206 summer (533 to 700 DD50 Miller and Danielson: Relative Susceptibility Among Arborvitae Cultivars to Arborvitae Leafminer ) (Adams 2015), mate, and females lay eggs in axils of branchlets or along leaf margins. There is one generation per year (Busck 1907; Davidson and Raupp 2010; Maier et al. 2011). Thuja occidentalis L. (American arborvitae) is native to the eastern United States and the Great Lakes area, and T. plicata Donn ex D. Don (west- ern red cedar) is native to the Pacific Northwest region. Both are important timber species, used primarily for shingles, shakes, siding, and poles. Both species, and hybrids of these with other species (e.g., T. standishii), include a number of small- to medium-sized evergreens valued for midwestern, eastern, and western landscaping. Thuja occidentalis cultivars ‘Smaragd’, ‘Nigra’, and ‘Techny’ in particular have high landscape value because of their dense growth and ability to maintain green foliage color in all seasons (Dirr 2009). Cultivars also have diverse growth hab- its and leaf color, including dwarf, round, globe, and narrow-upright forms with yellow, blue, and various shades of green foliage (Dirr 2009). Arborvitae tolerate a wide range of soil and cli- mate conditions, thriving in fertile, well-drained, moist soil and full sun, but they can also do well in light shade and somewhat dry locations. Thuja plicata is more shade-tolerant than T. occidentalis or T. (Platycladus) orientalis. Arborvitae is com- monly used as screens, windbreaks, groupings, accent plants, and boundary hedges (Dirr 2009). The relationship between adult female leafminer oviposition and larval survival is essential in understanding leafminer popula- tion dynamics. Since most adult Lepidopteran leafminers do not feed, ovipositional site selec- tion by the female should be driven by larval survival and fecundity and not by adult feeding preference, as is common by free-feeding insect herbivores (Auerbach and Simberloff 1989; Faeth 1991). In addition, being of limited mobility, larval leafminers are restricted to the area near where the adult female lays her eggs. Eggs laid on an unsuitable host, the wrong part of the host, or on a secondary host could negatively affect larval development and survival. It is reasonable to expect a positive correlation between oviposi- tional preference and larval performance. How- ever, according to Kagata and Ohgushi (2001), this is not always the case, even with leafminers. ©2017 International Society of Arboriculture The unique larval strategy of feeding inside the leaf (Auerbach et al. 1995; Connor and Tav- erner 1997) provides protection from various natural enemies, including the physical envi- ronment (e.g., rain, dew), some predators and parasitoids, premature leaf abscission, and some host-plant defenses (waxy cuticle) compared to external feeding herbivores. As an univoltine insect, ALM spends the majority of the leafmin- ing larval stage inside foliage (Davidson and Raupp 2010; Cornell Extension 2012) and prob- ably experiences minimal, if any, exposure to these natural enemies. In order to better under- stand the ovipositional preferences and relative susceptibility of Thuja spp., a study was initi- ated in the summer of 2000. The objectives of the study were to determine: ALM ovipositional preference on arborvitae species and cultivars; susceptibility of arborvitae species and cultivars to ALM feeding; what role, if any, plant growth characteristics (i.e., growth habit, height, canopy zone, density, and aspect) have in ALM ovipo- sitional preference, and to provide monitoring and management guidelines for nursery grow- ers and landscape management professionals. MATERIALS AND METHODS The study area was located in a 1.21 hectare arborvitae collection at The Morton Arbore- tum in Lisle, Illinois, U.S. The Morton Arbo- retum’s plant records indicate the study trees were planted between 1924 and 1973, and were approximately 30- to 80-years-old at the time of the study. There was no history of insecti- cide treatment, and all plants within a cultivar and/or species were approximately equal in height. The arborvitae leafminer (A. thui- ella) was the only leafminer species present. An ALM infestation was first observed in the collection in 1998 during routine plant health care (PHC) scouting. By summer of 2000, the infesta- tion had become more extensive and severe, with numerous adults observed (first seen at 702 DD50 accumulations starting 01 March) in the col- lection. Beginning in late summer 2000, a field study was initiated to evaluate the relative prefer- ence among Thuja species and cultivars for ALM oviposition and feeding. Study trees varied in height from 0.6 m to 12 m and had globose, pyra-
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