The dominant landscaping practice of recent centuries has been to create landscapes designed with ornamental plants that have been introduced from other countries. This practice has been so pervasive that nonnative plant species now outnumber native species in most urban, suburban, and rural landscapes (DeCandido et al., 2004; Dolan et al., 2011; McKinney, 2006, 2008; Qian and Rickleffs, 2006; Standley, 2003). For example, an extensive study of landscape plantings in Washington, DC, suburbs measured 55% nonnative plant biomass (Narango et al., 2017). This imbalance is perpetuated by nursery stock dominated by nonnative ornamentals. A study done at Mt. Cuba Center, a botanical garden dedicated to native plant horticulture and research in Hockessin, DE, found that large mid-Atlantic U.S. wholesale nurseries carried only 25% native species (Coombs and Gilchrist, 2017).
There is growing evidence that the preponderance of nonnative plants in built landscapes may impact the conservation of local biodiversity. Although nonnative plants can extend nectar resources late in the season (Salisbury et al., 2015), their exclusive use in pollinator gardens pose a threat to the dozens of native bee specialists that only reproduce on single native plant genera (Fowler, 2016). Moreover, because native plants support the growth and development of the insects that transfer the most energy from plants to vertebrate food webs far better than nonnative plants (Burghardt et al., 2010; Pearse et al., 2013; Tallamy, 2017; Tallamy and Shropshire, 2009), the lack of native species in urban ecosystems depresses populations of the wildlife that helps run those ecosystems (Burghardt et al., 2008; Narango et al., 2017). This is particularly true for the terrestrial birds in North America, 96% of which rear their young on insects [derived from Peterson (1980)].
There is increasing interest in gardening for wildlife, and consequently a growing market for native landscape plants (American Society of Landscape Architects, 2017; Brzuszeki and Harkess, 2009). This, in turn, has led to questions regarding what is “native” and what is not, what are appropriate constraints from plant provenance, and, most often, are cultivars of native plants (often called “nativars”) the ecological equivalents of the parent species from which they were selected? This last question is particularly important because the increased use of native plants is being driven more by their ecological function in the landscape than by their aesthetic roles. Moreover, the majority of native plants in the nursery trade are available as cultivars (Coombs and Gilchrist, 2017).
There are two primary ways cultivars may impact the insects that use plants for growth and reproduction: 1) A genetic change that creates a desired cultivar trait may alter the leaf chemistry of the plant to the point where insect herbivores no longer recognize the plant as a potential host or are repelled by an increase in distasteful feeding deterrents; 2) cultivars that change the color, shape, or phenology of a flower may inadvertently reduce the amount or quality of the pollen and nectar available for pollinators, or, in the case of double, sterile flowers, may eliminate pollen and nectar altogether. In this study, we focus exclusively on the potential impacts of native cultivars on insect herbivores and leave flower modifications for a later study. Specifically, using a common garden experiment and laboratory feeding preference trials, we measured how six common cultivar traits of trees and shrubs that modify leaf color, disease resistance, fruit size, and plant habit impact the feeding preference, abundance, and diversity of insect herbivores.
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