Do Cultivars of Native Plants Support Insect Herbivores?
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Do Cultivars of Native Plants Support Insect bee specialists that only reproduce on single native plant genera (Fowler, Herbivores? 2016). Moreover, because native plants support the growth and de- velopment of the insects that transfer 1,3 1,4,7 1,5 Emily C. Baisden , Douglas W. Tallamy ,DesireeL.Narango , the most energy from plants to ver- and Eileen Boyle2,6 tebrate food webs far better than nonnative plants (Burghardt et al., 2010; Pearse et al., 2013; Tallamy, ADDITIONAL INDEX WORDS. conservation, garden, biodiversity, anthocyanins 2017; Tallamy and Shropshire, 2009), the lack of native species in SUMMARY. Native plants are becoming widely used in built landscapes to help mitigate the loss of biodiversity caused by urbanization. The primary advantage of urban ecosystems depresses popula- native plant species over introduced ornamentals is their ability to support the tions of the wildlife that helps run development of the insects that fuel vertebrate food webs as well as specialist those ecosystems (Burghardt et al., pollinators. The horticultural industry has introduced many cultivars of native 2008; Narango et al., 2017). This is plants to improve their aesthetic value and disease resistance, but there has been particularly true for the terrestrial little work that measures the impact of these genetic changes on insect herbivores birds in North America, 96% of which and pollinators. Here we measure how six desirable traits in native woody plant rear their young on insects [derived cultivars (leaf color, variegation, fall color, habit, disease resistance, and fruit size) from Peterson (1980)]. compare with their wild types in terms of their ability to support insect herbivore There is increasing interest in development, abundance, and species richness. Using a common garden experi- ment, we quantified the abundance and diversity of insect herbivores using each gardening for wildlife, and conse- species and its cultivars for growth and development over a 2-year period, as well as quently a growing market for native cumulative feed damage over the entire season. We also conducted feeding tests with landscape plants (American Society evergreen bagworm (Thyridopteryx ephemeraeformis) to measure the preference of of Landscape Architects, 2017; Brzuszeki hatchling caterpillars for cultivars vs. straight species. We found that cultivars that and Harkess, 2009). This, in turn, had leaves altered from green to red, blue, or purple deterred insect feeding in all has led to questions regarding what is three experiments, a preference for variegated cultivars in one of our three ‘‘native’’ and what is not, what are experiments, but no consistent pattern of use among the species and cultivars chosen appropriate constraints from plant for other traits. These results suggest that the usefulness of native cultivars in provenance, and, most often, are restoring insect-driven food webs depends on the cultivar trait that has been selected. cultivars of native plants (often called ‘‘nativars’’) the ecological equivalents he dominant landscaping prac- in Washington, DC, suburbs mea- of the parent species from which they tice of recent centuries has sured 55% nonnative plant biomass were selected? This last question is Tbeen to create landscapes (Narango et al., 2017). This imbal- particularly important because the in- designed with ornamental plants that ance is perpetuated by nursery stock creased use of native plants is being have been introduced from other dominated by nonnative ornamen- driven more by their ecological func- countries. This practice has been so tals. A study done at Mt. Cuba tion in the landscape than by their pervasive that nonnative plant species Center, a botanical garden dedicated aesthetic roles. Moreover, the majority now outnumber native species in to native plant horticulture and re- of native plants in the nursery trade are most urban, suburban, and rural search in Hockessin, DE, found that available as cultivars (Coombs and landscapes (DeCandido et al., 2004; large mid-Atlantic U.S. wholesale Gilchrist, 2017). Dolan et al., 2011; McKinney, 2006, nurseries carried only 25% native spe- There are two primary ways cul- 2008; Qian and Rickleffs, 2006; cies (Coombs and Gilchrist, 2017). tivars may impact the insects that use Standley, 2003). For example, an ex- There is growing evidence that plants for growth and reproduction: tensive study of landscape plantings the preponderance of nonnative plants 1) A genetic change that creates a de- in built landscapes may impact the sired cultivar trait may alter the leaf We thank Mt. Cuba Center for financial, logistical, conservation of local biodiversity. chemistry of the plant to the point physical, and philosophical support for this project; George Coombs for invaluable help in locating sour- Although nonnative plants can ex- where insect herbivores no longer ces of appropriate cultivars; George Coombs, Amy tend nectar resources late in the recognize the plant as a potential host Highland, and four very thorough reviewers for help- ful comments on the manuscript; and Mt. Cuba staff season (Salisbury et al., 2015), their or are repelled by an increase in for assistance while planting our common garden exclusive use in pollinator gardens distasteful feeding deterrents; 2) cul- experiment. pose a threat to the dozens of native tivars that change the color, shape, or 1University of Delaware, Department of Entomology & Wildlife Ecology, 250 Townsend Hall, Newark, DE 19716 Units 2Mt. Cuba Center, 3120 Barley Mill Road, Hockessin, To convert U.S. to SI, To convert SI to U.S., DE 19707 multiply by U.S. unit SI unit multiply by 3Masters graduate. 0.4047 acre(s) ha 2.4711 4 Professor. 0.3048 ft m 3.2808 5Doctoral graduate. 0.0929 ft2 m2 10.7639 6Director of Education and Research. 3.7854 gal L 0.2642 2.54 inch(es) cm 0.3937 7 Corresponding author. E-mail: [email protected]. 25.4 inch(es) mm 0.0394 https://doi.org/10.21273/HORTTECH03957-18 6.4516 inch2 cm2 0.1550 596 • October 2018 28(5) phenology of a flower may inadver- of moths and butterflies (Lepidop- Silhouette’) was the same size as the tently reduce the amount or quality of tera) (Table 1). Thus, we evaluated straight species (10 ft tall), but half as the pollen and nectar available for the potential impact of six popular wide. Redosier dogwood (Cornus ser- pollinators, or, in the case of double, ornamental traits for our study: 1) icea ‘Farrow’) reached 3 ft during our sterile flowers, may eliminate pollen red, purple, or blue leaf color study whereas the straight species and nectar altogether. In this study, expressed during the growing sea- exceeded 6 ft. we focus exclusively on the potential son; 2) variegated (white- and green- After plantings were established, impacts of native cultivars on insect patterned) leaf color; 3) plants with we allowed them to grow and become herbivores and leave flower modifica- agrowthhabitmodifiedtolow- synchronized phenologically for 1 tions for a later study. Specifically, using growing or compact forms; 4) disease year before we sampled. Insects were a common garden experiment and resistance; 5) increased fruit size and sampled destructively once in June, laboratory feeding preference trials, yield; and 6) enhanced fall colors (i.e., July, and Aug. 2015 and 2016. This we measured how six common cultivar color expressed only during fall leaf sampling regime covered seasonal traits of trees and shrubs that modify senescence). Each trait was repre- shifts that drive insect abundance leaf color, disease resistance, fruit size, sented in the study by three different while preventing any excessive reduc- and plant habit impact the feeding species (three replicates). We paired tion of insect populations. Insects preference, abundance, and diversity the parent species (straight species) were collected via vacuum sampling of insect herbivores. with its cultivar(s) (Table 1; Supplemen- (Brook et al., 2008) and the total tal Figs. 1–10). Plants were grouped search technique (Wagner, 2005). Methods within the garden by species, with the Samples were standardized by time Common garden experiment straight species and its cultivar(s) ad- allocated to sampling effort as a result In May 2014 we planted a 1500-ft2 jacent to each other to facilitate insect of differences in size and growth common garden experiment at Mt. choice of plants. Five individuals of pattern of plant species and cultivars. Cuba Center. The site has more than each species or cultivar were spaced First we vacuumed plant foliage 50 acres of gardens surrounded by 4 ft apart to allow for growth and to (30 s/sample) using a leaf blower a deer exclusion fence and 500 acres provide a larger target for herbivores (Craftsman gasoline blow/vac, item of natural land that provided a diverse to locate and populate. no. 7179469; Sears, Hoffman Es- community of local insects with Cultivars used in the growth tates, IL) in vacuum mode fitted with which to evaluate insect herbivore habit comparisons differed from their a 5-gal paint strainer bag (Burghardt preference among plants within the straight species in the following ways. et al., 2010). The bags were labeled experimental garden. Eastern red cedar (Juniperus virgin- and put into jars containing ethyl We selected 10 common species iana ‘Gray Owl’) is a prostrate selec- acetate to kill the insects. After col- of ornamental trees and shrubs native tion that spreads but gets no taller lection, samples were labeled and to the mid-Atlantic U.S. area that than about 30 cm. In comparison, the stored in vials of 85% ethanol for later were available in the trade and also plants representing the straight spe- identification to the least possible serve as host plants to a variety of cies were 6 ft tall in our study. Sweet- taxonomic unit. We then searched common insects, particularly species gum (Liquidambar styraciflua ‘Slender leaves and stems of target plants Table 1. Treatments, plant species, and cultivars compared with straight species in terms of insect herbivory (abundance, species richness, and cumulative seasonal damage) in a common garden as well as with feeding preference experiments using evergreen bagworm hatchlings.