BOTTOM-UP EFFECT ON TOP-DOWN CONTROL IN A SUBURBAN LANDSCAPE. Erin Brown Reed

Transcription

1 BOTTOM-UP EFFECT ON TOP-DOWN CONTROL IN A SUBURBAN LANDSCAPE by Erin Brown Reed A thesis submitted to the Faculty of the University of Delaware in partial fulfillment of the requirements for the degree of Master of Science in Wildlife Ecology Fall 2010 Copyright 2010 Erin Brown Reed All Rights Reserved

2 BOTTOM-UP EFFECT ON TOP-DOWN CONTROL IN A SUBURBAN LANDSCAPE by Erin Brown Reed Approved: Douglas W. Tallamy, Ph.D. Professor in charge of thesis on behalf of the Advisory Committee Approved: Douglas W. Tallamy, Ph.D. Chair of the Department of Entomology and Wildlife Ecology Approved: Robin Morgan, Ph.D. Dean of the College of Agriculture and Natural Resources Approved: Charles G. Riordan, Ph.D. Vice Provost for Graduate and Professional Education

3 ACKNOWLEDGEMENTS I would be terribly remiss if I believed this has, in any way, been a solo effort. Many individuals had a hand in making this project a success, and in keeping me sane while doing it. First, I need to thank twelve homeowners who willingly allowed me to traipse about their properties for two years, with nothing but curious questions and nary a complaint in return. None of this would have been possible without their generosity. I also need to thank my committee: Greg Shriver, Charles Bartlett, Michael Smith, and especially the tireless efforts put forth by my advisor, Doug Tallamy. He remarked that I have come a long way since the time when I was a meek undergraduate, completely unsure how my future would unfold. And I think, for that, I have mostly him to thank. Additionally, Tallamy s Army, consisting of twelve undergraduates, two technicians, and three fellow graduate students, was an essential cog in the research machine. Particularly, to Kimberley Shropshire, Chris Philips, Brian Cutting, Kiri Cutting, and Karin Burghardt: thank you from the bottom of my heart. You were always there to lend a hand or offer advice and support, even while fighting your own research battles. For this, and for so many other things, I am forever grateful. Finally, to my family, friends, and the entire University of Delaware Department of Entomology & Wildlife Ecology, thank you for stimulating my mind, and occasionally my bar tab. This has been quite the ride, and I will never forget you. iii

4 TABLE OF CONTENTS LIST OF TABLES...v LIST OF FIGURES... vi ABSTRACT... viii Chapter 1 INTRODUCTION...1 The Role of Natives...3 Biotic Resistance Hypothesis...3 Understanding Extinction...5 How Aliens Outcompete Natives...6 Conservation Biological Control...9 Objectives METHODS...13 Study System...13 Project 1: Effectiveness of Natural Enemies...14 Herbivore Host Plant Systems...17 Statistical Analysis...17 Project 2: Arthropod Community Comparisons...18 Statistical Analysis...20 Project 3: Aesthetic Injury to Landscape Plants...21 Statistical Analysis RESULTS...23 Project 1: Effectiveness of Natural Enemies...23 Project 2: Arthropod Community Comparisons...27 Project 3: Aesthetic Injury to Landscape Plants DISCUSSION...40 REFERENCES...50 iv

5 LIST OF TABLES Table 1. Table 2. Table 3. Table 4. The herbivores, their host plants, and the number of individuals monitored to compare herbivore survival in native and conventional landscapes Survival (no. of days) of herbivores in native and conventional landscapes in 2008 and Herbivores were placed on host plants in native and conventional landscapes and counted until the remaining survivors reached 0. A P-value of 0.05 or less was considered significant (*) Survival (no. of days) of herbivores in native and conventional landscapes in 2008 and 2009, by property location. Herbivores were placed on host plants in native and conventional landscapes and counted until the remaining survivors reached 0. A P-value of 0.05 or less was considered significant (*). Tobacco hornworms were not monitored in Glenside in 2009 because of homeowner interference Average arthropod abundance and standard error / sample (in herbaceous samples) or 100 g leaf (dw) (in shrub and tree samples), diversity / sample (in herbaceous samples) or 1 g leaf (dw) (in shrub and tree samples), and species richness / sample (in herbaceous samples) or 100 g leaf (dw) (in shrub and tree samples) on conventional and native properties in July 2008, August 2008 and July A P-value of 0.05 or less was considered a significant difference (*) v

6 LIST OF FIGURES Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. The mean arthropod abundance (number) / sample (in herbaceous samples) or 100 g leaf (dw) (in shrub and tree samples) of arthropod trophic guilds (herbivores and natural enemies) sampled in July 2008 (A), August 2008 (B) and July 2009 (C) in conventional (gray bars) and native (white bars) properties. Note: Y-axes differ between years. A P-value less than 0.05 was considered a significant difference (*) The mean abundance (number) of herbivores and natural enemies (number) / sample (in herbaceous samples) or 100 g leaf (dw) (in shrub and tree samples) from native (gray triangle) and conventional (black circle) properties in July 2008 (A), August 2008 (B) and July 2009 (C). Note: Y-axes differ between years. A P-value less than 0.05 was considered a significant difference (*) The mean species richness (number of species) of herbivores and natural enemies (number) / sample (in herbaceous samples) or 100 g leaf (dw) (in shrub and tree samples) from native (gray triangle) and conventional (black circle) properties in July 2008 (A), August 2008 (B) and July 2009 (C). Note: Y-axes differ between years. A P-value less than 0.05 was considered a significant difference (*) The diversity, calculated by Simpson s diversity index, of herbivores and natural enemies / herbaceous sample from native (gray triangle) and conventional (black circle) properties in July 2008 (A), August 2008 (B) and July 2009 (C). A P-value less than 0.05 was considered a significant difference (*) The diversity, calculated by Simpson s diversity index, of herbivores and natural enemies / gram leaf (dw) of shrub and tree samples from native (gray triangle) and conventional (black circle) properties in July 2008 (A), August 2008 (B) and July 2009 (C). A P-value less than 0.05 was considered a significant difference (*) vi

7 Figure 6. Figure 7. Mean aesthetic injury (%) of conventional (gray bars) and native (white bars) properties in 2008 (A) and 2009 (B). The percent injury is presented as total damage and by injury type (discoloration and defoliation). A P- value less than 0.05 was considered a significant difference (*) between native and conventional landscapes within an injury category. Note: all levels of injury are below the Aesthetic Injury Threshold of 10% Mean aesthetic injury (%) of shrubs and trees in conventional (gray bars) and native (white bars) properties in 2008 (A) and 2009 (B). The percent injury is presented as total damage and by injury type (discoloration and defoliation). A P-value less than 0.05 was considered a significant difference (*) between native and conventional landscapes within an injury category. Note: all levels of injury are below the Aesthetic Injury Threshold of 10% vii

8 ABSTRACT Conservation biological control aims to attract and maintain populations of pestreducing natural enemies by way of alternative prey. By using plants with which native insect herbivores share an evolutionary history, homeowners may reduce the need for pesticides by supporting prey and thus, their enemies. To test this prediction, six pairs of properties, one landscaped with native ornamental plants and the other with non-natives, were studied. The abundance and diversity of arthropod communities, the effectiveness of natural enemies, and the degree of aesthetic injury to landscape plants were compared in 2008 and I found moderate evidence to support the prediction that pest populations will remain lower and more stable in native-based landscapes. Native properties supported a higher diversity of herbivores and natural enemies in the herbaceous layer than properties landscaped with non-native plants. Alternatively, a higher abundance of these guilds were found in non-native landscapes than native ones. These differences did not significantly affect insect herbivore survival; but aesthetic injury to landscape plants, especially that caused by leaf-chewing insects, was higher in properties landscaped with non-natives. This may be due to the attractiveness of native landscapes to birds, insect predators and parasitoids that suppress pest outbreaks. viii

9 Chapter 1 INTRODUCTION With housing developments and shopping centers rapidly replacing forests and wetlands, the suburban landscape is becoming a dominant ecosystem in North America. Over 80% of American citizens are urbanites, and, as such, urban land area has quadrupled over the last 60 years over twice the human population growth rate with land used specifically for residential purposes increasing by over five million hectares (29%) between 1997 and 2002 (Lubowski et al. 2006). In total, over sixty-two million hectares, an area larger than the size of California, are devoted to urban and suburban use in North America (Lubowski et al. 2006). Of that, over sixteen million hectares are covered by lawn alone, an area three times larger than that of any irrigated crop (Milesi et al. 2005), with many millions of dollars being spent by homeowners every year on its care and protection (Bormann et al. 1993). When replacing native ecosystems with non-native ornamentals in our yards, we not only remove native habitat, but we also increase the likelihood that these alien plants will escape cultivation and become invasive in the future, further threatening our native biodiversity by removing the native component of the first trophic level through competition and hybridization. Many species now known to be invasive, including Norway maple, purple loosestrife, kudzu, Japanese honeysuckle, mile-a-minute weed, 1

10 Oriental bittersweet, Japanese barberry, autumn olive, porcelain berry and English ivy were introduced to the U.S. by the ornamental horticulture industry. In fact, Reichard and White (2001) found 82% of invasive woody plants had been imported for use in landscapes as ornamentals. Even if a species seems innocuous today, we cannot assume it will never gain the ability to become invasive in the future. In fact, a plant can become aggressively invasive after being used ornamentally for over half a century (Pimentel et al. 2000). Butterfly bush, winged euonymus, and the grass, Miscanthus sinensis are just now escaping cultivation after being used in suburban landscapes for decades. Today, only five percent of the country s virgin habitats remain relatively uninfluenced by human activity (Rosenzweig 2003). The silver lining is that, unlike the land used in this country for agriculture, industry and roads, our residential areas are ones over which the individual homeowner has control. Homeowners can choose to share these spaces with biodiversity by restoring the natural habitat animals rely on for survival and reproduction. Moreover, because the urban and suburban ecosystem is quickly dominating the North American landscape, individual homeowners have more responsibility than ever to do so if we wish to preserve some remnant of our current biodiversity. There is mounting evidence that restoring native plant communities to managed landscapes is necessary to ensure that humans and the diverse array of organisms that used to live in eastern deciduous forests can coexist (Tallamy & Shropshire 2009). 2

11 The Role of Natives Native plants are the trophic foundation of all terrestrial ecosystems. They are the only organisms that can transfer energy captured from the sun into food that consumers can eat. They create the oxygen we breathe, filter our water, and cycle the nutrients we and other animals need to survive. A critical role of which we are becoming increasingly aware is native plants function in supporting native wildlife. Herbivorous insects, which comprise roughly 37% of all animal species (Weis & Berenbaum 1989), typically must share a long evolutionary history with their plant hosts in order to develop the adaptations necessary to circumvent the plants chemical and physical defenses (Ehrlich & Raven 1964). Therefore, insects, especially host plant specialists, may not be able to use nonnative plants, even those within the same genus as their native host (Tallamy 2004, Burghardt et al. 2010). If non-native plants are poor resources for insect herbivores, then the density and diversity of predators, parasites, and parasitoids that rely on insect herbivores for hosts including other arthropods, reptiles, amphibians, mammals, and 96% of terrestrial bird species (calculated from Dickinson 1999) are also likely to be reduced in habitats dominated by aliens (Tallamy 2004, Burghardt et al. 2009). Biotic Resistance Hypothesis Despite the role of native plants in supporting local food webs, the ornamental industry has been promoting the use of non-native plant species for over a century. Besides having larger, more colorful blooms, a longer growing season, and hardiness in 3

12 the face of drought and disease, non-native plants are also believed by some to be functionally beneficial to the ecosystem (Williams 1997, Harris et al. 1999, Kendle & Rose 2000) by providing structural diversification, food and shelter for wildlife, and modification of disturbance such as erosion and fires. It has also been proposed that nonnative plants create or fill niches by directly compensating for the loss of natives that were important for ecosystem function. Williams (1997) admits that, in anthropic systems, the functional benefit of non-natives in enhancing wildlife food and cover, controlling erosion or improving forage production are irrelevant and actually detrimental to the integrity of the ecosystem when natives exist to fulfill these roles. When restoring natural systems, however, non-natives are thought to aid in developing favorable microclimates, facilitating succession toward a management endpoint through carbon storage and nutrient fixation (Vitousek & Walker 1989, Lugo 1997, Williams 1997). Non-native plants have additionally been claimed to promote native ecosystem health by increasing biodiversity without causing native species extinction (Hokkanen & Pimentel 1989, Sax et al. 2002, Agrawal & Kotanen 2003, Colautti et al. 2004, Parker & Hay 2005, Parker et al. 2006, Sax & Gaines 2008). According to the biotic resistance hypothesis also known as the new associations or increased susceptibility hypotheses non-native plants that have not coevolved with native consumers will not have experienced selection from these consumers and will, therefore, not have the defenses needed to deter them (Elton 1958, Levine et al. 2004), thereby reducing the invasibility of the ecosystem. Thus, combined with the theory of island biogeography (MacArthur & Wilson 1967), an increase in immigration of species, non-native or otherwise, will lead to 4

13 an increase in species richness subglobally (Rosenzweig 2001, Davis 2003). When it comes to vascular plants, researchers have found that, so far, naturalizations far outweigh extinctions (Sax et al. 2002) because of a lack of competition-induced extinctions (Davis 2003, Sax & Gaines 2008). Understanding Extinction Despite any increase in local and regional species richness, the fact that biodiversity is declining globally is indisputable (Wilson 1992). A local boost in species richness is likely a temporary relict of alien naturalizations outpacing the time it takes for a species to become globally extinct. Even if extinction rates were to increase (they already exceed by many orders of magnitude what would be considered the background rate of extinction without human intervention (Wilson 1989)), the process of extinction, of a species being completely removed from the earth, may occur on a much longer time-scale than invasions (Sax et al. 2002). We may, therefore, be increasing local diversity temporarily, while simultaneously creating an extinction debt that may take thousands of years to come to completion (Tilman et al. 1994). Currently, habitat islands are roughly comprised of one naturalized non-native for every native species (Sax & Gaines 2008). This ratio, however, is likely to be transient and should approximate three non-natives for every two natives in the next fifty years (Sax & Gaines 2008). By continually altering these island biotas, we are merely exacerbating the extinction problem that has already been set in motion, further adding interest to the extinction debt that will be paid in the future. 5

14 It is also important to recognize the difference between global extinction, local extinction, and functional extinction. A species whose numbers have been reduced to the point where population density-dependent factors are negated, such as those that rely on a certain population size for survival and reproduction (Allee 1931), may be considered locally extinct though individuals still exist, because they are no longer propagating their species. Alternatively, the role performed by a species may no longer be accomplished when population numbers decrease to a threshold, despite the fact that many individuals may seem to be thriving. An increase in species richness after invasions by non-native species has little ecological meaning if the functionality of native species is decreased and ecosystem services are lost. For these reasons, we need not wait until an entire species disappears before we consider it functionally lost from our ecosystems. How Aliens Outcompete Natives The fact is that non-native species are becoming naturalized and our native habitats are suffering, and there has been no shortage of research attempting to determine why. Many hypotheses are not only non-mutually exclusive, but decidedly linked. In short, we can predict that non-natives become naturalized because (1) decreased abundance of native species has created vacant niches (Elton 1958, MacArthur 1970), (2) native habitat has been altered to become more favorable to aliens (Gray 1879, Baker 1974), (3) non-natives favor increasingly prevalent anthropogenic habitats (Byers 2002), (4) non-natives are competitively superior to natives (Vilà and Weiner 2004), and/or (5) 6

15 niches, especially in North America following glaciation, are not saturated (Sax et al. 2007). The leading hypothesis on why non-natives have been able to outcompete natives in their introduced range is the enemy release hypothesis (Darwin 1859, Williams 1954, Elton 1958). Many non-native species, upon being introduced to a new range, are liberated from specialist herbivores and pathogens that occupied their native range. These non-natives are, therefore, thought to gain an advantage because they are no longer suppressed by natural enemies and because they gain a competitive advantage over natives that suffer from native enemy attack. This hypothesis has been supported by decades of empirical evidence showing that insects require evolutionary time to specialize on native plants, developing the ability to detoxify, sequester, or excrete their noxious phytochemicals (Erhlich & Raven 1965, Rosenthal & Janzen 1979, Kennedy & Southwood 1984, Strong et al. 1984, Bell 1987, Bernays & Graham 1989, Berenbaum 1990, Scriber et al. 2008). Moreover, recent research has shown that successful invasive plants do suffer less herbivory than non-invasive plants (Carpenter & Cappuccino 2005) and have phytochemical defenses that are unique to the area the plants have invaded (Cappuccino & Arnason 2006). Thus, local insect herbivores do not have the necessary adaptations to use successful invaders as hosts. The biotic resistance hypothesis is based on the notion that generalists, though less diverse (Bernays & Graham 1989), are far more abundant than specialists (Futuyma & Gould 1979, Crawley 1989) and should therefore be able to feed and reproduce on non-native plants, but this assumes generalists are not restricted in host use. Studies that 7

16 have examined niche breadth in host use among generalists have found that generalists are often locally restricted in host use, despite the number of hosts recorded throughout their geographic range (Fox & Morrow 1981, Tallamy et al. 2010). Non-native plants, therefore, are not predicted to support as much insect biomass or diversity as natives, even when belonging to the same genus as native hosts (Burghardt et al. 2010). Because of the release from selective pressures in the novel environment, non-natives may experience rapid genetic changes, known as the evolution of invasiveness (Blossey & Nötzold 1995, Lee 2002, Stockwell et al. 2003) that, over time, may lead to the evolution of increased competitive ability (EICA) over natives (Blossey & Nötzold 1995). When released from enemy pressure, non-native plants may be able to reallocate resources from defense mechanisms into growth and development, thereby evolving to grow taller, produce more biomass, and yield more viable offspring than their native counterparts. Similarly, non-native plants may redistribute energy into increased production of competitive allelochemicals. Known as the novel weapons hypothesis (Callaway & Aschlehoug 2000, Bais et al. 2003), non-native plants may use interference competition from allelopathy by exuding noxious chemicals that inhibit naïve plants in recipient communities. In addition to enhanced competitive ability, non-natives may also gain an edge by utilizing resources not being used by natives, filling empty niches (Elton 1958; MacArthur 1970, 1972). This implies that species-rich communities will more completely use available resources and, therefore, lack empty niches, making them more resistant to invasion than species-poor communities. Thus, local disturbance that creates empty 8

17 niches promotes the establishment of non-native species because a community s susceptibility to invasion increases whenever the amount of unused resources in that community is increased. Finally, the disturbance hypothesis predicts that non-natives may be adapted to disturbances that are novel to natives; for instance, exotic ruderals often outperform native competitors in early successional stages (Gray 1879, Baker 1974). Conservation Biological Control Where non-native plants are not aggressively replacing natives on their own, humans are doing it for them. It is likely that the plants being introduced by landowners as ornamentals are selected in part because of their resistance to insect herbivory, as insect damage above a certain level diminishes aesthetic value (Sadof & Raupp 1987). As a result, instead of being a random sampling of non-native plants, these pest free species are favored and specifically selected by the ornamental industry because they are unpalatable to insects (Dirr 1998). The central tenet of conservation biological control is that low densities of herbivores representing a diversity of alternative prey must persist in a landscape to attract and retain natural enemies. When this is the case, predators (birds, rodents, spiders, predatory insects, etc.), parasitoids and pathogens are already present to prevent or suppress outbreaks when a potential pest enters the planting. Although somewhat counterintuitive, the notion that one must maintain pests in order to control pests is now well documented (Landis et al. 2000). It is precisely because populations of insect herbivores that serve as alternative prey for natural enemies are wildly erratic in urban 9

18 and suburban plantings that arthropod pests are rarely suppressed sustainably in these gardens by natural enemies (Hanks & Denno 1993, Shrewsbury & Raupp 2000, Tooker & Hanks 2000). The health of natural enemy populations and thus the sustainability of plantings without artificial pest control may be, in part, a function of the geographic origin of the plants comprising the landscape. If suburban plantings are too heavily biased toward nonnative plants that cannot support native herbivores (alternative prey), responsive communities of natural enemies should be absent from such plantings, increasing the risk and severity of pest outbreaks. In contrast, if suburban plantings consist of species with which native herbivores share an evolutionary history, a healthy community of natural enemies will be present and the herbivore community will be more stable. Natural enemies will be attracted to these landscapes by vegetative structural diversity that creates favorable microclimates and refuges from predators, along with alternative food resources including nectar, pollen, and prey (Landis et al. 2000, van Emden 2002, Langellotto & Denno 2004, Shrewsbury & Raupp 2006, Fielder & Landis 2007). The easiest way to maintain sustainable densities of alternative prey in ornamental plantings may be to provide adequate resources for these herbivores in the form of their ancestral host plants. Once established, the complex of native predators and parasitoids they support should prevent eruptive pest outbreaks without the use of pesticides (Gurr & Wratten 2000, Barbosa & Castellanos 2005). The use of integrated pest management (IPM) programs in urban landscape plantings is not only beneficial for the surrounding plants and animals, but also for the 10

19 homeowner as an individual. The cost of pest control can be dramatically lowered and the use of pesticides can be reduced as much as 94% when IPM approaches are adopted (Olkowski et al. 1978, Walker 1981, Brewer & Stevens 1983, Holmes & Davidson 1984, Smith & Raupp 1986). With pesticide use now being linked not only to reductions in natural enemy populations (Raupp et al. 2001), but also to water pollution and human health concerns, the use of IPM can be a winning alternative for everyone. Non-point source runoff has been identified as the primary source of water pollution in many aquatic systems (Russell & Shogren 1993), causing hypoxia or dead zones in which marine life cannot be supported. In addition, early childhood exposure to pesticides in and around the home has been identified as a cause of childhood leukemia (Lowengart 1987, Surgan et al. 2000). The use of pesticides should therefore be minimized, not only for the benefit of local animals, but also for the benefit of species hundreds of miles away as well as for human safety. Improving the effectiveness of natural enemies through the use of native plants should accomplish this goal. Objectives The primary objective of this study was to quantify the effectiveness of top-down control as it relates to the quality of the first trophic level. First, I measured the effectiveness of natural enemies in conventionally landscaped yards versus yards landscaped primarily with natives by investigating survival of simulated pest outbreaks. I predicted that properties dominated by native plants would have better control of pests by natural enemies than properties dominated by alien plants. Additionally, I quantified the 11

20 stability of herbivore populations by recording the degree to which landscape plants sustained insect-induced aesthetic damage. I used 10% total damage, produced by both leaf-chewing insects and leaf sucking insects, as an aesthetic injury threshold, as this is when studies have shown that homeowners initiate pest control (Raupp et al. 1989, Sadof & Alexander 1993, Sadof & Raupp 1996). Finally, I quantified the size and diversity of the herbivore and natural enemy arthropod communities as a whole in conventionally landscaped yards versus yards landscaped with natives. Yards landscaped with native plants should support more diverse communities of both insect herbivores and the natural enemies they attract. These herbivores, however, should be controlled at a level that will prevent aesthetic injury that would require human intervention. Alternatively, I predicted that properties landscaped conventionally, with a mix of native and alien plants, will support a less diverse community of herbivores that will, in turn, attract and sustain fewer natural enemies. Because the herbivore populations in these landscapes should be subject to less pressure from natural enemies, they are predicted to reach population levels that more frequently cause unacceptable levels of aesthetic injury. 12

21 Chapter 2 METHODS Study System To compare the arthropod communities of native and alien plantings and how these plantings influence conservation biological control, I conducted studies on properties typical of the suburban landscape in the piedmont region of southeastern Pennsylvania selected by Burghardt et al. (2009). Six pairs of properties (replicates) were selected, with one member of each pair dominated by native ornamentals at all vegetative strata (grasses, shrubs, understory, and canopy); and the other member landscaped conventionally, with mowed lawns of cool-season Eurasian grasses, Asian and European shrubs and understory, and a largely native canopy interspersed with mature aliens (i.e. Acer platanoides, Ailanthus altissima). A pair of properties was located in each of the following locations: Millersville, PA; Glenside, PA; Ambler, PA; Chester Springs, PA; Newtown Square, PA; and Media, PA. Properties ranged in size from 0.13 to 5.26 ha, with each pair member being within 1.6 km of each other. Pairs were matched according to area and surrounding landscape features that may provide a source of insect colonization or natural enemies: thus building cover, number of bordering woodlands and streams, bird boxes, and bird feeders were all controlled (Burghardt et al. 2009). Because suburban landscape design is 13

22 in most cases a matter of the homeowner s personal preference, exact vegetative uniformity among property pairs was impossible to achieve. Rather, variation existed among properties in terms of structural complexity of the vegetation as well as plant species richness and diversity. Because this could influence resident insect populations as well as their natural enemies, regardless of plant species origin, it was necessary to determine the degree of this variation. Burghardt et al. (2009) found the percent total vegetation cover was constant at all height strata (5 cm, 1 m, 4 m, >15m) between native and conventional sites, differing only in the percentage of native or alien plant species as defined by the USDA (USDA NRS While plant species richness was higher on native sites, a Simpson s diversity index did not reveal a significant difference in plant species diversity (Burghardt et al. 2009). Project 1: Effectiveness of Natural Enemies To test the hypothesis that the presence of non-native flora inhibits the effectiveness of natural enemies, I measured the survivorship of insect pest species in native and conventional landscapes. When the size, age structure, and resource base of initial pest populations and their host plants are comparable in native and conventional landscapes, any differences in survivorship that ensue can be credited to differential responses in the community of natural enemies associated with each garden type. I predicted that the greater abundance of alternative prey species in the native plantings would support a healthier natural enemy community, which would more effectively control a simulated pest outbreak. 14

23 Herbivore survival studies were conducted in all six of the property pairs 1 in 2008 and The herbivore species were all Lepidoptera larvae and were selected because they represent some of the most important pest species on native agricultural and ornamental plants. They often outpace natural enemies in ornamental settings and reach an eruptive phase of population growth. Caterpillars serve as representatives of external leaf feeders that commonly defoliate plants and have rich parasitoid and predator communities. The insect populations established on the plants were introduced as eggs or first instars to allow natural enemies access to all life stages. The number of eggs or larvae introduced (clutch size), the clumping patterns of the eggs or larvae, the area on the plant where the eggs or larvae were placed, the number, size, and species of host plant required, and the time of year they were introduced depended on the particular insect species and its life cycle and seasonal distribution. Table 1 lists the herbivore species used, their hosts, how many individual insects comprised each population, as well as the starting date and length of time of each survival trial. Each property was evaluated for insect pests before the establishment of my pest populations took place. All pest species on the properties that had overwintered from the previous season (e.g. bagworms) were located and removed so as not to influence my measurements of the populations I established. To respect the wishes of property owners, each host plant and respective pest species was established in an area they deemed aesthetically suitable. 1 With the exception of the tobacco hornworm trial at the Glenside properties in 2009, due to homeowner interference. 15

24 Table 1. The herbivores, their host plants, and the number of individuals monitored to compare herbivore survival in native and conventional landscapes. Herbivore Host Species No. Properties Host Density / Property Clutch Size / Host Total Population Size / Property Start Date (No. days of trial) cabbage looper bagworm tobacco hornworm tobacco hornworm whitemarked tussock moth cabbage July 31, 2008 (22) black cherry June 5, 2008 (33) tomato July 31, 2008 (21) tomato August 14, 2009 (32) black cherry September 8, 2009 (39) To compare survivorship, the number of remaining individuals was counted approximately every other day following the initial infestation until no survivors could be found. The dates on which populations were established were the same at the properties in each pair, but varied between pairs due to constraints in the number of herbivores that could be bought, reared or collected and the time and man-power needed to visit each property. Similarly, the frequency at which the populations were monitored (about every other day) also varied due to logistical constraints. Weather had no impact, as surveys were conducted rain or shine, alternating which property in the pair was visited first, but always within 30 minutes of each other. A count of zero individuals needed to be recorded at least twice in succession for the population to be considered eradicated. 16

25 Because no individuals reached their final instar in this study, I assumed that all individuals disappeared as a result of death as opposed to pupation. Herbivore/Host Plant Systems Four larval species representing the order Lepidoptera were used in the herbivore survival trials: cabbage looper (Trichoplusia ni (Hübner)), (Family: Noctuidae) on commercial garden cabbage (Brassica sp.), tobacco hornworm (Manduca sexta (Linnaeus)), (Family: Sphingidae) on commercial garden tomato (Solanum sp.), bagworm (Thyridopterix ephemeraeformis (Haworth)), (Family: Psychidae) on black cherry (Prunus serotina), and white-marked tussock moth (Hermerocampa leucostigma (Smith)), (Family: Lymantriidiae) also on black cherry (Prunus serotina). Cabbage looper and tobacco hornworm are common agricultural pests, while bagworm and whitemarked tussock moth are widespread in the suburban landscape, often deemed ornamental pests. Statistical Analysis The number of individual insects missing at each site was calculated from the number remaining each time the populations were monitored. A univariate survival analysis was performed to see if insects in one treatment native plantings live longer (more days) than insects in the other treatment conventional plantings. The data were blocked by location (SAS Institute 2009) and none of the observations were censored. As part of the Kaplan-Meier Survival Platform, the Log Rank and generalized Wilcoxon 17

26 Chi-square statistics were computed to test homogeneity of the estimated survival function across treatment groups. All reported results are untransformed means ± SEM (standard error of the mean). Project 2: Arthropod Community Comparisons Does a native landscape attract and retain a healthier and more diverse community of insect herbivores and their arthropodous natural enemies? To answer this question, a comprehensive sample of each property s arthropod community was taken three times, twice in the summer of 2008, and once in the summer of I hypothesized that a greater abundance and diversity of herbivorous insects would exist in landscapes dominated by native plants and that this community would sustain a healthy population of natural enemies. If a healthy community of natural enemies is needed to suppress pest populations, and if top-down control of phytophagous insects is affected by the bottomup quality of the first trophic level, then the number of phytophagous insects eaten and parasitized should be greater in habitats consisting of native plants than habitats dominated by aliens. To test whether arthropod predators and parasitoids are more numerous and diverse and thus better able to suppress more diverse populations of alternative prey in native plantings than in alien plantings, samples of arthropod communities on the vegetation in each of three strata were compared between landscape types. Ten random points, produced by a random number-generator, were located on each property. Using aerial photographs to locate the point, the herbaceous layer was sampled directly at the 18

27 randomly-generated point, while a predetermined number of leaves were sampled on the nearest shrub and tree. In 2008, no concern was given to whether the sampled tree was actually native or non-native. This practice was adjusted in 2009, with the native shrub and tree closest to the point being sampled in native properties, and the closest non-native being sampled in conventional properties when possible. A new set of ten points was generated for each sampling period so that, whenever possible, no plant was sampled twice in one season. Both the native and alien properties of each pair (replicate) were sampled on the same day and all six replicates were sampled within two weeks of each other. The host plants for the simulated pest infestation were not included in the arthropod community sampling. For herbaceous samples, a ring 1 meter in diameter was placed on the ground, encircling the vegetation to be sampled. Shrub and tree samples were taken by, first, counting and, if the plant was too large to be entirely sampled, demarcating a section of leaves to be sampled. A leaf-blower (Craftsman gasoline blow/vac, Item # ) set in reverse, its nozzle outfitted with an empty five-gallon paint strainer bag, was used to vacuum arthropods off the leaves and into the bag, which was then tied off with a rubberband, trapping the contents (Brook et al. 2008). The bags were placed in buckets containing ethyl acetate, permanently incapacitating the arthropods. Additionally, each plant was hand-searched for remaining arthropods, especially lepidopteran larvae (Wagner 2005). Collected specimens were then sorted from any vegetative debris and stored in labeled vials containing 70% ethyl acetate. Under magnification, they were then identified to trophic guild (herbivores or predators/parasitoids), counted, and the species 19

28 richness of each sample was measured using morphospecies (Species A, Species B, etc.). Simpson s diversity index was calculated per sample, as opposed to calculating the insect community diversity of the entire property. In herbaceous samples, these measures were calculated per sample, regardless of vegetation height or volume. Arthropod abundance and species richness data of shrub and tree samples, however, were standardized to the number of arthropods and arthropod species per 100 grams of dry leaf sampled to control for differences in plant size. This was done by multiplying the number of leaves sampled on each plant by the average dry weight of one leaf of that species. Simpson s diversity was calculated per gram of dry leaf weight (dw). Leaves sampled on plants were counted each sampling period. Statistical Analysis A one-way analysis of variance (ANOVA) and pooled t-test was used to compare the abundance, species richness, and diversity of herbivores and natural enemies (predators/parasitoids) in three vegetative strata (herbaceous, shrub, tree) in the native and conventional landscapes twice in 2008 and once in 2009 (SAS Institute 2009). The data were blocked by sample number to detect a difference in the plantings productivity across seasons. Additionally, each vegetative stratum was examined separately to see if differences in abundance, diversity, and species richness of herbivores and natural enemies existed in distinct layers of native and conventional landscapes. 20

29 Project 3: Aesthetic Injury to Landscape Plants To test the hypothesis that pest populations in native plants are less abundant and disruptive than those in traditional plantings dominated by aliens, I measured the damage pest species caused to the view-scape of the yard. Although natives are known to be beneficial for enhancing biodiversity, homeowners would be reluctant to plant species in their gardens if they sustain more visual damage than alien species. Therefore, in addition to determining if natives can suppress pest abundance in urban landscapes, I also measured how damage inflicted on natives by insect herbivores compares to damage sustained by plants in gardens comprised mostly of aliens. Previous studies have shown that homeowners associate 10% leaf defoliation and/or discoloration with aesthetic injury and initiate control at this time (Sadof & Raupp 1996). Therefore, injury from arthropod herbivores was monitored on the trees and shrubs selected in Project 2 once per year (just after insects were sampled in August) to compare damage levels and the length of time native and alien plants remain below the 10% damage threshold. For most plants, with the exception of those described below, leaves at four cardinal directions at two heights, either ¼ or ¾ of the distance from the ground (four locations / plant) were rated for injury. One branch was selected at each point and height combination. A pre-determined number of fully expanded leaves were then rated on each branch. Ratings began on leaves at the tip of the branch and moved toward the center of the plant. For each leaf, the percent of leaf area injured by insects was estimated. The estimations ranged from 0%, no injury, to 100%, when the leaf was completely damaged 21

30 by herbivores. Percent injury was recorded in 10% increments. The type of injury (defoliation by leaf-chewers or discoloration by leaf-suckers) was recorded. When more than one type of injury was found, the amount of each type of injury was recorded. Individual plant species had to be rated differently to take into account differences in leaf size, plant size, and growth habit. A standardized sampling procedure was developed for each species that determined the number of leaves and branches to be rated. The percent injury for most species was calculated from rating four branches, ten leaves per branch, for a total of 40 leaves. On species with compound leaves, ten leaflets on four compound leaves were rated. On species with needle leaves, ten two-inch sections of needles were examined in each of the four cardinal directions and given a score based on the percentage of needles with herbivore damage. Statistical Analysis An average percent injury was calculated for each of the 10 shrubs and 10 trees selected on each of the 12 properties. Data were then analyzed two ways. First, the average total percent injury of all plants combined in the native and conventional properties was compared in 2008 and 2009 using ANOVA (SAS Institute 2009). Second, the percent of area exhibiting each type of injury (defoliation caused by leaf-chewing insects and discoloration caused by leaf-sucking insects) in each treatment was compared using ANOVA. These data were further examined using the Tukey-Kramer honestly significant difference (HSD) test (SAS Institute 2009). All reported results are untransformed means ± SEM. 22

31 Chapter 3 RESULTS Project 1: Effectiveness of Natural Enemies In 2008, three larval insect species tobacco hornworm, cabbage looper, and bagworm were monitored for survival in each of the six property pairs (replicates), for a total of 12 monitored trials (Table 1). No significant differences in days until death occurred in any of the trials (Table 2). Tobacco hornworm larvae were again monitored in 2009, along with white-marked tussock moth larvae (Table 2). Differential survival occurred among tobacco hornworms, which survived 4.23 ± 0.11 days in conventional properties and 4.84 ± 0.25 days in native properties. A Log-Rank test showed this to be a significant difference in survival (χ 2 = 4.033, df = 1, P = ), while a Wilcoxon test did not (χ 2 = , df = 1, P = ). Survival of white-marked tussock moth did not differ (Table 2). 23

32 Table 2. Survival (no. of days) of herbivores in native and conventional landscapes in 2008 and Herbivores were placed on host plants in native and conventional landscapes and counted until the remaining survivors reached 0. A P-value of 0.05 or less was considered significant (*). Year Herbivore 2008 cabbage looper Average Days until Death ± SEM Log-Rank Wilcoxon Conventional Native Chi-square P-value Chi-square P-value 8.12 ± ± bagworm 11.4 ± ± tobacco hornworm 2009 tobacco hornworm 2009 whitemarked tussock moth 6.71 ± ± ± ± * ± ± When blocked by property location (Ambler, Glenside, Media, Chester Springs, Newtown Square, Millersville), bagworms in 2008 survived significantly longer on the conventional property than on the native property in Chester Springs, as well in Newtown Square, according to a Wilcoxon test (Table 3). Conversely, bagworms were sustained longer on the native property in Media, where they survived 14.9 ± 1.66 days, as opposed to 9.2 ± 0.52 days in the conventional property (Table 3). Differential survival occurred among cabbage loopers in 2008, with larvae surviving longer on the native properties in Chester Springs, Glenside, and Millersville and longer on the conventional properties in Ambler and Newtown Square (Table 3). No difference in survival of cabbage loopers existed in Media in 2008 (Table 3). Difference in survival of tobacco hornworms in 2008 occurred in one trial, with individuals surviving longer on the native Millersville property 24

33 than on the conventional property (Table 3). In 2009, hornworms were again sustained longer on the native property in Millersville, as well as in Ambler (by a Log-Rank test) and Newtown Square (Table 3). Only one trial led to differential survival of whitemarked tussock moths in 2009 individuals survived longer in the native Glenside property, ± 1.56 days, than in the conventional one, 3.1 ± 0.29 days (Table 3). Combining years, insect species, and locations, properties landscaped with natives provided a more hospitable environment for insect larvae than did those landscaped conventionally, with individuals surviving 7.70 ± 0.18 days versus 7.2 ± 0.15 days, respectively (Table 3). This difference was significant by a Log-Rank test (χ 2 = , df = 1, P = ), but not significant by a Wilcoxon test (χ 2 = , df = 1, P = ). 25

34 Table 3. Survival (no. of days) of herbivores in native and conventional landscapes in 2008 and 2009, by property location. Herbivores were placed on host plants in native and conventional landscapes and counted until the remaining survivors reached 0. A P-value of 0.05 or less was considered significant (*). Tobacco hornworms were not monitored in Glenside in 2009 because of homeowner interference. 26

35 Project 2: Arthropod Community Comparisons A total of 54,456 arthropods were collected during the course of this study. 11,088 herbivores and 12,232 natural enemies were collected from native properties, while conventional properties supported 18,058 herbivores and 13,078 natural enemies. Though not significant, native properties supported a higher abundance of natural enemies relative to herbivores than conventional properties in every sampling period. There was no significant difference in the abundance of predator/parasitoid arthropods between native and non-native landscapes in July 2008 (F 1,338 = , P = ). Herbivores (F 1,338 = , P = ), however, were significantly more abundant in conventionally landscaped properties than on those landscaped with natives (Figure 1A). 27

36 No. of arthropods/100g leaf (dw) No. of arthropods/100g leaf (dw) No. of arthropods/100g leaf (dw) A. July 2008 B. August * Herbivores Natural Enemies 0 Herbivores Natural Enemies C. July * * Herbivores Natural Enemies Figure 1 A,B,C. The mean arthropod abundance (number) / sample (in herbaceous samples) or 100 g leaf (dw) (in shrub and tree samples) of arthropod trophic guilds (herbivores and natural enemies) sampled in July 2008 (A), August 2008 (B) and July 2009 (C) in conventional (gray bars) and native (white bars) properties. Note: Y-axes differ between years. A P-value less than 0.05 was considered a significant difference (*). 28

37 When observed by stratum, it was only the herbaceous layer in conventional properties that produced more herbivorous arthropods than their native counterparts (Table 4). There was no significant difference in the abundance of arthropods between treatment landscapes in the shrub or tree layer, though abundance tended to be higher in conventional properties (Figure 2A). Arthropod diversity in the herbaceous layer in July 2008, however, was higher on native properties. Populations of herbivores (F 1,118 = , P = ), natural enemies (F 1,118 = , P = ), and subsequently the total diversity (F 1,118 = , P < ) of arthropods were significantly more diverse on native properties, despite being higher in abundance on conventional landscapes (Figure 4A). Species richness of these populations, with the exception of natural enemies, also significantly outweighed those in conventional properties (Figure 3A). No other significant differences in diversity or species richness existed in this sample, except for natural enemy populations in the shrub layer, which were more diverse in conventional properties than in native ones (F 1,103 = , P = )(Table 4). In August 2008, the diversity of natural enemies in the herbaceous layer was again higher in native properties, ± 0.036, than in conventional properties, ± (F 1,118 = , P = ). There were no other differences in the abundance, diversity, or species richness of herbivores, natural enemies, or total arthropods in the herbaceous, shrub, or tree layer between the treatment landscapes in August 2008 (Figure 2B, 3B, 4B, 5B, and Table 4). 29

38 No. arthropods No. arthropods No. arthropods A. July 2008 Conventional Native * Herbivores Natural Enemies Herbivores Natural Enemies Herbivores Natural Enemies Herbaceous Shrub Tree B. August 2008 Conventional Native Herbivores Natural Enemies Herbivores Natural Enemies Herbivores Natural Enemies Herbaceous Shrub Tree C. July 2009 Conventional Native Herbivores Natural Enemies * Herbivores Natural Enemies Herbivores Herbaceous Shrub Tree * Natural Enemies Figure 2 A,B,C. The mean abundance (number) of herbivores and natural enemies (number) / sample (in herbaceous samples) or 100 g leaf (dw) (in shrub and tree samples) from native (gray triangle) and conventional (black circle) properties in July 2008 (A), August 2008 (B) and July 2009 (C). Note: Y-axes differ between years. A P-value less than 0.05 was considered a significant difference (*). 30

39 No. arthropod species No. arthropod species No. arthropod species A. July 2008 Conventional Native * Herbivores Natural Enemies Herbivores Natural Enemies Herbivores Natural Enemies B. August 2008 Herbaceous Shrub Tree Conventional Native Herbivores Natural Enemies Herbivores Natural Enemies Herbivores Natural Enemies C. July 2009 Herbaceous Shrub Tree Conventional Native * * Herbivores Natural Enemies Herbivores Natural Enemies Herbivores Natural Enemies Herbaceous Shrub Tree Figure 3 A,B,C. The mean species richness (number of species) of herbivores and natural enemies (number) / sample (in herbaceous samples) or 100 g leaf (dw) (in shrub and tree samples) from native (gray triangle) and conventional (black circle) properties in July 2008 (A), August 2008 (B) and July 2009 (C). Note: Y-axes differ between years. A P- value less than 0.05 was considered a significant difference (*). 31

40 Arthropod diversity Arthropod diversity Arthropod diversity A. July 2008 Conventional Native B. August 2008 Herbivores * * Natural Enemies C. July 2009 Conventional Herbivores Native * Natural Enemies Conventional Herbivores Native * Natural Enemies Figure 4 A,B,C. The diversity, calculated by Simpson s diversity index, of herbivores and natural enemies / herbaceous sample from native (gray triangle) and conventional (black circle) properties in July 2008 (A), August 2008 (B) and July 2009 (C). A P-value less than 0.05 was considered a significant difference (*). 32

41 Arthropod diversity Arthropod diversity Arthropod diversity A. July 2008 Conventional Native * Herbivores Natural Enemies Herbivores Natural Enemies Shrub Tree B. August 2008 Conventional Native Herbivores Natural Enemies Herbivores Natural Enemies Shrub Tree C. July 2009 Conventional Native * * 0 Herbivores Natural Enemies Herbivores Natural Enemies Shrub Tree Figure 5 A,B,C. The diversity, calculated by Simpson s diversity index, of herbivores and natural enemies / gram leaf (dw) of shrub and tree samples from native (gray triangle) and conventional (black circle) properties in July 2008 (A), August 2008 (B) and July 2009 (C). A P-value less than 0.05 was considered a significant difference (*). 33

42 The herbaceous layer in native properties again supported a more diverse population of predator and parasitoid arthropods in July 2009 (F 1,118 = , P = )(Figure 4C). A new finding emerged, however, in that the shrub layer in conventional properties supported not only a higher abundance of arthropods, both phytophagous and creophagous, but also significantly more diverse populations and with higher species richness than their native counterparts (Figure 2C, 3C, 5C). When vegetative strata were combined, conventional properties also hosted higher abundances of arthropod herbivores, ± 9.48 individuals per 100 grams of leaves (dw)(f 1,353 = , P = ), natural enemies, ± 2.73 (F 1,353 = , P = ), and total arthropods, ± (F 1,353 = 8.25, P = ), than native treatments (23.55 ± 9.34, ± 2.69, and ± individuals per 100 grams of leaves (dw), respectively). All other differences in abundance, diversity, and species richness were not significant (Table 4). 34

43 Table 4. Average arthropod abundance and standard error / sample (in herbaceous samples) or 100 g leaf (dw) (in shrub and tree samples), diversity / sample (in herbaceous samples) or 1 g leaf (dw) (in shrub and tree samples), and species richness / sample (in herbaceous samples) or 100 g leaf (dw) (in shrub and tree samples) on conventional and native properties in July 2008, August 2008 and July A P-value of 0.05 or less was considered a significant difference (*). Sample July 2008 August 2008 July 2009 Trophic Guild Herbivores Natural Enemies Herbivores Natural Enemies Herbivores Natural Enemies Vegetative Average Abundance ± SEM Average Diversity (SEM) Average Species Richness (SEM) Stratum Conventional Native P-value Conventional Native P-value Conventional Native P-value Herbaceous ± ± * ± ± * 7.37 ± ± * Shrub ± ± ± ± ± ± Tree ± ± ± ± ± ± Herbaceous ± ± ± ± * ± ± Shrub ± ± ± ± * ± ± Tree ± ± ± ± ± ± Herbaceous ± ± ± ± ± ± Shrub ± ± ± ± ± ± Tree ± ± ± ± ± ± Herbaceous ± ± ± ± * ± ± Shrub ± ± ± ± ± ± Tree ± ± ± ± ± ± Herbaceous ± ± ± ± ± ± Shrub ± ± * ± ± * ± ± * Tree ± ± ± ± ± ± Herbaceous ± ± ± ± * ± ± Shrub ± ± * ± ± * ± ± * Tree ± ± ± ± ± ±

44 Project 3: Aesthetic Injury to Landscape Plants In 2008, the average combined aesthetic injury to plants in native properties was 7.24% ± 0.19, and 6.56% ± 0.19 in conventional properties (Figure 6A), which was a significant difference (F 1,9588 = , P = ). In 2009, however, plants were significantly more damaged on conventional properties (F 1,9589 = = P < )(Figure 6B). In fact, the results had reversed so much that, when averaged together, with the years combined, AIL was higher on conventional properties, 6.65% ± 0.14, than native properties, 6.17% ± 0.14, to a significant degree (F 1,19198 = , P = ). The percent of each type of aesthetic herbivore injury was compared between the native and conventional landscapes in 2008 and In 2008, there was significantly more discoloration damage caused by leaf-suckers in the native landscapes than in the conventional landscapes (F 1,9588 = , P < )(Figure 6A). Defoliation damage caused by leaf-chewers, however, was higher in conventional properties (F 1,9588 = , P = ). In 2009, both discoloration (F 1,9589 = , P < ) and defoliation (F 1,9589 = , P < ) damage were higher in conventional properties (Figure 6B). These findings were all significant via Tukey-Kramer HSD, and no average injury levels exceeded the 10% threshold. When separated by vegetative stratum shrub or tree the shrub layer in conventional properties sustained significantly more defoliation damage in 2008 (F 1,4798 = , P = )(Figure 7A). Trees on native properties, however, received significantly more discoloration damage (F 1,4788 = , P < 0.001), leading to total 36

45 damage being higher on these properties (F 1,4788 = , P < 0.001)(Figure 7A, 6A). Discoloration damage was not significantly different among property types in the shrub layer in 2008 (F 1,4798 = , P = ), and defoliation damage was not significantly different among property types in the tree layer (F 1,4788 = , P = )(Figure 7A). Overall, aesthetic damage to shrubs was not significantly different among property types in 2008 (F 1,4798 = , P = )(Figure 7A). In 2009, defoliation damage was again higher on shrubs in conventional properties than in native properties (F 1,4798 = , P = )(Figure 7B). Unlike 2008, however, trees on conventional properties also sustained significantly higher defoliation damage than those in native properties (F 1,4789 = , P = )(Figure 7B). Additionally, discoloration damage caused by leaf-sucking insects was significantly higher on conventionally landscaped properties trees than on native properties (F 1,4789, P < )(Figure 7B). No difference between property types in discoloration damage was seen on shrubs (F 1,4798 = , P = )(Figure 7B). Again, no average injury levels exceeded the aesthetic injury threshold of 10%. Data from all three projects can be found at 37

46 Aesthetic Injury (%) Aesthetic Injury (%) A * * Discoloration Defoliation Total * B * * * Discoloration Defoliation Total Figure 6 A,B. Mean aesthetic injury (%) of conventional (gray bars) and native (white bars) properties in 2008 (A) and 2009 (B). The percent injury is presented as total damage and by injury type (discoloration and defoliation). A P-value less than 0.05 was considered a significant difference (*) between native and conventional landscapes within an injury category. Note: all levels of injury are below the Aesthetic Injury Threshold of 10%. 38

47 Aesthetic Injury (%) Aesthetic Injury (%) A B * * * Discoloration Defoliation Total Discoloration Defoliation Total Shrub Tree * * * * * Discoloration Defoliation Total Discoloration Defoliation Total Shrub Tree Figure 7 A,B. Mean aesthetic injury (%) of shrubs and trees in conventional (gray bars) and native (white bars) properties in 2008 (A) and 2009 (B). The percent injury is presented as total damage and by injury type (discoloration and defoliation). A P-value less than 0.05 was considered a significant difference (*) between native and conventional landscapes within an injury category. Note: all levels of injury are below the Aesthetic Injury Threshold of 10%. 39