 1971). In contrast, a study by Bultman and Faeth (1988) on four leafminer species on Emory oak discovered that one of the species had higher sur- vivorship on shaded trees due to higher levels of parasitism on sun trees. Further, Barrett (1994)
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