Graduate Thesis Or Dissertation
 

Evaluating the Attractiveness of Pacific Northwest Native Plants to Insects and Gardeners

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  • Due to a growing body of literature documenting declines in pollinator populations, there has been increased public interest in pollinator conservation. In urban and suburban landscapes, many gardeners and urban land managers are interested in gardening for pollinators, particularly bees. Simultaneously, many homeowners and municipalities are interested in other aspects of ecological gardening, including drought tolerant plantings, and even gardening for general biodiversity to support arthropods amidst global insect declines. Native plants are of particular interest in habitat plantings, as their purported benefits include providing better quality habitat for native arthropods and needing fewer inputs due to water adaptation. However, in most regions of the United States, including the Pacific Northwest, there have been no empirical studies on the most attractive plants to bees. Better understanding the abundance and richness of bee communities attracted to native plants will help regional urban pollinator conservation programs. However, to truly be successful in urban pollinator plantings, native plants should also have aesthetic qualities that fit into ornamental gardens and landscapes. I conducted a three-year field study (2017-2019) to investigate the relative attractiveness of 23 native Pacific Northwest plant species to bees, and included four non-native garden plants commonly used in gardens as comparators. Additionally, I sought to characterize the abundance and richness of herbivores, predators, parasitoids, and detritivores supported by these plant species. I installed these plants in 1 m2 plots at Oregon State University’s North Willamette Research and Extension Center, with five replicates of each species and six meters between plots. During the week of peak bloom and the weeks preceding and following, I performed five-minute bee counts to assess bee abundance. To assess bee species richness, I used an insect vacuum to sample each plot, with four overhead passes to collect bee specimens for identification in the lab. In doing this, I also collected other insects present on the plant. In the lab, we identified bees to species and other insects to the lowest practical taxonomic resolution. To assess gardener preferences of our study plants, I designed an online survey disseminated via email listservs and social media. The first survey asked gardeners to rate how likely they would be to garden with 23 flowering plants native to the Pacific Northwest United States on a five-point Likert scale. I created a second survey that again asked gardeners to rank the likelihood they would garden with a subset of 11 of these 23 native plants on a five-point Likert scale, before and after receiving a one or two facts about the bee species each flower attracted. Most native plants were ranked relatively high by gardeners. Short messages about the bees associated with specific plants raised gardeners’ impressions of native plants. In Chapter 2, I analyze the bee communities associated with my study plants. Over the three years, Douglas’ aster (Symphyotrichum subspicatum), California poppy (Eschscholzia californica), varileaf phacelia (Phacelia heterophylla), Canada goldenrod (Solidago canadensis), farewell-to-spring (Clarkia amoena), globe gilia (Gilia capitata), and Oregon sunshine (Eriophyllum lanatum) all attracted high bee abundance and species richness. Additionally, several native plants were consistently more attractive to bees than the exotic comparator species. The bee communities associated with the study plants were significantly different. Thus, using a combination of these plants in landscape designs and plantings might maximize the ability of a garden to support a diverse community of bees. I analyze the communities of herbivores, natural enemies, and detritivores in Chapter 3. Significant differences were found between the insect communities supported by the various study plants. In particular, Douglas’ aster, goldenrod, pearly everlasting (Anaphalis margaritacea), yarrow, and farewell-to-spring attracted a high abundance and species richness of parasitoids and predators. These plants also supported a high overall herbivore abundance, indicating they may be good candidates to provide resources at the first trophic level for broader conservation plantings, or as insectary plants to promote biological control. In Chapter 4 I share the results of two gardener surveys assessing gardeners’ impressions of study plants. The first survey revealed a high level of acceptance of native plants by home gardeners. However, there was relatively little overlap between the plants most attractive to gardeners and those most attractive to bees (identified in Chapter 2). The second survey found that gardeners’ stated likelihood of planting significantly increased after seeing photos of and reading a small amount of information about the bees supported by each plant. Replies to open ended question boxes revealed gardeners were most concerned with the weediness of and aesthetics of candidate plants, and that they felt positively about the ecological benefits and the aesthetics of many species. Many gardeners wanted more information about the study plants, before committing to planting them in their garden. Finally, in Chapter 5, I created an Extension document to bridge that gap we identified, and provide the public with easily digestible information about the top bee plants via fact sheets and compelling visuals.
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