THE EFFECTS OF TURFGRASS CULTIVAR DIVERSITY AND COMPOSITION ON HERBIVORE FITNESS AND BEHAVIOR
By
ETHAN DOHERTY
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA 2018
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© 2018 Ethan Doherty
To my sister’s courage, my mother’s dedication, and the memory of the family we’ve lost these past few years
ACKNOWLEDGMENTS
I would like to express my gratitude to my advisor, Dr. Adam Dale, who gave me this opportunity, and was a tremendous help along the way. I would also like to thank my committee members, Dr. Heather McAuslane and Dr. Rob Meagher, for their guidance and support. Moreover, Amy Rowley’s assistance in providing fall armyworms was an enormous help throughout.
Thanks also go to the UF Turfgrass and Ornamental Entomology Lab.
Specifically, I would like to thank Rebecca Perry, Alex Locastro, Eddie Shuker, Jason
Chen, Hailee Smith and Tanner Felbinger for lending a hand, and Nicole Benda for her advice. I would also like to acknowledge Dr. James Colee for his assistance with these statistical analyses, and Mark Kann for managing our turf plots at Citra.
Lastly, the love of my friends, my family, and my partner creates associational resistance. Growing next to you has made me stronger - and less susceptible to pests.
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TABLE OF CONTENTS
page
ACKNOWLEDGMENTS ...... 4
LIST OF TABLES ...... 7
LIST OF FIGURES ...... 8
LIST OF ABBREVIATIONS ...... 10
ABSTRACT ...... 11
CHAPTER
1 LITERATURE REVIEW ...... 12
The Importance of Turfgrass ...... 12 Turfgrass Biology ...... 15 Turfgrass Insect Pest Management ...... 17 Fall Armyworm ...... 19 Plant Diversity ...... 20 Plant Diversity as a Management Tactic ...... 23
2 DIET MIXING OR ASSOCIATIONAL RESISTANCE: CULTIVAR DIVERSITY AFFECTS HERBIVORE FITNESS AND BEHAVIOR ...... 25
Introduction ...... 25 Method and Materials ...... 29 Study Organism ...... 29 Experimental Design ...... 30 No-Choice Laboratory Experiment ...... 30 Limited-Choice Greenhouse Experiment ...... 31 Spodoptera frugiperda Host Choice ...... 33 Statistical Analyses ...... 34 Results ...... 35 No-Choice Laboratory Experiment ...... 35 Limited-Choice Greenhouse Experiment ...... 37 Spodoptera frugiperda Host Choice ...... 38 Discussion ...... 38
3 TURFGRASS CULTIVARS INTERACT TO GENERATE ASSOCIATIONAL RESISTANCE OR SUSCEPTIBILITY TO AN INSECT HERBIVORE ...... 56
Introduction ...... 56 Methods and Materials...... 59 Study Organism ...... 59
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Study Design ...... 60 No-Choice Experiment ...... 61 Limited-Choice Experiment ...... 61 Statistical Analyses ...... 63 Results ...... 64 Effects of S. secundatum Cultivars in Monoculture ...... 64 No-choice laboratory experiment ...... 64 Limited-choice greenhouse experiment ...... 64 Effects of Diet Based on Presence of a Single Cultivar ...... 64 No-choice laboratory experiment ...... 64 Limited-choice greenhouse experiment ...... 65 Effects of Cultivar Pairs Relative to the Single Cultivar Components ...... 65 No-choice laboratory experiment ...... 65 Limited-choice greenhouse experiment ...... 66 Effects of Diet Based on Presence of Cultivar Pairs ...... 66 No-choice laboratory experiment ...... 66 Limited-choice greenhouse experiment ...... 67 Discussion ...... 67
4 CONCLUSIONS ...... 88
APPENDIX
A NO-CHOICE RESULTS ...... 90
B LIMITED-CHOICE RESULTS ...... 92
LIST OF REFERENCES ...... 94
BIOGRAPHICAL SKETCH ...... 104
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LIST OF TABLES
Table page
2-1 One-way ANOVA results for the effects of S. secundatum cultivar diversity on S. frugiperda fitness measures in laboratory no-choice feeding experiments. .... 43
2-2 One-way ANOVA results for the effects of S. secundatum cultivar diversity on male and female S. frugiperda fitness measures in laboratory no-choice feeding experiments...... 44
2-3 One-way ANOVA results for the effects of S. secundatum cultivar diversity on S. frugiperda fitness measures in greenhouse associational resistance experiments...... 45
3-1 All M1, M2, and M4 treatment cultivar or cultivar mixture compositions...... 72
3-2 Effects of S. secundatum cultivar monocultures on S. frugiperda life history traits in no-choice (top panel) and limited-choice (bottom panel) experiments. .. 73
3-3 Effects of S. secundatum cultivar monocultures on S. frugiperda herbivory in the limited-choice greenhouse experiment...... 74
A-1 All statistically significant and nearly significant compositional comparisons in the no-choice experiment...... 90
B-1 All statistically significant and nearly significant compositional comparisons in the limited-choice experiment...... 92
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LIST OF FIGURES
Figure page
2-1 Grass clippings placed in a rearing tray for the no-choice larval diet mixing experiment ...... 46
2-2 The process for producing mixed cultivar pots as used in limited-choice and host selection experiments ...... 47
2-3 Microcosms of the limited-choice experiment placed within a mesh cage ...... 48
2-4 Larval host choice experimental design with plantings arranged within buckets (A) and then covered with mesh (B) ...... 49
2-5 Oviposition choice experimental design with plantings arranged within the cage (A) and an example of an egg mass (B) ...... 50
2-6 Effects of S. secundatum cultivar diversity and S. frugiperda sex on S. frugiperda life history traits in no-choice laboratory experiments ...... 51
2-7 Female-biased sex ratio increases with cultivar diversity. Error bars represent standard error ...... 52
2-8 Mean percent S. frugiperda herbivory across S. secundatum cultivar diversity treatment groups ...... 53
2-9 Effects of S. secundatum cultivar diversity on S. frugiperda life history traits and herbivory in greenhouse experiments ...... 54
2-10 Mean number of colonizing S. frugiperda on each S. secundatum cultivar diversity treatment ...... 55
3-1 Mean S. frugiperda larval weight (A) and survival weight (B) measured across all S. secundatum diversity levels (M1, M2, M4) based on the presence of single cultivars ...... 75
3-2 Mean S. frugiperda survival rate (A) and percent herbivory (B) measured across all S. secundatum diversity levels (M1, M2, M4) based on the presence of single cultivars ...... 76
3-3 Mean larval weight of S. frugiperda when reared on each cultivar and each cultivar paired with ‘Classic’ ...... 77
3-4 Mean S. frugiperda days to pupation of larvae reared on mixtures that include ‘Classic’, ‘ Palmetto’ or the ‘Classic’-‘Palmetto’ pairing (A), and mean survival rate of S. frugiperda reared on ‘Classic’, ‘Floratam’ or ‘Classic’ and ‘Floratam’ (B) ...... 78
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3-5 Mean S. frugiperda larval weight measured across each S. secundatum cultivar, and its pairing with ‘Bitterblue’ ...... 79
3-6 (A) Mean days to eclose of S. frugiperda larvae reared on plantings containing ‘Captiva’ compared with that of larvae reared on plantings containing ‘Classic’ and ‘Captiva’; (B) mean survival rate of S. frugiperda larvae reared on ‘Seville’ compared to those reared on ‘Bitterblue’ and ‘Seville’ ...... 80
3-7 Mean percent herbivory measured across S. secundatum cultivars and those cultivars in pairs, with a focus on statically significant reductions in plant damage ...... 81
3-8 Mean S. frugiperda larval weight when reared on S. secundatum cultivar pairs that produced the largest and smallest larval weights ...... 82
3-9 Mean days to pupation of S. frugiperda larvae reared on the S. secundatum cultivar pairs that produced the longest and shortest days to pupation ...... 83
3-10 Mean survival rate of S. frugiperda larvae reared on S. secundatum cultivar pairs that produced the highest and lowest survival rates ...... 84
3-11 Mean survival rate of S. frugiperda larvae reared on S. secundatum cultivar pairs that produced the highest and lowest survival rates ...... 85
3-12 Mean percent herbivory of S. frugiperda larvae reared on S. secundatum cultivar pairs that yielded the most and least damage ...... 86
3-13 Mean percent herbivory of S. frugiperda larvae reared on S. secundatum mixtures and monocultures, with each cultivar represented by a letter: (a) ‘Bitterblue’, (b) ‘Captiva’, (c) ‘Classic’, (d) ‘Floratam’, (e) ‘Palmetto’, and (f) ‘Seville’ ...... 87
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LIST OF ABBREVIATIONS
ANOVA Analysis of variance
DF Numerator degrees of freedom
DFDen Denominator degrees of freedom
IPM Integrated pest management.
L/D Light/dark
NTEP National turfgrass evaluation program m/f Male/female
SEM Standard error of the mean
RH Relative humidity
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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science
THE EFFECTS OF TURFGRASS CULTIVAR DIVERSITY AND COMPOSITION ON HERBIVORE FITNESS AND BEHAVIOR
By
Ethan Doherty
August 2018
Chair: Adam G. Dale Major: Entomology and Nematology
Manipulating plant diversity and composition have been identified as potential cultural control tools that regulate arthropod communities in natural and agricultural landscapes. However, no studies have examined its application in warm season turfgrasses, a dominant plant type in southern U.S. urban landscapes. We examined the effects of St. Augustinegrass (Stenotaphrum secundatum) cultivar diversity and composition on fall armyworm (Spodoptera frugiperda). Cultivar diversity affected S. frugiperda life history traits through diet mixing, and herbivory through associational resistance. Cultivar diversity also affected S. frugiperda host choice, as larvae colonized monocultures more readily than mixtures of two cultivars or mixtures of four. Moreover, the composition of a cultivar mixture altered life history traits dependent upon specific cultivar interactions. Effects on larval weight, survival, sex ratio, development rate, and herbivory demonstrate that warm season turfgrass cultivar diversity and composition can affect herbivore response and warrant further investigation as an IPM tool.
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CHAPTER 1 LITERATURE REVIEW
The Importance of Turfgrass
Over 80% of people live in urban areas (United States Census Bureau 2010), and these landscapes are rapidly growing (Sukhdev et al. 2013). Turfgrasses (Poaceae) are arguably the most common plants in urban landscapes, and have become increasingly so in the U.S. since the progression of urban sprawl in the 1970s (Beard and Green 1994). These plants provide human health, economic, and ecological benefits by creating recreational space, increasing aesthetic and property value, and providing wildlife habitat (Beard and Green 1994, Beard 2000). Compared to impermeable surfaces that often dominate urban environments, turfgrasses provide valuable ecosystem services like heat dissipation, soil erosion control, carbon sequestration, dust stabilization, and reducing noise, light glare, and water runoff (Beard and Green 1994). Unfortunately, impervious surfaces are rapidly replacing vegetation in urban landscapes, which means maximizing the services provided by the vegetation that remains is critical (Nowak and Greenfield 2018).
Turfgrass production, sale, and management in the U.S. is a multibillion dollar industry, generating $62 billion and 825,000 jobs in 2005 (Haydu et al. 2006). Covering
164,000 km2 of the U.S., it is the largest irrigated crop by three-fold. Of this, 75% is residential, commercial, or institutional lawns (Milesi et al. 2005, Held and Potter 2012).
Due to high aesthetic standards and cultural requirements, turfgrasses are often highly managed through irrigation, fertilization, and pesticide applications (Held and Potter
2012). Improper turfgrass maintenance, like over-application of management inputs
(e.g. fertilizer), is not sustainable and can pose risks to humans and the environment
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(Innes 2013). This is particularly important in Florida, where the human population is rapidly increasing, the turfgrass industry is the largest in the country, and natural resource conservation is a top priority (Hodges and Stevens 2010, Chang et al. 2016).
Turfgrasses face many pests, including diseases, nematodes, weeds, and insects (Stowell and Gelernter 2001, Busey 2003, Crow 2005, Held and Potter 2012).
Aesthetic standards create a low tolerance for pest damage, which drives strict management. This is especially relevant in Florida’s warmer tropical/subtropical climate, where pest pressure often persists year-round (Diffenbaugh et al. 2008). Thus, professional management services frequently use calendar-based cover-spray applications of pesticides as preventive “insurance” treatments (Potter 2005). Several studies have shown these practices incur unnecessary costs (Reinert 1974, Raupp et al. 1992, 2001a, Muchovej and Rechcigl 1994, Potter 2005). For instance, Raupp et al.
(2001a) examined the effects of cover spray frequency on oak trees (Quercus palustris) and found that trees sprayed more frequently were more likely to suffer from reduced natural enemy populations and be infested with damaging scale insects. Over-irrigation and over-fertilization, often in response to pest damage, can strain natural resources, while introducing risks associated with nutrient runoff into waterways (nutrient saturation in water bodies; Muchovej and Rechcigl 1994). As Florida’s human population and urban land use expand, these problems will be compounded (Carr and Zwick 2016). A long-standing approach to more economically and environmentally sustainable pest control is integrated pest management (IPM), which manages pests with a combination of biological, chemical, and cultural control strategies (Bottrell 1979).
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Cultural control practices manipulate the environment to promote plant health and reduce pest pressure. Methods of cultural control include: site preparation, plant selection, irrigation, mowing practices, thatch removal, and nutrient management.
Improper cultural practices like over-fertilization can increase thatch accumulation and nitrogen content in plant tissue, providing habitat conducive to insect pests (Busey and
Snyder 1993, Davidson and Potter 1995). Among the most important cultural practices in urban landscapes are proper plant selection and the identification of key plants
(Raupp et al. 1985). Key plants are commonly used to serve aesthetic purposes, but are also known to be frequently attacked by pests, which requires management (Raupp et al. 1985). Identifying key plants can inform cultural practices by identifying specific plants or areas to target pest control efforts, which can reduce non-target risks.
Selecting plants that are more tolerant or resistant to known pests, can reduce pest habitat and food sources (Raupp et al. 1985). Unfortunately, when selecting commercially available turfgrasses to plant, pest resistant cultivars are rarely an option, particularly in warm season (C4 photosystem) species.
Despite progress in urban landscape IPM over recent decades, the turfgrass industry remains heavily reliant on insecticide use (Held and Potter 2012). This is due to several factors, including a dearth of scientific evidence supporting ecologically sound cultural management strategies that can be practically implemented. Most pest management operators and their clientele select management programs based on price and convenience. One challenge is that many IPM strategies require additional time commitments or attention to site-specific needs, which may not be practical for many businesses that manage multiple properties. Chemical control on the other hand can
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provide immediate relief of pests and is easy to understand as a management approach. While frequent, cover-spray pesticide applications may rid a lawn of the targeted pest, it can also eradicate non-target beneficial organisms, cause insecticide resistance within pest populations, and lead to secondary pest outbreaks, among other indirect effects (Ripper 1956, Potter 1994, Kunkel et al. 1999, Desneux et al. 2007). In addition, recent research suggests non-target effects of the most commonly used insecticides may pose risks to pollinators, which is driving social and regulatory changes
(Brittain et al. 2010, Potts et al. 2010, Larson et al. 2013, Goulson et al. 2015). Surveys suggest that consumers are environmentally concerned and open to alternative management approaches (Matheny et al. 2009). However, for new management strategies to be widely adopted, they must be cost effective, easy to implement, and meet industry and consumer needs.
Turfgrass Biology
The family Poaceae, grasses, has an estimated 600 genera and 7,500 species
(Gould and Shaw 1983). Warm-season grasses are heat-tolerant and best suited for growth during warmer periods due to their photosynthetic framework. During colder periods, warm-season grasses may be killed or enter dormancy. Conversely, cool- season grasses are cold-tolerant, and adapted to growth during cooler, moist periods of the year. Therefore, warm-season grasses are commonly grown in tropical and sub- tropical zones, while cool-season grass can be found in more temperate climates. Most grasses are heterozygous and self-incompatible, meaning they sexually reproduce but cannot self-reproduce, thus increasing their genetic diversity through cross-pollination
(Casler and Duncan 2003). In contrast, commercially produced warm-season grasses like zoysiagrass (Zoysia spp.), St. Augustinegrass [Stenotaphrum secundatum (Walt.) 15
Kuntz], bermudagrass [Cynodon dactylon (L)], and bahiagrass (Paspalum notatum
Flugge) are highly selected for and bred into groups of cultivated varieties (cultivars) commonly grown from vegetative cuttings. Cultivars that are maintained and mowed as turf are referred to as turfgrasses (Held and Potter 2012). These grasses are commonly grown where people live for environmental and human benefits like health, recreation, and economic value.
Stenotaphrum secundatum, St. Augustinegrass (also known as “buffalo grass”,
“Charleston grass”, and “San Augustin grass”) is the most common turfgrass in Florida, making up over 50% of all sod production (Hodges and Stevens 2010). The first records of St. Augustinegrass plantings occurred in Florida in 1880 (Casler and Duncan 2003).
By 2001, St. Augustinegrass was the primary turf grown in Florida, occupying 70% of lawns (Casler and Duncan 2003). The success of St. Augustinegrass can be attributed to a few key features. Under reduced sunlight, St. Augustinegrass has greater yield than other warm season alternatives including bahiagrass, bermudagrass, centipedegrass, and zoysiagrasses (Smith and Whiteman 1983). St. Augustinegrass can also grow in a wider variety of soil conditions and within a relatively wide pH range of 4.5-8.5 (Busey
1990). It is well-adapted to humid areas, and moderately resistant to weed establishment because of its tight leaf canopy.
While St. Augustinegrass can thrive in many conditions, it is not without weaknesses. It has no rhizomes (underground stems), leaving its stolons (aboveground horizontal plant stems) exposed and vulnerable to herbivores. As a result, damage to aboveground vegetation is not readily replaced, which slows recovery to herbivore damage and may allow weed invasion. In addition to poor defoliation recovery, St.
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Augustinegrass also exhibits poor traffic and compaction tolerance. To compensate for this, a few cultivars have been selected with shorter internodes that have higher wear tolerance (Casler and Duncan 2003). Additional irrigation is also often necessary during extended periods of drought to prevent plant death (Augustin and Peacock 2011).
Finally, St. Augustinegrass maintenance can be a challenge due to its numerous insect pests, which frequently require management.
Turfgrass Insect Pest Management
Turfgrass insect pests like mole crickets, caterpillars, and chinch bugs cause aesthetic and economic damage to lawns, golf courses, and other landscape types
(Held and Potter 2012). Some pest groups, such as Crambid grass moths, consume above-ground plant tissue, causing direct and rapid defoliation. Other damage is more obscure, like that caused by root-feeding beetle grubs, or sap-feeding chinch bugs.
Historically, few practical management options have been available other than chemical control. However, as we learn more about these insects, new IPM tactics can be developed. For example, Listronotus maculicollis Kirby, is a weevil pest of annual bluegrass (Poa annua L.) that overwinters as an adult in taller grass typical of golf course roughs or out-of-play areas, but moves to the edges of fairways as it prepares to oviposit (Diaz et al. 2008). By surveying for adults at course edges, turfgrass managers can take timely action to prevent larval damage to high value plant material. The black cutworm, Agrotis ipsilon (Hufnagel), typically oviposits on the tips of grass blades. By collecting grass clippings during regular mowing when adult moths are active, over 80% of eggs can be removed (Williamson and Potter 1997).
Integrated pest management strategies may also utilize turfgrass biology and symbioses to manage insect pests. Some cool-season grasses, like fescue (Festuca 17
arundinaceae L.), contain endophytic fungi that produce alkaloids and increase herbivorous insect resistance (Koppenhöfer and Fuzy 2003, Koppenhöfer et al. 2003).
Other grasses, like the cool-season perennial ryegrass (L. perenne L.), produce phenolic compounds that defend against herbivore attack (Moser et al. 1996). Although warm-season turfgrasses are not known to produce defensive alkaloids or phenolic compounds, some do have structural defenses that may increase the plant’s tolerance to herbivory (Rangasamy et al. 2009a, Pannkuk 2011). Some cultivars of St.
Augustinegrass in particular, defend against herbivores through thick-walled sclerenchyma cells, which can prevent sap-feeding herbivores from penetrating plant tissue (Rangasamy et al. 2009a). Studies have also identified other herbivore defense mechanisms in warm-season turfgrasses, including compensatory photosynthesis, defensive compounds, and oxidative stress tolerance (Heng-Moss et al. 2006,
Rangasamy et al. 2009a, 2009b).
Unfortunately, insect pest resistance does not often rank highly on the list of genotype selection criteria within turfgrass breeding programs like the National
Turfgrass Evaluation Program (Held and Potter 2012). This limited breeding effort limits the availability of commercially produced pest-resistant cultivars (Held and Potter 2012).
Despite this, there is a history of St. Augustinegrass breeding for pest resistance.
‘Floratam’ and ‘Floralawn’ St. Augustinegrass were both initially selected for their resistance to southern chinch bug, Blissus insularis Barber. However, 12 years after commercial release, southern chinch bug overcame this resistance, likely due to overplanting and widespread use of this cultivar (Busey and Center 1987). More recently, the University of Florida developed ‘Captiva’ St. Augustinegrass, which is also
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resistant to southern chinch bug (Rangasamy et al. 2009a). As of 2017, this resistance has not been overcome, although ‘Captiva’ is not widely used.
Fall Armyworm
The family Noctuidae, including the commonly known armyworms, cutworms, or owlet moths, is a diverse group that occupies a great variety of habitats. In North
America, there are an estimated 2,900 species (Lotts and Naberhaus 2014). Although the majority are not economic pests, many are, which affects multiple industries. The corn earworm/cotton bollworm, Helicoverpa zea (Boddie), is one of the most economically damaging noctuid pests, as the larvae attack and reduce yield in cotton, corn, soybeans, and grain sorghum (Quaintance and Brues 1905). Similarly,
Trichoplusia ni (Hübner) is an agricultural pest that feeds on broccoli, cabbage, cauliflower, Chinese cabbage, and many other vegetable crops (Robinson et al. 2002).
Both pests present ecological and economic costs, as they can disrupt agricultural production and existing ecosystems. Within turfgrasses there are several noctuid pests, including, Agrotis ipsilon (Hufnagel), Peridroma saucia (Hübner), Nephelodes minians
Guenée, and Mythimna (Pseudaletia) unipuncta (Haworth) (Potter and Braman 1991).
The most damaging noctuid moth in Florida turfgrasses is the fall armyworm,
Spodoptera frugiperda (J.E. Smith).
Spodoptera frugiperda is broadly distributed in the Americas, from New York to
Argentina (Capinera 2014). The species is present year-round in southern Florida, but
S. frugiperda does not undergo diapause, so populations decline in temperate regions during the winter (Sparks 1979). There are two strains of S. frugiperda, commonly called “corn strain” and “rice strain”. Rice strain S. frugiperda larvae behave as generalists as they feed on a variety of crops, but commonly prefer grasses (Sparks 19
1979). Rice strain larvae attack many species and cultivars of cool and warm season turfgrasses, including Stenotaphrum secundatum (Reinert et al. 1997), leaving large holes or completely defoliating grass leaves, which can lead to lawn death and weed invasion (Capinera 2014). Therefore, preventive insecticide applications are commonly used against this insect to avoid damage to lawns.
Plant Diversity
Effective preventive measures are necessary to preserve the services provided by St. Augustinegrass lawns and reduce management inputs. Current preventive management includes frequent broadcast insecticide applications (Held and Potter
2012). Several insecticide products can provide effective control within a short time, which makes chemical management an easy choice for professionals and satisfying for consumers (Reinert 1974). However, more sustainable alternatives may exist to reduce the need for insecticide applications, while improving turfgrass quality and services.
Pest-resistant plants can provide an effective and environmentally friendly tool for managing herbivorous insect pests. However, no warm season turfgrass cultivars resistant to S. frugiperda are known, and though pest resistance is an excellent IPM tool, it alone is not a long-term solution. This is because pests can overcome resistance as seen with ‘Floratam’ and the southern chinch bug (Busey and Center 1987). In addition, most resistant grasses are pest-specific, rather than resistant to multiple pests
(Held and Potter 2012). Therefore, a more integrated approach to pest management is needed.
Ecological evidence suggests that in both agricultural and natural settings, increasing intra- or interspecific diversity can provide broader resilience to pests or abiotic stresses and increase ecological stability (Tahvanainen and Root 1972, Root 20
1973, Riihimäki et al. 2005, Tooker and Frank 2012). Cool-season turfgrasses commonly used in temperate climates are regularly produced and planted as species or cultivar mixtures to improve turf quality and resilience to foot traffic or diseases (Fushtey and Taylor 1983, Dunn et al. 1994). Therefore, increasing the diversity of warm-season turfgrasses within a lawn may also provide benefits. However, the pest management potential of warm season turfgrass mixtures is little studied and no studies have investigated the effects of mixing warm season turfgrass cultivars.
Within agricultural systems, monocultures (one species or genotype) of crops are often planted for economic, aesthetic, or logistic reasons (Cook and Weller 2004, Ata et al. 2012, Dobbs and Potter 2013). Crop producers typically seek the most profitable species and cultivars for production, but also those that facilitate mass production and management. A single cultivar is typically selected for mass production, rather than smaller productions of several cultivars because different varieties may have different nutritional and management needs or harvest requirements (Cook and Weller 2004, Ata et al. 2012, Dobbs and Potter 2013). These differences can create financial and physical limitations, which disincentivize the use of polycultures (multiple species or genotypes).
Plant diversity is the variability within and among species of plants (Andow 1991).
When compared to monocultures, polycultures can be more ecologically stable and less prone to herbivore outbreaks (rapid increases in abundance; Elton 1958, Pimentel
1961). The mechanisms behind this were first described by Tahvanainen and Root
(1972) as “associational resistance”, which is the reduction in a focal plant’s vulnerability to herbivores due to its proximity to another plant (Tahvanainen and Root 1972). This
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may result from the neighboring plant hiding the focal plant visually (Rausher 1981), by diluting chemical signals (Schoonhoven et al. 1981), or other mechanisms. Neighboring plants can also hinder herbivore movement, reducing the herbivore’s ability to access the focal plant (Holmes and Barrett 1997). Associational resistance can be a useful pest management tool for agronomic crops, turfgrasses, and ornamental plants (Tooker and
Frank 2012).
While the benefits of plant diversity were being explored, the costs of monocultures were also receiving attention. Root (1973) proposed the Resource
Concentration Hypothesis, which postulates that mixed stands of plants will incur less herbivory than monocultures, or stands with less diversity. Moreover, specialized herbivores will attain greater relative densities in less diverse stands, resulting in a high herbivore abundance, as was determined by investigating a specialist flea beetle
(Tahvanainen and Root 1972). Larger patches of monocultures will exhibit larger immigration rates, while smaller patches will exhibit larger emigration rates (Root 1973).
These effects may occur because:
1. Herbivores whose life requirements are met within the stand are more likely to
remain and reproduce (e.g. specialists), while herbivores with broad host
ranges are more likely to disperse (e.g. generalists).
2. Concentrated hosts provide conditions that make well-suited herbivores more
likely to remain.
In contrast, diverse plantings can also lead to associational susceptibility, where a focal plant’s vulnerability to herbivores increases due to its neighbors (Barbosa et al.
2009). For instance, Utsumi et al. (2011) explored how increasing the diversity of tall
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goldenrod (Solidago altissima L.) plantings affected aphid populations and found an increase in aphid abundance. They proposed this was due to the source-sink hypothesis, where aphids moved from a plant with high rates of aphid reproduction to one with lower rates. Moreover, plant diversity can also improve herbivore fitness by giving them access to nutrients that their preferred hosts do not provide (Mody et al.
2007, Kotowska et al. 2010). Kotowska et al. (2010) examined the effect of intraspecific diversity in Arabidopsis thaliana L. on the cabbage looper, T. ni (Hübner), and found that the insect’s biomass and survival rate increased with the diversity of its plant diet.
Plant Diversity as a Management Tactic
Herbivore outbreaks are more common within urban landscapes than their surrounding rural or natural areas, though the reasons remain unclear (Raupp et al.
2010). One potential reason is that plant communities in urban landscapes are highly manipulated and often dominated by one or few species (Raupp et al. 2006, 2010).
Raupp et al. (2001b) suggests that vegetation diversity is a more influential factor on ecological stability than host abundance. There are positive correlations between plant diversity and pest species richness, but the strength of the correlation seems dependent on whether herbivores are generalists or specialists (Raupp et al. 2001b).
While most studies examining plant diversity have investigated the effects of plant species diversity, others have examined the effects of plant genotypic diversity and found similar results (Mody et al. 2007, Kotowska et al. 2010, Grettenberger and
Tooker 2016, 2017). For instance, Mody et al. (2007) investigated why Chrysopsyche imparilis Aurivillius seemed to regularly switch between host trees of the same species.
They found that when the diet of C. imparilis included leaves from multiple Combretum fragrans F. Hoffm. genotypes, there was a significant increase in fecundity. In contrast, 23
host plant intraspecific diversity can also hinder herbivore population growth (Shoffner and Tooker 2013), and thus have applications as a pest management tactic.
In addition to diversity, some studies have shown that the composition of a plant mixture can be a more important factor driving herbivore response than plant diversity per se (Finch et al. 2003, Grettenberger and Tooker 2017). For example, increasing wheat (Triticum aestivum L.) genotypic diversity can reduce aphid (Rhopalosiphum padi
L.) reproduction rates. However, the presence of this effect was often dependent upon the varieties of wheat included in the mixture (Grettenberger and Tooker 2017).
Therefore, if plant genotypic diversity is to be used as a pest management tool, investigating the effects of genotypic mixture composition may help optimize its use.
In this study, we investigated the effects of S. secundatum cultivar, cultivar diversity, and cultivar mixture composition upon S. frugiperda life history, herbivory, and behavior. Moreover, we make a distinction between cultivar diet diversity and cultivar planting diversity. Diet diversity indicates the cultivars consumed by an herbivore, while cultivar planting diversity indicates plants grown together and available for herbivore consumption. We hypothesized that as diet diversity increased, S. frugiperda fitness would also increase. Second, we hypothesized that as cultivar planting diversity increased, S. frugiperda fitness and herbivory would decrease. Finally, we hypothesized that these measures would be differentially affected by the compositon of cultivars present in different mixtures.
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CHAPTER 2 DIET MIXING OR ASSOCIATIONAL RESISTANCE: CULTIVAR DIVERSITY AFFECTS HERBIVORE FITNESS AND BEHAVIOR
Introduction
Turfgrasses occupy over three times the surface area of any other irrigated crop in the continental U.S. (Milesi et al. 2005). Not only do these plants have a substantial footprint, but they provide human health, economic, and ecological benefits by creating recreational space, improving aesthetics, increasing property values, reducing soil erosion, and dissipating heat (Beard and Green 1994, Beard 2000). Unfortunately, turfgrasses are frequently damaged by herbivorous insects, which reduces plant benefits and introduces environmental risks (Held and Potter 2012). For example, pest- damaged lawns do not reduce surface runoff or filter nutrients as efficiently as healthy, actively growing turfgrass, and therefore require supplemental maintenance to provide these services (Mugaas et al. 1997, Telenko et al. 2015, Shaddox et al. 2016). These problems are further exacerbated in warmer climates, where pest pressure persists year-round and damage can occur rapidly (Diffenbaugh et al. 2008). As a result, pest management professionals frequently make calendar-based, cover-spray applications of pesticides to preventively control pests (Potter 2005, Held and Potter 2012). Such indiscriminate use of pesticides can harm beneficial insects, cause pesticide resistance, and trigger secondary pest outbreaks (Reinert 1974, Raupp et al. 1992, 2001a,
Muchovej and Rechcigl 1994, Potter 2005).
Cultural control practices, like proper plant selection, can effectively reduce pests and reliance on pesticides. Among the most important cultural management practices in urban landscapes are the identification of key plants that require recurrent management
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and key pests that repeatedly cause plant damage and require control (Raupp et al.
1985). Proper plant selection by avoiding key plants or using them appropriately can reduce pests and management inputs while improving plant health (Raupp et al. 1985,
Dale et al. 2016). Unfortunately, not only are urban plant communities often composed of key plants, but in many cases are dominated by one or few species (Raupp et al.
2006, 2010). Therefore, concentrated plantings of one or few hosts may create conditions that facilitate herbivore outbreaks and predispose landscapes to environmental and economic losses (Root 1973, Raupp et al. 2006, 2010). This was illustrated among U.S. urban forests following the invasion and spread of emerald ash borer (Agrilus planipennis Fairmaire) and Dutch elm disease, Ophiostoma ulmi Brasier, which had major economic and human health impacts (Raupp et al. 2006, Donovan et al. 2013). Thus, the negative effects of plant pests can be reduced if urban landscapes are composed of greater plant diversity (Elton 1958, Pimentel 1961, Santamour 2002).
Plant diversity, or the variability within and among species, can also have direct effects on herbivorous arthropods (Andow 1991, Raupp et al. 2010). The mechanisms behind this were first described by Tahvanainen and Root (1972) as “associational resistance”, where a host plant’s vulnerability to an herbivore is reduced due to its proximity to a neighboring plant (Tahvanainen and Root 1972). This may result from neighboring plants obscuring the focal plant visually (Rausher 1981), chemically
(Schoonhoven et al. 1981), or by other mechanisms (Holmes and Barrett 1997). Root
(1973) also proposed the Resource Concentration Hypothesis, which postulates that mixed stands of plants will incur less herbivory than monocultures, as specialized herbivores will attain greater relative densities in less diverse stands (Tahvanainen and
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Root 1972), and larger patches of monocultures will exhibit larger immigration rates
(Root 1973). In contrast, herbivore performance can also increase with diet diversity
(Bernays et al. 1994, Hägele and Rowell-Rahier 1999), as a more diverse plant diet could provide nutrients that preferred hosts do not provide in isolation. For example,
Kotowska et al. (2010) examined the effect of intraspecific diversity in Arabidopsis thaliana on Trichoplusia ni (Hübner) and found that the insect’s biomass and survival rate increased with the genotypic diversity of its plant diet.
Turfgrasses are arguably the most common plant type in urban and residential landscapes throughout the United States. Warm season turfgrasses (C4 photosystem) dominate lawns in warmer climates like the southern U.S. (Held and Potter 2012), and are selectively bred to exhibit superior aesthetics, growth rates, and maintenance needs
(Fraser et al. 2012). To conserve these traits, and in the absence of viable seeds, warm season turfgrasses are produced through vegetative propagation and planted as sod or plugs (Casler and Duncan 2003), which creates lawns of genotypic monocultures.
Several cool season (C3 photosystem) turfgrass species are produced or planted as inter- or intraspecific mixtures as a tactic to enhance traffic (Newell et al. 1996) or disease tolerance (Dunn et al. 2002). However, warm season turf species are never intentionally planted as mixtures. Simmons et al. (2011) found that interspecific mixtures of up to seven warm season grasses improved leaf density and resistance to weed invasions. However, because aesthetic and maintenance uniformity drive the turfgrass industry, planting different species together is less marketable and likely more difficult to produce and maintain.
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Fortunately, evidence from agricultural and natural systems suggests that intraspecific diversity can also provide pest management and agronomic benefits
(Riihimäki et al. 2005, Tooker and Frank 2012, Grettenberger and Tooker 2016). For instance, Grettenberger and Tooker (2016) found that intraspecific diversity in wheat
(Triticum aestivum L.) reduced aphid [Rhopalosiphum padi (L.)] size and reproduction.
Moreover, mixing cultivars of the same species is more likely to meet turfgrass consumer and industry standards due to logistics associated with producing and maintaining different species, and the fact that commercially available cultivars have been bred to meet these standards (Tooker and Frank 2012).
Warm season turfgrasses in the U.S. are primarily composed of 10 species within seven genera (Trenholm and Unruh 2005, Haydu et al. 2006). A single species,
Stenotaphrum secundatum (Walt.) Kuntz, was first planted in Florida in 1880 and now makes up 70% of lawns (Casler and Duncan 2003) and over 50% of commercial sod produced in the state, the largest turfgrass industry in the U.S. (Hodges and Stevens
2010). Importantly, over 80% of S. secundatum grown in Florida is a single cultivar,
‘Floratam’, which suggests that over 170 km2 of Florida lawns are highly genetically similar (Satterthwaite et al. 2009). We used S. secundatum as our host plant species because our results may inform immediate integrated pest management (IPM) strategies with high economic and environmental impact.
We investigated the effects of increasing cultivar diversity of S. secundatum, the most common warm season turfgrass (Satterthwaite et al. 2009), on the fitness and host selection of a key herbivorous turfgrass pest, Spodoptera frugiperda (J.E. Smith)
(Lepidoptera: Noctuidae). We hypothesized that under no-choice conditions, S.
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frugiperda fitness would increase with S. secundatum genotypic diversity due to diet mixing. In contrast, we hypothesized that when reared in a stand of planted S. secundatum, S. frugiperda fitness would decrease with increasing plant diversity due to associational resistance. Finally, based on the resource concentration hypothesis, we predicted that host colonization would be biased towards monocultures over more diverse plantings.
Method and Materials
Study Organism
Fall armyworm is a common pest of turfgrasses throughout the U.S. and a frequently used model organism for ecological research (Erb et al. 2011, Unbehend et al. 2013, Jakka et al. 2014). We used S. frugiperda (rice strain), which were reared at the United States Department of Agriculture, Agricultural Research Service (Gainesville,
FL) on the soybean-based “multiple species diet” (Southland Products, Lake Village,
AR) in rearing trays (Frontier Agricultural Science, Newark, DE; model RT32W) at 25°C under a 16/8 (L/D) photoperiod. Upon egg hatch, first instar neonates were moved into growth chambers (Percival Scientific, Perry, IA) maintained at 27C, 70% RH, and 14:10
(L/D) at the University of Florida (Gainesville, FL).
The most commonly produced S. secundatum cultivars in Florida, USA are
‘Captiva’, ‘Classic’, ‘Floratam’, ‘Bitterblue’, ‘Seville’, and ‘Palmetto’, which make up over
98% of S. secundatum produced in the state (Satterthwaite et al. 2009). In 2016, we obtained sod (ca. 40 m2) of each cultivar from a certified Florida sod producer (JB
Farms, Lake Placid, FL) and planted each as standalone 40 m2 plots at the University of
Florida Plant Science Research and Education Unit (Citra, FL). Throughout the duration of this experiment, we collected plant material from these established cultivar plots and 29
planted it in pots (15 cm diameter) within a greenhouse (min: 21C, max: 43C) for use in laboratory and greenhouse experiments. Potting media was one-part potting soil
(SunGro Profession Growing Mix, Agawam, MA) and one-part pure sand. Plants were watered as needed and fertilized once every two weeks (MiracleGro, 24-8-16 %N-P-K).
Experimental Design
Using the six previously listed S. secundatum cultivars, we created three different treatment groups, referred to henceforth as M1, M2, and M4. The M1 treatment consisted of three replicates of all cultivars planted or provided in monoculture (n=18).
The M2 treatment consisted of all unique combinations of two cultivars planted together or provided in sequence from the pool of six, which resulted in 15 unique combinations
(n=15). The M4 treatment consisted of all combinations of four cultivars planted together or provided in sequence from the pool of six, also resulting in 15 unique combinations
(n=15). The effects of plant diversity were evaluated based on the diversity treatment, rather than the cultivar composition of each treatment replicate.
No-Choice Laboratory Experiment
To determine the effect of diet mixing on S. frugiperda fitness, we performed a no-choice feeding experiment in growth chambers (Percival Scientific, Perry, IA) maintained at 27C, 70% RH, and 14:10 (L/D). First, we assigned treatments to individual 4x4x1 cm cells in rearing trays (Frontier Agricultural Science, Newark, DE) and placed one randomly selected first instar S. frugiperda larva into each cell (Figure
2-1). We assigned one larva to each treatment replicate described above, resulting in
18 M1, 15 M2, and 15 M4 individuals. Next, we provided caterpillars with grass clippings of their respective treatment. Clippings came from S. secundatum monocultures planted
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in pots and maintained in a greenhouse as described above. Clippings in each cell were replaced every two days with fresh plant material. Caterpillars in the M2 and M4 treatments received one cultivar at a time in a random order, such that each caterpillar in the M2 treatment fed on two different cultivars every four days and each caterpillar in the M4 treatment fed on four different cultivars over an eight-day period. Caterpillars were always given more clippings than they could consume within the two days. To control for the physical disruption of changing diets, caterpillars in the monoculture treatment also had their diet replenished every two days, but with the same cultivar that was replaced. This experiment was repeated four times, totaling 72 M1, 60 M2, and 60
M4 S. frugiperda individuals.
To determine the effects of intraspecific S. secundatum diet mixing on S. frugiperda fitness, we recorded larval body weight, days to pupation, pupal weight, days to eclosion, and the sex of each individual. We measured larval body weight (mg) after nine days of feeding, as it can be an important indicator of fecundity and survival
(Berger et al. 2008). Development rate is also important as it can affect the timing and application of pest control efforts, the duration of feeding and associated damage that may occur, as well as the rate at which populations can increase. Therefore, we recorded development time from first instar larva to pupa and first instar larva to adult for each individual. We also measured sex ratio and survival rate for all treatments since these factors can affect population composition and growth rates.
Limited-Choice Greenhouse Experiment
To conduct a more realistic evaluation of the effects of S. secundatum cultivar diversity on S. frugiperda fitness, we designed a greenhouse experiment where we planted each diversity treatment in pots (15 cm diameter) using rooted cuttings of each 31
S. secundatum cultivar. As with their source pots, soil was composed of one-part potting mix (SunGro Profession Growing Mix, Agawam, MA) and one-part sand. Plants were watered as needed and fertilized once every two weeks (MiracleGro, Marysville, OH,
24-8-16 %N-P-K). These plantings mirrored the M1, M2, and M4 treatments used in the growth chamber no-choice feeding experiment (Figure 2-2).
Potted plants were trimmed weekly to approximately 7.5 cm above the soil to promote new and lateral growth. Once planted cuttings had established and filled in each pot (3-6 weeks after planting), we placed two second instar S. frugiperda larvae into each pot and enclosed them within a microcosm made of a cylindrical (10.1 cm height, 14.3 cm top diameter) 32 oz supermarket container (Genpak, Charlotte, NC) with mesh (Casa Collection Organza Fabric, Joann, Hudson, OH) hot-glued in place of the closed end. These containers were adhered to the rim of each pot with Parafilm
(Pechiney, Chicago, IL). Six to nine potted plants (two to four of each treatment) were placed into a 60x60x60 cm PCV pipe cage covered with mesh (Casa Collection
Organza Fabric, Joann, Hudson, OH) and kept within a greenhouse (min: 21C, max:
43C) (Figure 2-3).
As in the no-choice diet mixing laboratory experiment, we measured multiple S. frugiperda life history traits to determine if there was an effect of cultivar diversity on insect fitness. Eleven to twelve days after hatching, we measured larval weight by placing individual larvae in weigh boats on a scale. We recorded the number of days to eclosion, sex, and survival for each individual. To evaluate the effect of S. frugiperda feeding on cultivar diversity treatments, we also recorded visual estimates of percent herbivory for each pot 11 to 12 days after egg hatch. Estimates were made in
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increments of 10% covering the whole plant, noting window feeding and chewing damage. Chewing damage began from the side of a blade, so missing parts of the blade were apparent. When chewing damage removed the top half of a blade, estimates were informed by the remains’ similarities to the expected shape of a full blade. We repeated this experiment four times. A fifth experimental replicate was started, but not completed. Thus, we recorded sex ratios, survival rates, and development rates for 72 M1, 60 M2, and 60 M4 replicates. Larval weights and percent herbivory were recorded for 90 M1, 75 M2, and 75 M4 replicates.
Spodoptera frugiperda Host Choice
After determining effects of S. secundatum genotypic diversity on S. frugiperda life history traits, we sought to identify how cultivar diversity may affect larval and adult host choice. To test this, we planted rooted cuttings of each S. secundatum cultivar in
9.5 cm diameter pots as in the previously described greenhouse development experiment. This included 15 M1 pots with no single cultivar represented more than three times, and one pot of each unique cultivar combination for M2 (n=15) and M4
(n=15) treatments, totaling 45 pots.
To evaluate larval host choice, we randomly selected one pot of each treatment without replacement and placed it into a bucket (30 cm diameter) filled with pure sand.
Each buck contained three pots and the top rim of each pot was flush with the sand and
24 cm below the lip of the bucket. Pots were placed at the edges of the bucket equidistant from each other, resulting in approximately 6 cm from the center of the bucket to each planting. We repeated this process until all pots were used across 15 buckets, each containing one pot of each cultivar diversity treatment (Figure 2-4). Next, we placed 10 third-instar S. frugiperda larvae onto the sand in the center of each bucket 33
equidistant from the pots. We then recorded the number of larvae on each S. secundatum treatment planting 10 minutes, 24 hours, 48 hours, and 72 hours after introduction.
To determine if S. secundatum diversity affects S. frugiperda adult oviposition choice, we used the same treatment arrangement as in the larval host choice, however, instead of buckets, each set of three pots (15 cm diameter) were placed 33 cm apart in a 60x60x60 cm PCV pipe cage covered with white mesh (Casa Collection Organza
Fabric) and kept within a greenhouse (min: 21C, max: 43C) (Figure 2-5). We placed
10 newly eclosed adults (5 male, 5 female) into the center of each mesh cage. After 72 hours, we recorded the number of egg masses on each treatment, weighed each mass, and calculated total egg mass weight per pot. This experiment and the larval host choice experiment were each repeated three times.
Statistical Analyses
We used analyses of variance (ANOVA) to determine the effect of cultivar diversity on each measure of S. frugiperda fitness during the lab no-choice and greenhouse limited-choice experiments (JMP Pro 13.1, SAS Institute, Cary, NC).
Replicate was used as a random covariate in all analyses. We used logistic regression and Chi2 to test for effects of cultivar diversity on S. frugiperda sex ratios. To detect effects of S. secundatum diversity level, time, and their interaction during the host choice experiments, we conducted repeated-measures ANOVA on the number of larvae present per treatment at each time interval. Paired t-tests were performed for post-hoc pairwise means comparisons of host choice for each treatment level at each time point.
ANOVAs were also used to examine the effect of treatment on oviposition measures
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(egg mass quantity, average egg mass weight, total egg mass weight). When ANOVA identified a significant main effect, we used Tukey’s HSD post-hoc tests for pairwise means comparisons to detect differences between treatments. Alpha was set at 0.05 for all analyses.
Results
No-Choice Laboratory Experiment
In the laboratory no-choice feeding experiment, S. frugiperda nine-day old larval weight ranged from 106.4 mg to 576.4 mg with a mean (SEM) of 320.4 (6.02) mg.
Pupal weight ranged from 115.5 mg to 247.0 mg with a mean of 189.2 (1.54) mg. We found that when provided with a known diet of S. secundatum clippings, cultivar diversity significantly affected larval weight such that body size decreased as cultivar diversity increased (Table 2-1, Figure 2-6a). More specifically, larval weight was over
10% greater in the M1 treatment (n= 72, Mean=341.110.61 mg) compared to M2 (n=
60, Mean=308.810.62 mg) and M4 (n= 60, Mean=303.710.31 mg) treatments.
Although larvae from both mixed cultivar treatments were smaller than M1 larvae, only the M4 treatment was significantly less (Table 2-1). There was a nearly significant effect following the same trend of cultivar diversity on pupal weight (Table 2-1, Figure 2-6b).
Overall sex ratio across all treatments was slightly female biased (n=156,
Mean=0.430.04 m/f). Sex ratio was nearly significantly affected by cultivar diversity, with a trend suggesting an increasing female bias as S. secundatum cultivar diversity increased (Table 2-1). The M1 treatment had a relatively balanced sex ratio (n=60,
Mean=0.55 0.06 m/f), while the M2 (n=46, Mean=0.380.07 m/f) and M4 treatments
(n=50, Mean=0.35 0.07 m/f) were female biased (Figure 2-7). The population
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percentage of females steadily increased with cultivar diversity (M1: 39%; M2: 48%; M4:
53%), while the percentage of surviving males dropped sharply between monoculture and mixed cultivar treatments (M1: 45%; M2: 28%; M4: 28%).
Interestingly, when separated by sex, the trend for larval weight persisted for females only. Average female larval weight was 305.6 (8.41) mg. Larval weight in the
M1 (n= 28, Mean=339.316.90 mg) group was significantly higher than both the M2 (n=
29, Mean=282.811.33 mg) and M4 groups (n=32, Mean=296.813.62 mg). Male larvae were about 10% larger than females on average (Mean=344.710.71 mg), but were not affected by S. secundatum cultivar diversity. Male pupae (n= 67,
Mean=194.022.16) were also larger than female pupae (n= 89, Mean=186.352.22 mg), but neither were affected by cultivar diversity.
On average, S. frugiperda larvae took 13.9 (0.09) days to develop from a first instar larva to a pupa and 22.3 (0.12) days to an adult. Diversity level did not affect days to pupation (Table 2-1, Figure 2-6c). However, there was a significant effect of cultivar diversity on days to eclosion. Larvae developing on the M2 treatment developed most rapidly (n= 46, Mean=21.90.23 days), while larvae developing on the M1 group took the longest (n= 60, Mean=22.60.19 days), and M4 fell in the middle (n= 50,
Mean=22.30.19 days) (Figure 2-6d). When separated by sex, there was no significant difference in days to pupation or eclosion. On average, males took the same amount of time to pupate (Mean=13.90.12 days, Mean=13.70.09 days) as females, but took nearly two days longer than females (Mean=21.60.14 days) to eclose
(Mean=23.20.13 days).
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Limited-Choice Greenhouse Experiment
In the greenhouse feeding experiment, percent herbivory ranged from 10-90% with a mean of 471.40%. Herbivory was significantly different between treatments
(Table 2-3) and decreased with increasing cultivar diversity (Figure 2-8). On average,
M1 plantings had 12% more herbivory than M4 plantings and 5% more herbivory than
M2 plantings.
Larval weight ranged from 13.70 mg to 607.10 mg, with a mean of 314.04 10.70 mg. Larval weight was also significantly affected by treatment, such that M2 larvae
(n=51, Mean=353.2417.49 mg) were larger than those from the M4 treatment (n= 61,
Mean=298.1117.72 mg) (Figure 2-9a). The M1 treatment (n= 62, Mean=297.4719.26 mg) was no different from either diversity group. Development rate was affected by diversity level as well. Spodoptera frugiperda feeding on the M4 treatment
(Mean=24.880.35 days) took significantly longer to eclose than those feeding on M2
(Mean=24.020.33 days), although this difference was less than one day. Time to eclosion for M1 S. frugiperda was no different from either treatment group
(Mean=24.50.35 days). Survival rate of S. frugiperda was not associated with S. secundatum diversity level (Table 2-3).
Sex ratio was affected by S. secundatum cultivar diversity, with treatment populations becoming less female biased as cultivar diversity increased. The M1 S. frugiperda had the greatest female-bias (n=61, Mean=0.360.06 m/f), while the M4 group had the most balanced sex ratio (n=52, Mean=0.480.06 m/f). The M2 group had a minor female-bias (n=50, Mean=0.410.06 m/f). The percentage of surviving males was greater in the M2 and M4 treatments compared to the M1 treatment (M1: 19%; M2:
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28%; M4: 27%); however, the percentage of surviving females was consistent across treatment groups (M1: 35%; M2: 38%; M4: 32%).
Spodoptera frugiperda Host Choice
On average S. frugiperda larval colonization declined as S. secundatum cultivar diversity increased. Count totals often fell short of 10 as not all larvae colonized pots.
Spodoptera frugiperda larval host choice was significantly affected by S. secundatum cultivar diversity (F2,478.6=46.97, p<0.001). Moreover, the number of colonizing larvae did not change with time (F3,480.1=0.15, p=0.928) and this was the case for all treatment levels as there was no interaction (F6,478.6=0.41, p=0.871). This treatment difference was apparent 10 minutes after larval introduction and persisted for the duration of the 72 hour experiment (Figure 2-10). Significantly more larvae colonized the M1 plantings than the M2 or M4 plantings, and more larvae colonized M2 than M4 (Figure 2-10).
On average, adult female S. frugiperda deposited 74.33 egg masses per experimental replicate and 1.65 masses per planting. Average egg mass weight was
10.95 mg and average total egg mass weight per planting was 25.80 mg. There were no apparent trends in egg number or mass across S. secundatum diversity levels. Thus, we did not detect an effect of S. secundatum cultivar diversity on the number
(F2,88=0.53, p=0.590), average weight (F2,64.36=0.21, p=0.809), or total weight
(F2,70.03=0.04, p=0.959) of egg masses deposited by adult female S. frugiperda.
Discussion
Previous research has identified positive (Kotowska et al. 2010) and negative
(Grettenberger and Tooker 2017) effects associated with increasing host plant genotypic diversity on herbivores via diet mixing and associational resistance, respectively. We pitted these hypotheses against each other to determine the
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methodological and biological significance of increasing the genotypic diversity of an herbivore’s diet. Interestingly, we found that experimental methodology (forced vs. optional diet mixing) yielded different results, which has implications for future studies.
More importantly, we found that manipulating S. secundatum cultivar diversity affected
S. frugiperda herbivory and life history traits by changing body size, development rates, colonization rates, and sex ratios. Although we did not measure these effects through multiple generations, each of these results may have important implications for insect biology and pest management in turfgrass lawns, a ubiquitous vegetation type in urban ecosystems.
We hypothesized that when S. frugiperda is forced to diet mix, its fitness would increase (Mody et al. 2007, Kotowska et al. 2010). Increasing S. secundatum genotypic diversity affected S. frugiperda life history traits, but primarily in females and sometimes in ways that suggested reduced fitness. Our results seem to mirror those of Wetzel and
Thaler (2018), who found that diet mixing by Colorado potato beetle [Leptinotarsa decemlineata (Say)] reduced body mass. Through diet mixing, female larval weight decreased in the M2 and M4 treatments compared to monocultures. Additionally, though only nearly significant (p=0.0697), sex ratios became increasingly female-biased as cultivar diversity increased. This may have been driven by a trend of increased female survivorship and reduced male survivorship. Increasing cultivar diversity also reduced development time from larva to adult; however, this effect was only apparent when males and females were combined for analysis. Female S. frugiperda eclose more quickly than males (Pashley et al. 1995). Thus, the overall effect on development
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rate may, in part, be driven by the female-biased sex ratios in the M2 and M4 treatments.
When S. frugiperda developed on a monoculture of a S. secundatum cultivar, female larvae grew larger, but fewer survived to adults. Therefore, diet mixing seems to have triggered a tradeoff: increased survivorship at the cost of reduced body size. This tradeoff was not apparent in S. frugiperda males where survivorship sharply declined in response to diet mixing and larval weight was unchanged. Ultimately, our results suggest that diet mixing creates female-biased populations composed of smaller individuals. These findings support those in Grettenberger and Tooker (2016, 2017), where increased genotypic diversity reduced female aphid body size and population growth rates. However, our observed effects became less apparent and changed when
S. frugiperda larvae were not forced to diet mix, but freely fed among diverse plantings.
We hypothesized that when S. secundatum cultivars were planted together and larvae could choose what to feed upon, increasing genotypic diversity would negatively affect life history traits due to associational resistance (Barbosa et al. 2009).
Interestingly, effects of cultivar diversity did not become more apparent as cultivar diversity increased, but rather were only apparent when larvae developed on M2 plantings. Larvae in the M2 plantings were significantly larger and developed faster than those in the M4 plantings, while neither cultivar diversity level was different than M1.
Therefore, our results suggest that a high level of cultivar diversity is detrimental to S. frugiperda fitness, but low levels of genotypic diversity may be beneficial. Further investigation into the mechanisms behind this pattern may help explain if these trends would persist in actual planted lawns.
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The level of diet mixing that occurred within each treatment is one potential explanation of our results. Our lab study found that forcing diet mixing to a 50/50 ratio
(in M2) or a 25/25/25/25 ratio (in M4) was detrimental. However, we did not determine if diet mixing to a lesser degree (e.g. 75/25) has a different effect. It has been shown that when herbivores are able to choose hosts, they can reduce the negative effects of diet mixing by optimizing the composition of their diet (Wetzel and Thaler 2018). If a species can receive benefits from diet mixing, herbivore choice is one avenue for optimizing that diet. Within our limited-choice study, S. frugiperda may have selected an ideal balance in the M2 plantings, maximizing the beneficial effects of diet mixing by primarily feeding upon one cultivar and lightly feeding on a second. In the M4 treatment, which supports smaller quantities of more cultivars, feeding is more similar to the forced diet mixing experiment. Therefore, S. frugiperda must feed on more cultivars, effectively reducing any beneficial effects of diet mixing ratios.
It is unclear why larvae developing on M2 plantings were larger and developed more rapidly than the other two levels of diversity, but the resource availability hypothesis provides another potential explanation (Wilson and Tilman 1993, Endara and Coley 2011, Gianoli and Salgado-Luarte 2017). Cultivars in competition may devote more resources to growth, when they would otherwise devote resources to defense.
This reduction in plant defenses could aid S. frugiperda fitness, but at the same time the increased plant growth rate could compensate and mask herbivory, which aligns well with our observations. Moreover, while our no-choice study found a trend of increasing female-bias with genotypic diversity, our limited-choice study found an increased female bias in monoculture plantings. This shift was driven by improved male survival in the M2
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and M4 treatments, another potential effect of associational susceptibility (Barbosa et al.
2009, Wetzel and Thaler 2018).
The resource concentration hypothesis predicts that reducing the stand size of a preferred host planting by increasing plant diversity may reduce herbivore immigration, retention, and herbivory (Root 1973). Within a microcosm, reduced retention may appear as less time spent feeding and more time performing other behaviors. The time spent performing non-foraging behaviors due to the presence of multiple hosts would suggest associational resistance and result in less herbivory (Barbosa et al. 2009). In support of this, we found that S. frugiperda herbivory declined with increasing cultivar diversity such that M4 plantings had 12% less damage than M1 plantings (Figure 2-8).
Though herbivory has been shown to increase with diversity in other systems (Brezzi et al. 2017), our results are particularly important for S. secundatum lawns because less herbivore immigration and herbivory translates to greater aesthetic quality and less need for pesticides.
Other studies have found that plant composition is more important than diversity in its effects on herbivores (Finch et al. 2003, Grettenberger and Tooker 2017). For example, while Finch et al. (2003) found that many non-host plant species could effectively change herbivore behavior when paired with Brassica oleracea L., some were far more effective than others. Therefore, further experimentation and evaluation of our results may identify underlying factors contributing to the trends we observed. As it stands, our results suggest that increasing warm season turfgrass cultivar diversity could be an effective IPM tactic for lawn managers that may reduce the economic impact of insect pests and the environmental impact of pest management.
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Table 2-1. One-way ANOVA results for the effects of S. secundatum cultivar diversity on S. frugiperda fitness measures in laboratory no-choice feeding experiments. Parameter N DF DFDen F Ratio p-value Larval Weight 191 2 185 4.18 0.0167* Pupal Weight 178 2 172.1 2.49 0.0859 Days to Pupation 178 2 172 0.88 0.4159 Days to Eclosion 158 2 152.1 3.70 0.0270* Survival 192 2 186 0.49 0.6161 Sex Ratio 158 2 (X2) 5.22 0.0735 Asterisks (*) Indicate statistically significant results (p<0.05).
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Table 2-2. One-way ANOVA results for the effects of S. secundatum cultivar diversity on male and female S. frugiperda fitness measures in laboratory no-choice feeding experiments. Sex Parameter N DF DFDen F Ratio p-value Male Larval Weight 67 2 62.75 0.35 0.7044 Pupal Weight 67 2 62.43 1.25 0.2933 Days to Pupation 67 2 61.44 0.70 0.5026 Days to Eclosion 67 2 62.70 0.21 0.8116 Female Larval Weight 89 2 83.49 5.35 0.0065* Pupal Weight 89 2 83.99 0.65 0.5233 Days to Pupation 89 2 83.10 2.36 0.1012 Days to Eclosion 89 2 83.21 1.91 0.1552 Asterisks (*) indicate statistically significant results (p<0.05).
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Table 2-3. One-way ANOVA results for the effects of S. secundatum cultivar diversity on S. frugiperda fitness measures in greenhouse associational resistance experiments. Parameter N DF DFDen F Ratio p-value Larval Weight 170 2 167 3.61 0.0291* Days to Eclosing 163 2 157 3.86 0.0231* Percent Survival 192 2 186 1.77 0.1731 Sex Ratio 163 4 (X2) 14.00 0.0073* Percent Herbivory 240 2 233 13.29 <.0001* Asterisks (*) indicate statistically significant results (p<0.05).
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Figure 2-1. Grass clippings placed in a rearing tray for the no-choice larval diet mixing experiment.
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Figure 2-2. The process for producing mixed cultivar pots as used in limited-choice and host selection experiments.
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Figure 2-3. Microcosms of the limited-choice experiment placed within a mesh cage.
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Figure 2-4. Larval host choice experimental design with plantings arranged within buckets (A) and then covered with mesh (B).
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Figure 2-5. Oviposition choice experimental design with plantings arranged within the cage (A) and an example of an egg mass (B).
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Figure 2-6. Effects of S. secundatum cultivar diversity and S. frugiperda sex on S. frugiperda life history traits in no-choice laboratory experiments. Fitness measures include, nine-day larval weight (a), pupal weight (b), days to pupation (c), and days to eclosion (d). Error bars represent standard error. Different letters of the same case indicate statistical differences between treatment means (p<0.05). Asterisks over bars indicate a statistical difference between S. frugiperda sexes within a treatment. *p<0.05 **p<0.01 ***p<0.001.
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Figure 2-7. Female-biased sex ratio increases with cultivar diversity. Error bars represent standard error.
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Figure 2-8. Mean percent S. frugiperda herbivory across S. secundatum cultivar diversity treatment groups. Error bars represent standard error and different letters indicate statistical differences between treatment means (p<0.05).
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Figure 2-9. Effects of S. secundatum cultivar diversity on S. frugiperda life history traits and herbivory in greenhouse experiments. Fitness measures include, 11 or 12-day larval weight (A), days to eclosion (B), survival rate (C), and treatment population sex ratio (D). Error bars represent standard error. Different letters indicate statistical differences between treatments (p<0.05).
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Figure 2-10. Mean number of colonizing S. frugiperda on each S. secundatum cultivar diversity treatment. Error bars represent standard error. One asterisk (*) indicates M1 is statistically different from M2 and M4. Two asterisks (**) indicates a statistical difference between all three treatment groups.
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CHAPTER 3 TURFGRASS CULTIVARS INTERACT TO GENERATE ASSOCIATIONAL RESISTANCE OR SUSCEPTIBILITY TO AN INSECT HERBIVORE
Introduction
Cultural pest control tactics such as pest resistant plants or promoting plant health and defense through proper plant selection are critical components of integrated pest management (IPM) (Quisenberry 1990, Reinert et al. 2004a, Dale et al. 2016, Just et al. 2018). When used properly, cultural control strategies can reduce pests, pesticide use, and associated risks (Raupp et al. 1985). Unfortunately, as of 2018, there is only one commercially produced warm season turfgrass used in residential landscapes that has been documented as resistant to an herbivore pest (Rangasamy et al. 2009a,
2009b). This is important because Florida has the largest turfgrass industry in the U.S., and rapidly expanding urban landscapes, where lawns are the foremost vegetation
(Brown et al. 2005). An alternative tactic to pest resistant plants is manipulating plant diversity to increase a planting’s resilience or resistance to pests (Root 1973, Barbosa et al. 2009). By planting specific combinations of species or cultivars, growers can create associational resistance, where a less-preferred neighboring plant reduces herbivory or colonization on a favored host (Tahvanainen and Root 1972, Barbosa et al.
2009). In contrast, plant diversity can also create associational susceptibility, where a non-host plant makes a host more apparent to pests, or facilitates the pest’s consumption of a host (White and Whitham 2000, Barbosa et al. 2009). Therefore, promoting plant diversity may not be as effective as promoting specific combinations of plants.
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Identifying plant species and cultivars that are susceptible or resistant to herbivorous pests is an important tool for promoting plant health and ecosystem services in urban and residential landscapes (Raupp et al. 1985, Dale et al. 2016).
Plants that are less frequently colonized by pests or support fewer pests require less frequent pesticide applications and reduce non-target human and environmental health risks. Several studies have identified turfgrass species and genotypes resistant to key arthropod pests (Reinert et al. 1997, 2004b, Reinert and Engelke 2010). For example,
Reinert and Read (2004) found that multiple cultivars of Poa pratensis L., a cool season grass, were susceptible to Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), but also that many were highly resistant, causing over 89% mortality. Reinert et al.
(1997) found that several warm season turfgrass species and cultivars increased S. frugiperda mortality. However, all species and cultivars evaluated are either not used in residential lawns or are no longer commercially produced in the southeastern U.S.
Therefore, a limited number of species and cultivars of pest resistant plants are available, and the existing commercially available cultivars have unknown susceptibility.
Warm season turfgrasses (C4 photosystem) are selectively bred for qualities like pest resistance, aesthetic quality, and tolerance to environmental stressors (Brilman
2005, Fraser et al. 2012). These breeding efforts require years of research (Morris and
Shearman 1998), but often result in several unique genotypes of a given species that have adaptive and morphological variations. These chromosome-associated differences can affect the genotype’s susceptibility to herbivores and its ability to support other arthropods (Johnson et al. 2002, Casler and Duncan 2003). Thus, some cultivars may be highly susceptible to a pest, while others are resistant (Reinert et al. 2004a). To
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conserve the highly selected-for traits, all warm season turfgrasses are produced, planted, and maintained as cultivar (genotypic) monocultures. However, recent evidence suggests that increasing cultivar diversity of warm season turfgrasses may affect herbivore fitness and behavior (Chapter 2).
In agricultural and natural systems, there is substantial evidence suggesting that plant diversity reduces pests and promotes plant productivity (Finch et al. 2003,
Kotowska et al. 2010, Tooker and Frank 2012, Grettenberger and Tooker 2016). Finch et al. (2003) found that planting Brassica oleracea with non-host plants created associational resistance, which reduced the number of eggs oviposited by Delia radicum L. and reduced the pest’s ability to locate the host plant. However, some non- host plant species were more effective at this than others. Similar associational resistance can occur in genotypic mixtures. Grettenberger and Tooker (2016) found that increasing wheat (Triticum aestivum L.) genotypic diversity reduced aphid
(Rhopalosiphum padi L.) performance and reproductive fitness. On the other hand,
Kotowska et al. (2010) found genotypic diversity of Arabidopsis thaliana increased
Trichoplusia ni (Hübner) survival rates, but also plant productivity. Authors from both studies found that the effects of genotypic diversity on herbivore fitness were dependent on the identity of plant genotypes in the mixture. In fact, genotypic composition was more important than diversity, per se.
A logical approach to generating associational resistance is planting known resistant cultivars or species in combination with susceptible ones. However, no studies have documented the relative susceptibility or resistance for cultivars of the most common warm season lawn species, Stenotaphrum secundatum (Walt.) Kuntz, to an
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important herbivore pest, S. frugiperda. Here, we investigate the effects of commercially produced S. secundatum cultivars on S. frugiperda life history traits and herbivory to quantify relative susceptibility and resistance. Using the same S. secundatum cultivars, we investigated if cultivar composition within mixed cultivar plantings generates associational resistance or susceptibility to S. frugiperda. We hypothesized S. frugiperda fitness would be differentially affected by S. secundatum cultivars and that this effect would be conserved when planted in mixtures of other cultivars. More specifically, resistant cultivars would convey associational resistance, while highly susceptible cultivars would create associational susceptibility. We tested our hypotheses through a combination of laboratory and greenhouse experiments to determine if developing specific turfgrass cultivar mixture recommendations could be an important component of using diverse cultivar plantings as an IPM approach for managing urban and residential lawns.
Methods and Materials
Study Organism
Stenotaphrum secundatum is the most common turfgrass species in the southern
U.S. and makes up over 70% of Florida lawns (Casler and Duncan 2003, Hodges and
Stevens 2010). Within the largest turfgrass industry in the U.S. (Florida), over 80% of S. secundatum produced is one cultivar, ‘Floratam’ (Satterthwaite et al. 2009). However, there are several other commercially produced cultivars, including: ‘Bitterblue’, ‘Captiva’,
‘Classic’, ‘Palmetto’, and ‘Seville’. We obtained sod of each cultivar from a certified
Florida sod producer (JB Farms, Lake Placid, FL), and planted them in 40 m2 monoculture plots at the University of Florida Plant Science Research and Education
Unit (Citra, FL). For all laboratory and greenhouse experiments, we collected turfgrass
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from these field plots and planted it in pots (15 cm diameter) kept in a greenhouse
(ranging from 21-43C).
Spodoptera frugiperda is a well-studied model organism and a common herbivorous pest of S. secundatum (Erb et al. 2011, Unbehend et al. 2013, Jakka et al.
2014). Rice strain S. frugiperda were reared at the United States Department of
Agriculture, Agricultural Research Service (Gainesville, FL) at 25°C under a 16/8 (L/D) photoperiod in trays (Frontier Agricultural Science, Newark, DE; model RT32W) on a
“multiple species diet” (Southland Products, Lake Village, AR). After eggs hatched, neonates were stored in growth chambers (Percival Scientific, Perry, IA) until needed.
Growth chambers were kept at 27C, 70% RH, and 14:10 (L/D) at the University of
Florida (Gainesville, FL).
Study Design
Using the six S. secundatum cultivars listed above, we created three treatment groups: monocultures (M1), mixtures of two cultivars (M2), and mixtures of four cultivars
(M4). The M1 treatment resulted in a total of 18 M1 replicates (three replicates for each of the six cultivars). The M2 treatment included each unique combination of two cultivars from the pool of six, resulting in 15 M2 replicates. The M4 treatment included each unique combination of four cultivars from the pool of six, resulting in 15 M4 replicates (Table 3-1). Experiments were each replicated four times, resulting in 12 replicates of each S. secundatum cultivar planted in monoculture and four replicates of each unique cultivar combination of two and four. In result, each S. secundatum cultivar is represented among 16 different plant mixture compositions containing one, two, or four cultivars (Table 3-1).
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No-Choice Experiment
To determine the direct effects of turfgrass cultivar and cultivar mixture composition on S. frugiperda fitness, we performed a no-choice feeding experiment.
Treatments were randomly assigned to 4x4x1 cm cells in rearing trays (Frontier
Agricultural Science, Newark, DE; model RT32W) and included the 18 M1, 15 M2, and
15 M4 treatments described above. First instar S. frugiperda were randomly assigned to rearing cells containing S. secundatum clippings. Trays were placed into growth chambers (Percival Scientific, Perry, IA) maintained at 27C, 70% RH, and 14:10 (L/D).
Larvae were fed freshly cut S. secundatum clippings corresponding to their treatment every two days. Larvae in the M1 treatment received clippings from their single cultivar repeatedly, while M2 larvae received clippings from alternating two cultivars every two days. Larvae in the M4 treatment received one of four cultivars every two days, such that they received all four of their assigned cultivars after eight days. Larvae were always given more clippings than they could consume within the two-day period. This feeding sequence continued through S. frugiperda development from first instar larva to pupa. Pupae were left in rearing trays until they eclosed as an adult. To determine the effects of diet cultivar composition, we recorded S. frugiperda life history traits including larval body weight, time to pupation, time to eclosion, and survival rates. This experiment was repeated four times, resulting in 12 replicates of each cultivar planted in monoculture and four replicates of each cultivar mixture combination.
Limited-Choice Experiment
No-choice diet mixing is unlikely to represent field-relevant results, so we also measured S. frugiperda development and fitness in microcosms of mixed plantings, where S. frugiperda larvae could select what to feed on within a planting and cultivars 61
could interact. The M1 treatment was created by taking eight rooted cuttings from the source pot of a single cultivar and planting them together in a pot. The M2 treatment was created by taking four rooted cuttings from each of two cultivar source pots and planting them together. The M4 treatment was created by taking two rooted cuttings from each of four different cultivar source pots and planting them together. Therefore, each planting contained an equal initial representation of each cultivar contained within a mixture. Plantings were watered as needed and fertilized once every two weeks
(MiracleGro, Marysville, OH, 24-8-16 %N-P-K). Potting media was one-part potting soil
(SunGro Profession Growing Mix, Agawam, MA) and one-part pure sand. Pots were grown in for 3-6 weeks after planting, at which point they filled their pot and were ready for use in experiments.
Two second instar S. frugiperda larvae were placed into M1, M2, or M4 plantings in pots capped with a 32 oz supermarket container (Genpak, Charlotte, NC) with mesh
(Casa Collection Organza Fabric, Joann, Hudson, OH) hot-glued in place of the closed end. This container was secured to the rim of the pot by Parafilm (Pechiney, Chicago,
IL) to prevent larvae from escaping. Microcosms were kept in 60x60x60 cm PVC pipe cages covered with mesh (Casa Collection Organza Fabric, Joann, Hudson, OH) and kept in a greenhouse (min: 21C, max: 43C). Six to nine microcosms were kept in a cage, with two to three microcosms of each treatment randomly selected and placed in each cage. Eleven to twelve days after egg hatch, larval weight was measured and visual percent S. secundatum herbivory estimates were recorded. Upon eclosion, development time, and survival rate were recorded. This experiment was repeated four times, resulting in 12 replicates of each cultivar planted in monoculture and four
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replicates of each cultivar mixture combination. A fifth experimental replicate was started, but not completed beyond day 11. Thus, larval weight and percent herbivory were recorded for the fifth replicate, but not sex ratio, survival rate, and development rate.
Statistical Analyses
To determine the effects of S. secundatum cultivar on S. frugiperda, we conducted analyses of variance (ANOVA) on mean life history trait measures of larvae that developed on monoculture plantings. If ANOVA indicated a significant main effect, we conducted Tukey’s HSD post-hoc means comparisons to detect differences between
S. secundatum cultivars. To determine if cultivar effects were conserved or changed when planted with other cultivars (e.g. associational resistance or susceptibility), we created linear mixed effects models with turfgrass composition as a fixed effect and block as a random effect. Linear contrasts were then used to test for specific differences in S. frugiperda life history traits between turfgrass cultivars, cultivar pairings, and the presence or absence of each in mixtures. Specifically, we used data from M1, M2, and
M4 to determine the effects of diet based on the presence of a single cultivar in any mixture or monoculture. We also evaluated M2 and M4 mixtures to determine if cultivar pairings magnified or suppressed the effect of their single cultivar components. Finally, we compared S. secundatum cultivar pairs to determine which pairings generated associational resistance or susceptibility. JMP Pro 13.1 (SAS Institute, Cary, NC) was used for all analyses and alpha was set at 0.05.
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Results
Effects of S. secundatum Cultivars in Monoculture
No-choice laboratory experiment
Using data from the M1 treatment, we separated the effects of each S. secundatum cultivar provided as a monoculture (n=12). We found no effect of cultivar on S. frugiperda larval weight, time to eclosion, or survival rate. However, we did find an effect on time to pupation (F5,58.02=5.72, p=0.002), where larvae feeding on ‘Bitterblue’ took significantly longer to pupate than those reared on ‘Captiva’, ‘Floratam’, ‘Palmetto’, or ‘Seville’, but not ‘Classic’ (Table 3-2).
Limited-choice greenhouse experiment
We did not detect any differences between S. secundatum cultivars and their effects on S. frugiperda life history traits when planted in monoculture (M1) (Table 3-2,
Table 3-3).
Effects of Diet Based on Presence of a Single Cultivar
Due to the magnitude of means comparisons between cultivars and cultivar pairings, all results presented are statistically significant unless otherwise stated
(Appendix A-1; Appendix B-1).
No-choice laboratory experiment
The effect of each cultivar on S. frugiperda life history traits was analyzed based on its presence, regardless of the diversity treatment or the mixture composition in which it appeared. We found that under no-choice conditions, S. frugiperda larvae reared on diets containing ‘Classic’ developed into significantly smaller individuals compared to those reared on diets containing ‘Bitterblue’, ‘Captiva’, ‘Palmetto’, or
‘Seville’, and were nearly significantly smaller than those reared on ‘Floratam’
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(F1,139.4=3.83, p=0.052) (Figure 3-1). On average, larvae reared on diets containing
‘Classic’ were 40.91 mg smaller than larvae developing on any other cultivar, 13% of the average S. frugiperda body weight. In addition, S. frugiperda survival rate was highest when fed any diet containing ‘Floratam’ compared to any diet containing ‘Seville’
(Figure 3-1). There were no single cultivar-dependent differences in time to pupation, or time to eclosion through diet mixing.
Limited-choice greenhouse experiment
Analyses comparing cultivar effects (regardless of treatment or mixture) found no significant differences in larval weight or days to eclosion. Comparisons of survival showed that S. frugiperda reared on plantings that contained ‘Bitterblue’ had a mean survival rate of 54.864.10%, which was significantly lower than the survival rates of larvae reared on plantings that contained ‘Captiva’ or ‘Seville’ (Mean=63.894.10%,
Mean=63.893.98%, respectively). Plantings that contained ‘Floratam’ and ‘Palmetto’ received approximately 5% and 4% less herbivory, than ‘Captiva’ and ‘Seville’, respectively.
Effects of Cultivar Pairs Relative to the Single Cultivar Components
Due to the magnitude of means comparisons between cultivars and cultivar pairings, all results presented are statistically significant unless otherwise stated
(Appendix A-1; Appendix B-1).
No-choice laboratory experiment
We found that S. frugiperda larvae were significantly smaller when reared on mixtures containing ‘Classic’ (Figure 3-3). Nine days into development, larvae that fed on cultivar mixtures without ‘Classic’ averaged 327.9811.33 mg, while larvae that fed
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on mixtures containing ‘Classic’ were over 13% smaller, averaging 284.768.71 mg.
Larvae that were fed diets containing both ‘Classic’ and ‘Palmetto’ had significantly shorter development time to pupation than larvae fed ‘Classic’ without ‘Palmetto’ (Figure
3-4a), while larvae fed the ‘Classic’-‘Floratam’ pairing had the highest survival rates, significantly higher than those fed ‘Classic’ without ‘Floratam’ (Figure 3-4b).
Limited-choice greenhouse experiment
Spodoptera frugiperda larvae developed into significantly smaller individuals when reared on plantings that contained ‘Bitterblue’ compared to plantings that did not.
This was true for every cultivar pairing with ‘Bitterblue’, except ‘Seville’ (Figure 3-5).
When larvae were reared on plantings containing ‘Captiva’ and ‘Classic’, they developed more rapidly than when reared on ‘Classic’ or ‘Captiva’ not planted together.
Spodoptera frugiperda survival rate significantly decreased when reared on plantings of the ‘Seville’-‘Bitterblue’ pairing compared to plantings containing ‘Seville’ without
‘Bitterblue’.
Percent herbivory was commonly reduced in plantings that contained ‘Floratam’
(Figure 3-7). Herbivory was also reduced in plantings that contained ‘Classic’ by mixing it with ‘Palmetto’ (Figure 3-7).
Effects of Diet Based on Presence of Cultivar Pairs
Due to the magnitude of means comparisons between cultivars and cultivar pairings, all results presented are statistically significant unless otherwise stated
(Appendix A-1; Appendix B-1).
No-choice laboratory experiment
Spodoptera frugiperda larvae were largest when reared on a diet containing both
‘Captiva’ and ‘Bitterblue’, or ‘Captiva’ and ‘Seville’, but smallest on mixtures containing
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‘Classic’ in combination with ‘Palmetto’ or ‘Seville’ (Figure 3-8). Larvae developed most rapidly when reared on mixtures containing ‘Classic’ and ‘Palmetto’, but slowest when reared on mixtures containing ‘Classic’ and ‘Seville’ (Figure 3-9). Spodoptera frugiperda reared on any mixture containing the ‘Classic’ and ‘Floratam’ pair had the highest survival rate from larva to adult compared to all other combinations of cultivar pairs in
M2 or M4 mixtures, closely followed by ‘Captiva’-’Palmetto’. Conversely, S. frugiperda survival rate was lowest when fed cultivar mixtures containing the ‘Bitterblue’ and
‘Seville’ pair (Figure 3-10).
Limited-choice greenhouse experiment
When comparing the effects of cultivar pairings on S. frugiperda life history traits, a few important trends appeared regarding survival rate and herbivory. On the M2 and
M4 plantings that contained ‘Bitterblue’-‘Floratam’ pairings, S. frugiperda larvae had the lowest survival rates (Mean=48.27.02%), which was significantly lower than the three pairings with the highest survival rates (Figure 3-11). In addition, percent herbivory was significantly reduced in M2 and M4 plantings that contained ‘Floratam’-‘Palmetto’ and
‘Floratam’-‘Classic’ pairings (Mean=36.0 3.10%, Mean= 38.03.17%, respectively)
(Figure 3-12).
Discussion
Plant diversity is often recommended as a cultural control tactic to reduce herbivore abundance and herbivory (Finch et al. 2003, Barbosa et al. 2009,
Grettenberger and Tooker 2017). However, several investigations have found that effects on herbivores often depend on the right kind of diversity (Karban and Maron
2002, Atsatt and Dowd 2017). Thus, we hypothesized that S. frugiperda fitness would
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be differentially affected by S. secundatum cultivars and that these effects would be conserved when planted with other cultivars to generate associational resistance or susceptibility. Surprisingly, all S. secundatum cultivars evaluated were equally susceptible to S. frugiperda when planted or provided in monoculture. However, more interestingly, our results suggest that when planted together, susceptible S. secundatum cultivars interact to affect S. frugiperda life history traits and herbivory. In multiple instances the presence of a cultivar or cultivar pair significantly changed herbivore life history traits compared to that cultivar in isolation. For example, the most apparent effect occurred when ‘Classic’ appeared in any no-choice mixture and reduced larval weight compared to the other cultivar without ‘Classic’. This effect was apparent no matter what cultivar ‘Classic’ was paired with in the no-choice study.
The presence of some cultivars exhibited associational resistance and susceptibility depending on the cultivar(s) they were mixed with. For example, in the no- choice diet mixing experiment, monocultures of ‘Bitterblue’ generated larvae with the longest development time to pupation. Development time was reduced when ‘Bitterblue’ was mixed with most other cultivars, but increased in some pairings. Larvae feeding on
‘Bitterblue’-‘Palmetto’ pairings had the shortest days to pupation, while larvae feeding on ‘Bitterblue’-‘Seville’ or ‘Bitterblue’-‘Captiva’ had some of the longest days to pupation in the no-choice study. This dynamic effect of cultivar composition is generally representative of our results in both studies, which indicates evidence for both associational resistance and associational susceptibility (Barbosa et al. 2009). Although identifying the mechanisms behind these dynamic effects is beyond the scope of this
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study, there are several possible explanations for the increase or decrease in effects on herbivores.
The no-choice study examines the effects of mixing cultivars in S. frugiperda diets, while the limited-choice study introduces two additional variables: S. frugiperda behavior and plant-plant interactions. In the limited-choice study, S. frugiperda can avoid cultivars if they choose, and feed on another. Although biomass may be relatively equal among cultivars, different cultivar phenotypes can express that biomass in different regions of the plant (Arief and Delacy 2009). In terms of consumable plant tissue, this differential expression could effectively alter the relative abundance of cultivars, and subsequently change herbivore behavior or response to feeding (Barbosa et al. 2009). Additionally, Wetzel and Thaler (2018) found that negative effects of diet mixing could be reduced when herbivores were capable of choosing their hosts within mixed stands. Perhaps the negative effects of ‘Classic’ on S. frugiperda observed in the no-choice study are reduced in the limited-choice study through a similar manner.
Plant-plant interactions can also generate associational resistance/susceptibility, and one route for this is competition. Gold et al. (1990) examined populations of whiteflies (Hemiptera: Aleyrodidae), Aleurotrachelus socialis Bondar and Trialeurodes variabilis (Quaintance), on cassava (Manihot esculenta Crantz) intercropped with non- host plants. They found that plant competition slowed cassava growth and reduced plant size. In turn, this created associational resistance by reducing whitefly populations.
Similarly, competitive interactions between S. secundatum cultivars may increase or decrease susceptibility to herbivores. Differences in the relative strength of competitors could then result in the dynamic changes seen in our studies.
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Regardless of the mechanisms driving these changes, our previous research suggests the effects of cultivar diversity on S. frugiperda life history traits are not as apparent as their effects on herbivory. Therefore, similar to other studies, cultivar composition may be more important in its effects on S. frugiperda biology (Finch et al.
2003, Grettenberger and Tooker 2016). For example, trends associated with S. frugiperda life history traits were apparent in mixtures containing ‘Bitterblue’. Larvae feeding on this cultivar in the limited-choice experiment had the smallest larval weights.
In multiple cases, pairing ‘Bitterblue’ with another cultivar reduced larval weight and survival. Having ‘Bitterblue’ in a mixture generated the lowest survival rate across all diversity levels. Moreover, the lowest survival rate among paired cultivars occurred in mixtures containing ‘Bitterblue’-‘Floratam’. This trend associated with ‘Bitterblue’ on S. frugiperda survival also appeared in our no-choice study. Therefore, the effect of
‘Bitterblue’ on S. frugiperda seems to be conserved between true diet mixing and mixed plantings. Since ‘Bitterblue’ still influences survival rate in the limited-choice study, larvae may continue to diet mix with ‘Bitterblue’ even when given the option to do otherwise. Our findings align with Reinert et al. (2009), who found ‘Bitterblue’ increased tropical sod webworm (Herpetogramma phaeopteralis Guenée) mortality, another key
Lepidopteran pest of warm season turfgrasses.
The reductions in damage seen in plantings that contained ‘Floratam’ cannot be undervalued since lawn aesthetic quality and uniformity is a primary driver of the turfgrass industry. This damage was further reduced when paired with ‘Palmetto’. In our no-choice study, ‘Classic’ incurred less herbivory when paired with ‘Palmetto’ or
‘Floratam’. In fact, ‘Classic’-‘Floratam’ pairings had the second lowest percent herbivory
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of any cultivar pair. Moreover, adding ‘Bitterblue’ to this mix (‘Classic’-‘Bitterblue’-
‘Floratam’-‘Palmetto’) is the composition that received the least herbivory in the limited- choice experiment (Mean=246%) (Figure 3-13). These reductions in herbivory, along with other effects on survival and larval weight, suggest that turfgrass managers and producers should further investigate mixed cultivar plantings of ‘Classic’-‘Bitterblue’-
‘Floratam’-‘Palmetto’.
From an experimental design perspective, it is important to note that results may differ based on the method used to detect the effects of diet on herbivores. From a fundamental perspective, it is interesting and important that cultivars within a species may have no differential effects on an herbivore in isolation, but interactions, either prior to or after consumption, can affect herbivore fitness. Although this study generates several questions, it also identifies important interactions that will aid in developing cultivar-specific recommendations for turfgrass maintenance professionals. These recommendations could then be used in residential, commercial, and industrial lawns, thereby reducing pest pressure and pesticide inputs in urban landscapes.
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Table 3-1. All M1, M2, and M4 treatment cultivar or cultivar mixture compositions. M1 M2 M4 'Bitterblue' 1 'Bitterblue'-'Captiva' 'Bitterblue'-'Captiva'-'Classic'-'Floratam' 'Bitterblue' 2 'Bitterblue'-'Seville' 'Bitterblue'-'Captiva'-'Classic'-'Seville' 'Bitterblue' 3 'Bitterblue'-'Floratam' 'Bitterblue'-'Captiva'-'Floratam'-'Seville' 'Captiva' 1 'Bitterblue'-'Classic' 'Bitterblue'-'Classic'-'Floratam'-'Palmetto' 'Captiva' 2 'Bitterblue'-'Palmetto' 'Bitterblue'-'Classic'-'Palmetto'-'Seville' 'Captiva' 3 'Captiva'-'Palmetto' 'Bitterblue'-'Captiva'-'Classic'-'Palmetto' 'Classic' 1 'Captiva'-'Floratam' 'Bitterblue'-'Floratam'-'Palmetto'-'Seville' 'Classic' 2 'Captiva'-'Seville' 'Bitterblue'-'Captiva'-'Palmetto'-'Seville' 'Classic' 3 'Captiva'-'Classic' 'Bitterblue'-'Classic'-'Floratam'-'Seville' 'Floratam' 1 'Classic'-'Seville' 'Bitterblue'-'Captiva'-'Floratam'-'Palmetto' 'Floratam' 2 'Classic'-'Palmetto' 'Captiva'-'Classic'-'Palmetto'-'Seville' 'Floratam' 3 'Classic'-'Floratam' 'Captiva'-'Classic'-'Floratam'-'Palmetto' 'Palmetto' 1 'Floratam'-'Palmetto' 'Captiva'-'Classic'-'Floratam'-'Seville' 'Palmetto' 2 'Floratam'-'Seville' 'Captiva'-'Floratam'-'Palmetto'-'Seville' 'Palmetto' 3 'Palmetto'-'Seville' 'Classic'-'Floratam'-'Palmetto'-'Seville' 'Seville' 1 'Seville' 2 'Seville' 3
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Table 3-2. Effects of S. secundatum cultivar monocultures on S. frugiperda life history traits in no-choice (top panel) and limited-choice (bottom panel) experiments. S. secundatum Larval weight Days to pupation Days to Survival Rate cultivar (mg) eclosion No-choice 'Classic' 339.27 (23.34) 14.17 (0.32)ab 22.45 (0.53) 91.67 (8.33) 'Bitterblue' 385.98 (28.29) 14.91 (0.34)a 23.11 (0.45) 66.67 (14.21) 'Captiva' 318.73 (28.49) 13.64 (0.28)b 22.82 (0.35) 91.67 (8.33) 'Floratam' 320.03 (18.45) 13.55 (0.16)b 22.1 (0.43) 83.33 (11.24) 'Palmetto' 348.99 (32.80) 13.83 (0.32)b 22.5 (0.48) 83.33 (11.24) 'Seville' 333.4 (22.43) 13.6 (0.31)b 22.6 (0.48) 83.33 (11.24) Limited-choice 'Classic' 298.93 (38.01) NA 23.83 (0.67) 66.67 (7.11) 'Bitterblue' 290.03 (47.63) NA 25.25 (0.87) 50 (8.70) 'Captiva' 358.48 (56.07) NA 24.45 (0.70) 58.33 (10.36) 'Floratam' 245.32 (43.29) NA 24.56 (1.05) 41.67 (10.36) 'Palmetto' 354.06 (51.21) NA 25.35 (1.10) 45.83 (7.43) 'Seville' 254.92 (41.71) NA 24.59 (0.84) 66.67 (9.40) Different letters indicate statistical differences (p<0.05). No letters in a column indicate no statistical differences.
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Table 3-3. Effects of S. secundatum cultivar monocultures on S. frugiperda herbivory in the limited-choice greenhouse experiment. S. secundatum cultivar Percent Herbivory 'Classic' 58.00 (5.87) 'Bitterblue' 51.33 (5.10) 'Captiva' 54.67 (5.10) 'Floratam' 58.00 (5.87) 'Palmetto' 50.00 (4.78 'Seville' 51.33 (4.56)
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Figure 3-1. Mean S. frugiperda larval weight (A) and survival weight (B) measured across all S. secundatum diversity levels (M1, M2, M4) based on the presence of single cultivars. Error bars represent standard error. Different letters indicate statistical differences (p<0.05).
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Figure 3-2. Mean S. frugiperda survival rate (A) and percent herbivory (B) measured across all S. secundatum diversity levels (M1, M2, M4) based on the presence of single cultivars. Error bars represent standard error. Different letters indicate statistical differences (p<0.05).
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Figure 3-3. Mean larval weight of S. frugiperda when reared on each cultivar and each cultivar paired with ‘Classic’. Error bars represent standard error. Different letters indicate statistical differences (p<0.05).
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Figure 3-4. Mean S. frugiperda days to pupation of larvae reared on mixtures that include ‘Classic’, ‘ Palmetto’ or the ‘Classic’-‘Palmetto’ pairing (A), and mean survival rate of S. frugiperda reared on ‘Classic’, ‘Floratam’ or ‘Classic’ and ‘Floratam’ (B). Error bars represent standard error. Different letters indicate statistical differences (p<0.05).
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Figure 3-5. Mean S. frugiperda larval weight measured across each S. secundatum cultivar, and its pairing with ‘Bitterblue’. Error bars represent standard error. Different letters indicate statistical differences (p<0.05).
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Figure 3-6. (A) Mean days to eclose of S. frugiperda larvae reared on plantings containing ‘Captiva’ compared with that of larvae reared on plantings containing ‘Classic’ and ‘Captiva’; (B) mean survival rate of S. frugiperda larvae reared on ‘Seville’ compared to those reared on ‘Bitterblue’ and ‘Seville’. Error bars represent standard error. Different letters indicate statistical differences (p<0.05).
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Figure 3-7. Mean percent herbivory measured across S. secundatum cultivars and those cultivars in pairs, with a focus on statically significant reductions in plant damage. Error bars represent standard error. Different letters indicate statistical differences (p<0.05).
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Figure 3-8. Mean S. frugiperda larval weight when reared on S. secundatum cultivar pairs that produced the largest and smallest larval weights. Error bars represent standard error. Different letters indicate statistical differences (p<0.05).
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Figure 3-9. Mean days to pupation of S. frugiperda larvae reared on the S. secundatum cultivar pairs that produced the longest and shortest days to pupation. Error bars represent standard error. Different letters indicate statistical differences (p<0.05).
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Figure 3-10. Mean survival rate of S. frugiperda larvae reared on S. secundatum cultivar pairs that produced the highest and lowest survival rates. Error bars represent standard error. Different letters indicate statistical differences (p<0.05).
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Figure 3-11. Mean survival rate of S. frugiperda larvae reared on S. secundatum cultivar pairs that produced the highest and lowest survival rates. Error bars represent standard error. Different letters indicate statistical differences (p<0.05).
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Figure 3-12. Mean percent herbivory of S. frugiperda larvae reared on S. secundatum cultivar pairs that yielded the most and least damage. Error bars represent standard error. Different letters indicate statistical differences (p<0.05).
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Figure 3-13. Mean percent herbivory of S. frugiperda larvae reared on S. secundatum mixtures and monocultures, with each cultivar represented by a letter: (a) ‘Bitterblue’, (b) ‘Captiva’, (c) ‘Classic’, (d) ‘Floratam’, (e) ‘Palmetto’, and (f) ‘Seville’. Error bars represent standard error.
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CHAPTER 4 CONCLUSIONS
Manipulating Stenotephrum secundatum cultivar diversity and composition has the potential to be an effective IPM strategy if properly implemented. Our results found that cultivar diversity affected herbivore life history traits through diet mixing and associational resistance. Specifically, we found that mixtures of four S. secundatum cultivars can reduce S. frugiperda larval weight when diet mixing is enforced and reduce herbivory otherwise. Furthermore, we observed changes in sex ratio based on cultivar diversity and sex specific responses to cultivar diversity. Diet mixing can produce smaller, female-biased populations, while mixed plantings may result in more balanced sex ratios. While we did not investigate generational changes in Spodoptera frugiperda populations, our results suggest it is a possibility. Future studies may be able to identify if cultivar diversity affects populations in turfgrass plantings as it does in other systems
(Tahvanainen and Root 1972, Brezzi et al. 2017, Grettenberger and Tooker 2017).
Herbivore life history traits were also affected by cultivar composition. ‘Classic’ significantly reduced larval weight in no-choice studies, but this effect was minimized in mixed plantings. Instead, ‘Classic’ reduced herbivory when paired with ‘Floratam’ and
‘Palmetto’, and ‘Classic’-‘Bitterblue’-‘Floratam’-‘Palmetto’ mixtures demonstrated the least herbivory of any composition in our limited-choice study. Further research might examine if these effects are retained in lawns, as this composition could act as a ground work for turfgrass breeders and producers to invest in the idea of cultivar diversity as a pest management strategy.
Cool season turfgrasses (C3 photosystem), commonly grown in temperate climates, are regularly produced and planted as inter- or intra-specific mixtures to 88
improve lawn quality and resilience to stress (Fushtey and Taylor 1983, Dunn et al.
1994). These mixtures can be planted and maintained by continuing to spread seed over time. Because warm season turfgrasses are grown vegetatively, overseeding is not an option. A lawn of mixed warm season cultivars could result in a homogenous mixture, which may retain the benefits seen in our limited-choice study. However, different cultivars vary in their tolerance to shade, drought, soil moisture, and other factors that over time could result in a lawn of heterogeneous patches of monocultures.
This could present challenges in producing and maintaining diverse warm-season turfgrass lawns and may affect the applicability of intraspecific diversity as a pest management tool.
Researching these mixtures and establishing a cultivar composition recommendation system could strongly benefit the turfgrass industry. However, the variety of herbivore responses dependent on diversity level and cultivar-specific interactions makes identification of the most effective mixtures a challenge. Future studies should also examine the effects of cultivar diversity and mixture composition on other economically important Stenotephrum secundatum pests like Blissus insularis and
Herpetogramma phaeopteralis. Nevertheless, this IPM strategy has the potential to reduce pesticide inputs and herbivory rates, thereby helping our ever-growing urban landscapes retain the ecosystem services provided by turfgrasses.
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APPENDIX A NO-CHOICE RESULTS
Table A-1. All statistically significant and nearly significant compositional comparisons in the no-choice experiment. Compared Parameter Lower Higher Num Den F ratio P DF DF value Single- Larval 'Classic' 'Bitterblue' 1.00 152.00 4.12 0.044* Single Weight Single- Larval 'Classic' 'Captiva' 1.00 152.00 7.71 0.006* Single Weight Single- Larval 'Classic' 'Floratam' 1.00 152.00 3.57 0.061 Single Weight Single- Larval 'Classic' 'Palmetto' 1.00 152.00 4.10 0.045* Single Weight Single- Larval 'Classic' 'Seville' 1.00 152.00 5.00 0.027* Single Weight Single- Survival 'Seville' 'Floratam' 1.00 153.00 4.62 0.033* Single Pair- Days to 'Classic'- 'Classic' 1.00 139.00 6.39 0.013* Single Pupation 'Palmetto' Pair- Larval 'Bitterblue'- 'Bitterblue' 1.00 152.00 5.20 0.024* Single Weight 'Classic' Pair- Larval 'Captiva'- 'Captiva' 1.00 152.00 4.36 0.039* Single Weight 'Classic' Pair- Larval 'Classic'- 'Palmetto' 1.00 152.00 4.87 0.029* Single Weight 'Palmetto' Pair- Larval 'Classic'- 'Seville' 1.00 152.00 6.78 0.010* Single Weight 'Seville' Pair- Survival 'Classic' 'Classic'- 1.00 153.00 4.75 0.031* Single 'Floratam' Pair-Pair Days to 'Bitterblue'- 'Bitterblue' 1.00 139.00 3.61 0.059 Pupation 'Palmetto' -'Captiva' Pair-Pair Days to 'Bitterblue'- 'Bitterblue' 1.00 139.00 4.36 0.039* Pupation 'Palmetto' -'Seville' Pair-Pair Days to 'Bitterblue'- 'Classic'- 1.00 139.00 6.33 0.013* Pupation 'Palmetto' 'Seville' Pair-Pair Days to 'Classic''- 'Classic'- 1.00 139.00 12.79 0.001* Pupation Palmetto' 'Seville' Pair-Pair Days to 'Classic'- 'Bitterblue' 1.00 139.00 3.83 0.052 Pupation 'Palmetto' -'Captiva' Pair-Pair Days to 'Classic'- 'Bitterblue' 1.00 139.00 4.21 0.042* Pupation 'Palmetto' -'Seville' Pair-Pair Days to 'Classic'- 'Captiva'- 1.00 139.00 3.80 0.053* Pupation 'Palmetto' 'Seville' * Indicates statistically significant results
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Table A-1. Continued Compared Parameter Lower Higher Num Den F ratio P DF DF value Pair-Pair Larval 'Bitterblue'- 'Bitterblue' 1.00 152.00 7.02 0.009* Weight 'Classic -'Captiva' Pair-Pair Larval 'Bitterblue'- 'Captiva'- 1.00 152.00 3.82 0.052 Weight 'Classic' 'Floratam' Pair-Pair Larval 'Bitterblue'- 'Captiva'- 1.00 152.00 4.52 0.035* Weight 'Classic' 'Seville' Pair-Pair Larval 'Bitterblue'- 'Bitterblue' 1.00 152.00 3.50 0.063* Weight 'Floratam' -'Captiva' Pair-Pair Larval 'Captiva'- 'Bitterblue' 1.00 152.00 4.16 0.043* Weight 'Classic' -'Captiva' Pair-Pair Larval 'Captiva'- 'Captiva'- 1.00 152.10 3.95 0.049* Weight 'Classic' 'Seville' Pair-Pair Larval 'Classic'- 'Bitterblue' 1.00 152.00 5.48 0.021* Weight 'Floratam' -'Captiva' Pair-Pair Larval 'Classic'- 'Captiva'- 1.00 152.00 6.82 0.010* Weight 'Floratam' 'Floratam' Pair-Pair Larval 'Classic'- 'Captiva'- 1.00 152.00 3.67 0.057 Weight 'Floratam' 'Floratam' Pair-Pair Larval 'Classic'- 'Captiva'- 1.00 152.00 5.29 0.023 Weight 'Floratam' 'Seville' Pair-Pair Larval 'Classic'- 'Palmetto'- 1.00 152.00 3.58 0.060 Weight 'Floratam' 'Seville' Pair-Pair Larval 'Classic'- 'Captiva'- 1.00 152.00 3.54 0.062 Weight 'Palmetto' 'Palmetto' Pair-Pair Larval 'Classic'- 'Captiva'- 1.00 152.00 4.36 0.039* Weight 'Palmetto' 'Seville' Pair-Pair Larval 'Classic'- 'Palmetto'- 1.00 152.00 4.22 0.042* Weight 'Palmetto' 'Seville' Pair-Pair Larval 'Classic'- 'Bitterblue' 1.00 152.00 5.02 0.027* Weight 'Seville' -'Captiva' Pair-Pair Larval 'Classic'- 'Captiva'- 1.00 152.00 4.13 0.044* Weight 'Seville' 'Floratam' Pair-Pair Larval 'Classic'- 'Captiva'- 1.00 152.10 7.17 0.008* Weight 'Seville' 'Seville' Pair-Pair Larval 'Classic'- 'Palmetto'- 1.00 152.00 4.13 0.044* Weight 'Seville' 'Seville' Pair-Pair Survival 'Bitterblue'- 'Captiva'- 1.00 153.00 4.71 0.032* 'Seville' 'Palmetto' Pair-Pair Survival 'Bitterblue'- 'Classic'- 1.00 153.00 6.41 0.012* 'Seville' 'Floratam' * Indicates statistically significant results
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APPENDIX B LIMITED-CHOICE RESULTS
Table B-1. All statistically significant and nearly significant compositional comparisons in the limited-choice experiment. Compared Parameter Lower Higher Num Den F P DF DF Ratio value Single-Single Damage 'Floratam' 'Captiva' 1.00 199.00 5.64 0.019* Single-Single Damage 'Floratam' 'Seville' 1.00 199.00 5.45 0.021* Single-Single Damage 'Palmetto' 'Captiva' 1.00 199.00 4.90 0.028 Single-Single Damage 'Palmetto' 'Seville' 1.00 199.00 4.57 0.034* Single-Single Survival 'Bitterblue' 'Captiva' 1.00 153.00 4.75 0.031* Single-Single Survival 'Bitterblue' 'Classic' 1.00 153.00 3.73 0.055 Single-Single Survival 'Bitterblue' 'Seville' 1.00 153.00 5.31 0.023* Pair-Single Damage 'Bitterblue'- 'Bitterblue' 1.00 199.00 3.55 0.061 'Floratam' Pair-Single Damage 'Classic'- 'Classic' 1.00 199.00 6.78 0.010* 'Floratam' Pair-Single Damage 'Classic'- 'Classic' 1.00 199.00 4.61 0.033* 'Palmetto' Pair-Single Damage 'Floratam'- 'Floratam' 1.00 199.00 6.88 0.009* 'Palmetto' Pair-Single Damage 'Floratam'- 'Palmetto' 1.00 199.00 7.86 0.006* 'Palmetto' Pair-Single Damage 'Palmetto'- 'Seville' 1.00 199.00 6.88 0.009* 'Seville' Pair-Single Days to 'Captiva' 'Captiva'- 1.00 124.00 5.35 0.022* Eclosion 'Classic' Pair-Single Larval 'Bitterblue'- 'Captiva' 1.00 133.90 8.91 0.003* Weight 'Captiva' Pair-Single Larval 'Bitterblue'- 'Classic' 1.00 133.90 6.03 0.015* Weight 'Classic' Pair-Single Larval 'Bitterblue'- 'Bitterblue' 1.00 133.90 3.84 0.052 Weight 'Floratam' Pair-Single Larval 'Bitterblue'- 'Palmetto' 1.00 134.00 7.32 0.008* Weight 'Palmetto' Pair-Single Survival 'Bitterblue'- 'Seville' 1.00 153.00 4.55 0.035* 'Seville' Pair-Pair Damage 'Classic'- 'Bitterblue' 1.00 199.00 3.91 0.049* 'Floratam' -'Captiva' Pair-Pair Damage 'Classic'- 'Classic'- 1.00 199.00 4.00 0.047* 'Floratam' 'Seville' Pair-Pair Damage 'Floratam'- 'Bitterblue' 1.00 199.00 6.66 0.011* 'Palmetto' 'Captiva' * Indicates statistically significant results
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Table B-1. Continued Compared Parameter Lower Higher Num Den F P DF DF Ratio value Pair-Pair Damage 'Floratam'- 'Bitterblue' 1.00 199.00 5.39 0.021* 'Palmetto' -'Seville' Pair-Pair Damage 'Floratam'- 'Captiva'- 1.00 199.00 6.39 0.012* 'Palmetto' 'Palmetto' Pair-Pair Damage 'Floratam'- 'Captiva'- 1.00 199.00 4.62 0.033* 'Palmetto' 'Seville' Pair-Pair Damage 'Floratam'- 'Classic'- 1.00 199.00 5.00 0.027* 'Palmetto' 'Seville' Pair-Pair Survival 'Bitterblue'- 'Captiva'- 1.00 153.00 5.87 0.017* 'Floratam' 'Classic' Pair-Pair Survival 'Bitterblue'- 'Captiva'- 1.00 153.00 7.27 0.008* 'Floratam' 'Floratam' Pair-Pair Survival 'Bitterblue'- 'Floratam'- 1.00 153.00 8.80 0.004* 'Floratam' 'Seville' * Indicates statistically significant results
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LIST OF REFERENCES
Andow, D. A. 1991. Vegetational diversity and arthropod population response. Annual Review of Entomology 36:561–566.
Arief, V., and I. H. Delacy. 2009. Characterization of commercial cultivars and naturalized genotypes of Stenotaphrum secundatum (Walter) Kuntze in Australia. International Turfgrass Society 11:549–561.
Ata, B., D. Lee, and M. H. Tongarlak. 2012. Got Local Food? Harvard Business School:1–40.
Atsatt, P. R., and D. J. O. Dowd. 2017. Plant defense guilds. American Association for the Advancement of Science 193:24–29.
Augustin, B. J., and C. H. Peacock. 2011. St. Augustinegrass for Florida lawns. Florida Horticulture 5:1–11.
Barbosa, P., J. Hines, I. Kaplan, H. Martinson, A. Szczepaniec, and Z. Szendrei. 2009. Associational resistance and associational susceptibility: having right or wrong neighbors. Annual Review of Ecology Evolution and Systematics 40:1–20.
Beard, J. B. 2000. Turfgrass benefits and the golf environment. Fate and Management of Turfgrass Chemicals 743:36–44.
Beard, J. B., and R. L. Green. 1994. The role of turfgrasses in environmental protection and their benefits to humans. Journal of Environment Quality 23:452.
Berger, D., R. Walters, and K. Gotthard. 2008. What limits insect fecundity? Body size- and temperature-dependent egg maturation and oviposition in a butterfly. Functional Ecology 22:523–529.
Bernays, E. A., K. L. Bright, N. Gonzalez, and J. Angel. 1994. Dietary mixing in a generalist herbivore: tests of two hypotheses. Ecology 75:1997–2006.
Bottrell, D. 1979. Integrated Pest Management. Council on Environmental Quality.
Brezzi, M., B. Schmid, P. A. Niklaus, and A. Schuldt. 2017. Tree diversity increases levels of herbivore damage in a subtropical forest canopy: evidence for dietary mixing by arthropods? Journal of Plant Ecology 10:13–27.
Brilman, L. A. 2005. Turfgrass breeding in the United States: public and private, cool and warm season. International Turfgrass Society Research Journal 10:508–514.
94
Brittain, C. A., M. Vighi, R. Bommarco, J. Settele, and S. G. Potts. 2010. Impacts of a pesticide on pollinator species richness at different spatial scales. Basic and Applied Ecology 11:106–115.
Busey, P. 1990. Polyploid Stenotaphrum germplasm: resistance to the polyploid damaging population southern chinch bug. Crop Science 30:588–593.
Busey, P. 2003. Cultural management of weeds in turfgrass: a review. Crop Science 43:1899–1911.
Busey, P., and B. J. Center. 1987. Southern chinch bug (Hemiptera: Heteroptera: Lygaeidae) overcomes resistance in St. Augustinegrass. Journal of Economic Entomology 80:608–611.
Busey, P., and G. H. Snyder. 1993. Population outbreak of the southern chinch bug is regulated by fertilization. International Turfgrass Society Research Journal 7:353– 357.
Capinera, J. L. 2014. Fall Armyworm, Spodoptera frugiperda (J . E . Smith)(Insecta: Lepidoptera: Noctuidae). UF/IFAS Extension Service 98:1–6.
Carr, M. H., and P. D. Zwick. 2016. Water 2070: Mapping Florida’s Future - Alternative Patterns of Water Use in 2070.
Casler, M., and R. Duncan. 2003. Turfgrass biology, genetics, and breeding. John Wiley & Sons, Hoboken, NJ.
Chang, S., W. D. Graham, S. Hwang, and R. Muñoz-Carpena. 2016. Sensitivity of future continental United States water deficit projections to general circulation models, the evapotranspiration estimation method, and the greenhouse gas emission scenario. Hydrology and Earth System Sciences 20:3245–3261.
Cook, R., and D. Weller. 2004. In Defense of Crop Monoculture. Proceedings of the Fourth International Crop Science Congress 4:1–11.
Crow, W. T. 2005. Plant-parasitic nematodes on golf course turf. Outlooks on Pest Management 16:10–15.
Dale, A. G., E. Youngsteadt, and S. Frank. 2016. Forecasting the effects of heat and pests on urban trees: impervious surface thresholds and the “pace-to-plant” technique. Arboriculture and Urban Forestry 42:181–191.
Davidson, A. W., and D. A. Potter. 1995. Response of plant-feeding, predatory, and soil- inhabiting invertebrates to acremonium endophyte and nitrogen fertilization in tall fescue turf. Journal of Economic Entomology 88:367–379.
95
Desneux, N., A. Decourtye, and J. M. Delpuech. 2007. The sublethal effects of pesticides on beneficial arthropods. Annual Review of Entomology 52:81–106.
Diaz, M. D. C., M. Seto, and D. C. Peck. 2008. Patterns of variation in the seasonal dynamics of Listronotus maculicollis (Coleoptera: Curculionidae) populations on golf course turf. Environmental Entomology 37:1438–1450.
Diffenbaugh, N. S., C. H. Krupke, M. A. White, and C. E. Alexander. 2008. Global warming presents new challenges for maize pest management. Environmental Research Letters 3:1–9.
Dobbs, E., and D. A. Potter. 2013. Enhancing beneficial insect biodiversity and biological control in turf: mowing height, naturalized roughs, and operation pollinator. University of Kentucky Theses and Dissertations--Entomology 5:1–94.
Donovan, G. H., D. T. Butry, Y. L. Michael, J. P. Prestemon, A. M. Liebhold, D. Gatziolis, and M. Y. Mao. 2013. The relationship between trees and human health: evidence from the spread of the emerald ash borer. American Journal of Preventive Medicine 44:139–145.
Dunn, J. H., E. H. Ervin, and B. S. Fresenburg. 2002. Turf performance of mixtures and blends of tall fescue, kentucky bluegrass, and perennial ryegrass. HortScience 37:214–217.
Dunn, J. H., D. D. Minner, B. F. Fresenburg, and S. S. Bughrara. 1994. Bermudagrass and cool-season turfgrass mixtures: response to simulated traffic. Agronomy Journal 86:10–16.
Elton, C. S. 1958. The Ecology of Invasions by Animals and Plants. University of Chicago Press.
Endara, M. J., and P. D. Coley. 2011. The resource availability hypothesis revisited: a meta-analysis. Functional Ecology 25:389–398.
Erb, M., C. A. M. Robert, B. E. Hibbard, and T. C. J. Turlings. 2011. Sequence of arrival determines plant-mediated interactions between herbivores. Journal of Ecology 99:7–15.
Finch, S., H. Billiald, and R. H. Collier. 2003. Companion planting - Do aromatic plants disrupt host-plant finding by the cabbage root fly and the onion fly more effectively than non-aromatic plants? Entomologia Experimentalis et Applicata 109:183–195.
Fraser, M., M. Kenna, D. Karcher, D. Kopec, K. Diesburg, S. Ebdon, and K. Morris. 2012. National Turfgrass Evaluation Program 2000-3.
96
Fushtey, S., and D. Taylor. 1983. The effect of wear stress on survival of turfgrass in pure stands and in mixtures. Canadian Journal of Plant Science 63:317–322.
Gianoli, E., and C. Salgado-Luarte. 2017. Tolerance to herbivory and the resource availability hypothesis. Biology Letters 13:1–4.
Gold, C. S., M. A. Altieri, and A. C. Bellotti. 1990. Direct and residual effects of short duration intercrops on the cassava whiteflies Aleurotrachelus socialis and Trialeurodes variabilis (Homoptera: Aleyrodidae) in Colombia. Agriculture, Ecosystems and Environment 32:57–67.
Gould, F. W., and R. B. Shaw. 1983. Grass systematics. Brittonia 35:301–301.
Goulson, D., E. Nicholls, C. Botías, and E. L. Rotheray. 2015. Bee declines driven by combined stress from parasites, pesticides, and lack of flowers. Science 347:1-9
Grettenberger, I. M., and J. F. Tooker. 2016. Inter-varietal interactions among plants in genotypically diverse mixtures tend to decrease herbivore performance. Oecologia 182:189–202.
Grettenberger, I. M., and J. F. Tooker. 2017. Variety mixtures of wheat influence aphid populations and attract an aphid predator. Arthropod-Plant Interactions 11:133– 146.
Hägele, B. F., and M. Rowell-Rahier. 1999. Dietary mixing in three generalist herbivores: nutrient complementation or toxin dilution? Oecologia 119:521–533.
Haydu, J. J., A. W. Hodges, and C. R. Hall. 2006. Economic impacts of the turfgrass and lawncare industry in the United States. UF/IFAS Extension Service 632:1–35.
Held, D. W., and D. A. Potter. 2012. Prospects for managing turfgrass pests with reduced chemical inputs. Annual Review of Entomology 57:329–354.
Heng-Moss, T., T. Macedo, L. Franzen, F. Baxendale, L. Higley, and G. Sarath. 2006. Physiological responses of resistant and susceptible buffalograsses to Blissus occiduus (Hemiptera: Blissidae) feeding. Journal of Economic Entomology 99:222– 228.
Hodges, A. W., and T. J. Stevens. 2010. Economic contributions of the turfgrass industry in Florida. University of Florida, Final Project Report to the Florida Turfgrass Association:1–39.
Holmes, D. M., and G. W. Barrett. 1997. Japanese beetle (Popillia japonica) dispersal behavior in intercropped vs monoculture soybean agroecosystems. American Midland Naturalist 137:312–319.
97
Innes, R. 2013. Economics of agricultural residuals and overfertilization: chemical fertilizer use, livestock waste, manure management, and environmental impacts. Encyclopedia of Energy, Natural Resource, and Environmental Economics 2:50– 57.
Jakka, S. R. K., V. R. Knight, and J. L. Jurat-Fuentes. 2014. Fitness costs associated with field-evolved resistance to Bt maize in Spodoptera frugiperda (Lepidoptera: Noctuidae). Journal of Economic Entomology 107:342–351.
Johnson, A. W., M. E. Snookb, and B. R. Wisemanc. 2002. Green leaf chemistry of various turfgrasses: differentiation and resistance to fall armyworm. Crop Science 42:2004–2010.
Just, M., S. Frank, and A. Dale. 2018. Impervious surface thresholds for urban tree site selection. Urban Forestry & Urban Greening 34:141–146.
Karban, R., and J. Maron. 2002. The fitness consequences of interspecific eavesdropping between plants. Ecology 83:1209–1213.
Koppenhöfer, A. M., R. S. Cowles, and E. M. Fuzy. 2003. Effects of turfgrass endophytes (Clavicipitaceae: Ascomycetes) on white grub (Coleoptera: Scarabaeidae) larval development and field populations. Environmental Entomology 32:895–906.
Koppenhöfer, A. M., and E. M. Fuzy. 2003. Effects of turfgrass endophytes (Clavicipitaceae: Ascomycetes) on white grub (Coleoptera: Scarabaeidae) control by the entomopathogenic nematode Heterorhabditis bacteriophora (Rhabditida: Heterorhabditidae). Environmental Entomology 32:392–396.
Kotowska, A. M., J. F. Cahill, and B. A. Keddie. 2010. Plant genetic diversity yields increased plant productivity and herbivore performance. Journal of Ecology 98:237–245.
Kunkel, B. A., D. W. Held, and D. A. Potter. 1999. Impact of halofenozide, imidacloprid, and bendiocarb on beneficial invertebrates and predatory activity in turfgrass. Journal of Economic Entomology 92:922–930.
Larson, J. L., C. T. Redmond, and D. A. Potter. 2013. Assessing insecticide hazard to bumble bees foraging on flowering weeds in treated lawns. PLoS ONE 8:1-7
Lotts, K., and T. Naberhaus. 2014. Butterflies and moths of North America | collecting and sharing data about Lepidoptera. http://www.butterfliesandmoths.org/species/Vanessa-cardui.
Matheny, A. L., J. Traunfeld, M. J. Raupp, and A. E. Brown. 2009. Gardeners and gardening trends in the United States. Outlooks on Pest Management 20:51–56.
98
Milesi, C., S. W. Running, C. D. Elvidge, J. B. Dietz, B. T. Tuttle, and R. R. Nemani. 2005. Mapping and modeling the biogeochemical cycling of turf grasses in the United States. Environmental Management 36:426–438.
Mody, K., S. B. Unsicker, and K. E. Linsenmair. 2007. Fitness related diet-mixing by intraspecific host-plant-switching of specialist insect herbivores. Ecology 88:1012– 1020.
Morris, K., and R. Shearman. 1998. NTEP turfgrass evaluation guidelines.
Moser, L. E., D. R. Buxton, M. D. Casler, M. D. Casler, J. F. Pedersen, G. C. Eizenga, and S. D. Stratton. 1996. Germplasm and cultivar development. Pages 413–469 Cool-Season Forage Grasses, Agronomy Monograph.
Muchovej, R., and J. Rechcigl. 1994. Impacts of nitrogen fertilization of pastures and turfgrasses on water quality. Pages 91–135. Soil Processes and Water Quality, Boca Raton: Lewis Publishers.
Mugaas, R. J., M. L. Agnew, and N. E. Christians. 1997. Turfgrass management for protecting surface water quality. Minnesota Extension Service.
Newell, A., F. E. M. Crossley, and A. C. Jones. 1996. Selection of grass species, cultivars and mixture for lawn tennis court. Journal of the Sports Turf Research Institute 42:42–60.
Nowak, D. J., and E. J. Greenfield. 2018. Declining urban and community tree cover in the United States. Urban Forestry & Urban Greening 32:32–55.
Pannkuk, T. R. 2011. A taxonomic key for selected turf-type bermudagrasses. The Texas Journal of Agriculture and Natural Resource 24:83–87.
Pashley, D. P., T. N. Hardy, and A. M. Hammond. 1995. Host effect on developmental and reproductive traits in fall armyworm strain (Lepidoptera: Noctuidae). Annals of the Entomological Society of America 88:748–755.
Pimentel, D. 1961. Species diversity and insect population outbreaks. Annals of the Entomological Society of America 54:76–86.
Potter, D. A. 1994. Effects of pesticides on beneficial invertebrates in turf. Boca Raton, FL: CRC Press.
Potter, D. A. 2005. Prospects for managing destructive turfgrass insects without protective chemicals. International Turfgrass Society Research Journal 10:42–54.
Potter, D. A., and S. K. Braman. 1991. Ecology and management of turfgrass insects. Annual Review Entomology 36:383–406.
99
Potts, S. G., J. C. Biesmeijer, C. Kremen, P. Neumann, O. Schweiger, and W. E. Kunin. 2010. Global pollinator declines: trends, impacts and drivers. Trends in Ecology and Evolution 25:345–353.
Quaintance, A., and C. Brues. 1905. The Cotton Bollworm. Page US Department of Agriculture, Bureau of Entomology Bulletin.
Quisenberry, S. 1990. Plant resistance to insects and mites in forage and turf grasses. Florida Entomologist 73:411–421.
Rangasamy, M., B. Rathinasabapathi, H. J. McAuslane, R. H. Cherry, and R. T. Nagata. 2009a. Role of leaf sheath lignification and anatomy in resistance against southern chinch bug (Hemiptera: Blissidae) in St. Augustinegrass. Journal of Economic Entomology 102:432–439.
Rangasamy, M., B. Rathinasabapathi, H. J. McAuslane, R. H. Cherry, and R. T. Nagata. 2009b. Oxidative responses of St. Augustinegrasses to feeding of southern chinch bug, Blissus insularis Barber. Journal of Chemical Ecology 35:796–805.
Raupp, M., J. Davidson, J. Homes, and J. Hellman. 1985. The concept of key plants in integrated pest management for landscapes. Journal of Arboriculture 11:317–322.
Raupp, M. J., A. B. Cumming, and E. C. Raupp. 2006. Street tree diversity in eastern North America and its potential for tree loss to exotic borers. Arboriculture and Urban Forestry 32:297–304.
Raupp, M. J., J. J. Holmes, C. Sadof, P. Shrewsbury, and J. A. Davidson. 2001a. Effects of cover sprays and residual pesticides on scale insects and natural enemies in urban forests. Journal of Arboriculture 27:203–214.
Raupp, M. J., C. S. Koehler, and J. A. Davidson. 1992. Advances in implementing integrated pest management for woody landscape plants. Annual Review of Entomology 37:561–585.
Raupp, M. J., P. M. Shrewsbury, and D. Herms. 2010. Ecology of herbivorous arthropods in urban landscapes. Annual Review of Entomology 55:19–38.
Raupp, M. J., P. M. Shrewsbury, J. J. Holmes, and J. A. Davidson. 2001b. Plant species diversity and abundance affects the number of arthropod pest in residential landscapes. Journal of Arboriculture 27:222–229.
Rausher, M. D. 1981. The effect of native vegetation on the susceptibility of Aristolochia reticulata (Aristolochiaceae) to herbivore attack. Ecology 62:1187–1195.
Reinert, J. A. 1974. Tropical sod webworm and southern chinch bug control in Florida. The Florida Entomologist 57:275–279.
100
Reinert, J. A., and M. C. Engelke. 2010. Resistance in zoysiagrass (Zoysia spp .) to the fall armyworm (Spodoptera frugiperda) (Lepidoptera : Noctuidae). Florida Entomological Society 93:254–259.
Reinert, J. A., M. C. Engelke, A. D. Genovesi, A. Chandra, and J. E. Mccoy. 2009. Resistance to tropical sod webworm (Herpetogramma phaeopteralis) (Lepidoptera: Crambidae) in St. Ausgustinegrass and zoysiagrass. International Turfgrass Society Research Journal 11.
Reinert, J. A., M. C. Engelke, and J. C. Read. 2004a. Host resistance to insects and mites, a review - a major IPM strategy in turfgrass culture. Acta Horticulturae 661:463–486.
Reinert, J. A., M. C. Engelket, J. C. Readt, S. J. Maranz, and B. R. Wiseman. 1997. Susceptibility of cool and warm season turfgrass to fall armyworm, Spodoptera frugiperda. International Turfgrass Society Research 8:1003–1011.
Reinert, J. A., J. C. Read, and R. Meyers. 2004b. Resistance to fall armyworm (Spodoptera frugiperda) among Kentucky bluegrass (Poa pratensis) cultivars. Acta Horticulturae 661:525–530.
Riihimäki, J., P. Kaitaniemi, J. Koricheva, and H. Vehviläinen. 2005. Testing the enemies hypothesis in forest stands: the important role of tree species composition. Oecologia 142:90–97.
Ripper, W. E. 1956. Effect of pesticides on balance of arthropod populations. Annual Review of Entomology 1:403–438.
Robinson, G. S., P. R. Ackery, I. J. Kitching, G. W. Beccaloni, and L. M. Hernandez. 2002. Hostplants of the moth and butterfly caterpillars of America north of Mexico. Page Memoirs fo the American Entomological Institute. American Entomological Institute, Gainesville, FL.
Root, R. B. 1973. Organization of a plant-arthropod association in simple and diverse habitats: the fauna of collards (Brassica oleracea). Ecological Monographs 43:95– 124.
Santamour, F. S. 2002. Trees for urban planting: diversity, uniformity, and common sense. Proceedings of the 7th Conference of the Metropolitan Tree Improvement Alliance 7:57–66.
Satterthwaite, L. N., A. W. Hodges, J. J. Haydu, and J. L. Cisar. 2009. An agronomic and economic profile of Florida’s sod industry in 2007. University of Florida, IFAS, Florida Agriculture Experiment Stations, Florida Cooperative Extension Service Gainesville, FL.
101
Schoonhoven, A. V., C. Cardona, J. Garcia, and F. Garzon. 1981. Effect of weed covers on Empoasca kraemeri Ross and Moore populations and dry bean yields. Environmental Entomology 10:901–907.
Shaddox, T. W., J. B. Unruh, L. E. Trenholm, P. C. McGroary, and J. L. Cisar. 2016. Nitrogen rate required for acceptable St. Augustinegrass and associated nitrate leaching. Crop Science 56:439–451.
Shoffner, A. V., and J. F. Tooker. 2013. The potential of genotypically diverse cultivar mixtures to moderate aphid populations in wheat (Triticum aestivum L.). Arthropod- Plant Interactions 7:33–43.
Simmons, M., M. Bertelsen, S. Windhager, and H. Zafian. 2011. The performance of native and non-native turfgrass monocultures and native turfgrass polycultures: an ecological approach to sustainable lawns. Ecological Engineering 37:1095–1103.
Smith, M., and P. Whiteman. 1983. Evaluation of tropical grasses in increasing shade under coconut canopies. Experimental Agriculture 19:153–161.
Sparks, A. L. 1979. A review of the biology of the fall armyworm. The Florida Entomologist 62:82–87.
Stowell, L. J., and W. D. Gelernter. 2001. Diagnosis of turfgrass diseases. Annual Review of Phytopathology 39:135–155.
Sukhdev, P., T. Elmqvist, M. Fragkias, J. Goodness, B. Güneralp, P. J. Marcotullio, R. I. Mcdonald, S. Parnell, M. Schewenius, M. Sendstad, K. C. Seto, and C. Wilkinson. 2013. Urbanization, Biodiversity and Ecosystem Services: Challenges and Opportunities A Global Assessment. Dordrecht, NL.
Tahvanainen, J. O., and R. B. Root. 1972. The influence of vegetational diversity on the population ecology of a specialized herbivore, Phyllotreta cruciferae (Coleoptera: Chrysomelidae). Oecologia 10:321–346.
Telenko, D. E. P., T. W. Shaddox, J. Bryan Unruh, and L. E. Trenholm. 2015. Nitrate leaching, turf quality, and growth rate of ‘Floratam’ St. Augustinegrass and common centipedegrass. Crop Science 55:1320–1328.
Tooker, J. F., and S. D. Frank. 2012. Genotypically diverse cultivar mixtures for insect pest management and increased crop yields. Journal of Applied Ecology 49:974– 985.
Trenholm, L. E., and J. B. Unruh. 2005. The Florida lawn handbook: best management practices for your home lawn in Florida. University Press of Florida.
102
Unbehend, M., S. Hänniger, R. L. Meagher, D. G. Heckel, and A. T. Groot. 2013. Pheromonal divergence between two strains of Spodoptera frugiperda. Journal of Chemical Ecology 39:364–376.
United States Census Bureau. 2010. 2010 census urban and rural classification and urban area criteria.
Utsumi, S., Y. Ando, T. P. Craig, and T. Ohgushi. 2011. Plant genotypic diversity increases population size of a herbivorous insect. Proceedings of the Royal Society: Biolocial Sciences 278:3108–3115.
Wetzel, W. C., and J. S. Thaler. 2018. Host-choice reduces, but does not eliminate, the negative effects of a multi-species diet for an herbivorous beetle. Oecologia 186:483–493.
White, J. A., and T. G. Whitham. 2000. Associational susceptibility of cottonwood to a box elder herbivore. Ecology 81:1795–1803.
Williamson, R. C., and D. A. Potter. 1997. Turfgrass species and endophyte effects on survival, development, and feeding preference of black cutworms (Lepidoptera: Noctuidae). Journal of Economic Entomology 90:1290–1299.
Wilson, S. D., and D. Tilman. 1993. Plant competition and resource availability in response to disturbance and fertilization. Ecology 74:599–611.
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BIOGRAPHICAL SKETCH
Ethan Doherty got his start in biology at the College of Wooster. He investigated intraguild predation and biological control at the Ohio Agricultural Research and
Development Center, while he worked toward his Bachelor of Arts. Afterwards, he studied lemur scent marking, and foraging behavior at the Duke Lemur Center. While at
Duke, he also assisted the Amboseli Baboon Project with their investigation into the genetic drivers of behavior. He briefly studied blueberry pests at the University of
Georgia before he transferred to the University of Florida. While earning his Master of
Science, he served as an adviser to a Duke undergraduate student investigating lemur cognition and puzzle solving. In August 2018, he completed his MS degree in entomology and nematology.
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