The Ecology of Cactoblastis Cactorum (Berg) (Lepidoptera

Home , Chelinidea vittiger, Consolea moniliformis, Cylindropuntia fulgida, Melitara prodenialis, Opuntia aurantiaca, Opuntia cochenillifera

THE ECOLOGY OF CACTOBLASTIS CACTORUM (BERG) (LEPIDOPTERA:

PYRALIDAE) IN FLORIDA

Kristen Erica Sauby

A Thesis Submitted to the Faculty of Mississippi State University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Biological Sciences in the Department of Biological Sciences

Mississippi State, Mississippi

August 2009

Kristen Erica Sauby

2009

THE ECOLOGY OF CACTOBLASTIS CACTORUM (BERG) (LEPIDOPTERA:

PYRALIDAE) IN FLORIDA

Kristen Erica Sauby

Approved:

Christopher P. Brooks Richard L. Brown Assistant Professor of Biological Sciences Professor of Entomology (Director of Thesis) (Committee Member)

Gary N. Ervin Gary N. Ervin Associate Professor of Biological Sciences Graduate Coordinator of the Department of (Committee Member) Biological Sciences

Gary L. Myers Dean, College of Arts and Sciences

Name: Kristen Erica Sauby

Date of Degree: August 8, 2009

Institution: Mississippi State University

Major Field: Biological Sciences

Major Professor: Dr. Christopher P. Brooks

Title of Study: THE ECOLOGY OF CACTOBLASTIS CACTORUM (BERG) (LEPIDOPTERA: PYRALIDAE) IN FLORIDA

Pages in Study: 90

Candidate for Degree of Master of Science

I used a theoretical model to determine the conditions under which Cactoblastis cactorum populations would be expected to experience positive population growth.

Results from simulations suggest that host species richness, host quality, and the C. cactorum death rate interact to determine the probability of C. cactorum positive population growth. I also studied the influence of host diversity empirically.

Cactoblastis cactorum prevalence was significantly higher when O. stricta was present in the community. Also, higher species richness within host assemblages led to a higher prevalence of infestation than in single-species host assemblages. Finally, I explored co- occurrence patterns of native cactus-feeding insects in an effort to document the impact of C. cactorum to native insect assemblages. The presence of C. cactorum in a community did not appear to affect the structure of native cactus-feeding insect assemblages.

Key Words: amplification effect, biodiversity, Cactoblastis cactorum, Chelinidea vittiger,

Dactylopius, host quality, insect herbivore, invasive species, Melitara prodenialis, net reproductive rate, null model, Opuntia, prickly pear cacti, species co-occurrence

DEDICATION

I dedicate this work to my parents.

ACKNOWLEDGEMENTS

I would like to thank my advisor, Christopher Brooks, for making this project possible by providing ideas, advice, and criticism as well as financial support through his funding from the Mississippi State University Department of Biological Sciences and the

Office of Research & Economic Development. I would also like to thank my committee member, Richard Brown, for providing advice and suggestions as well as financial support through a Research Assistantship in the Mississippi Entomological Museum. I would like to thank my committee member, Gary Ervin, for identifying Christopher

Brooks as a potential advisor for me in addition to providing advice and suggestions as well as financial support through his grants from the U.S. Geological Survey Biological

Resources Discipline (04HQAG0135 and 08HQAG0139). I would also like to thank the

Department of Biological Sciences at Mississippi State University for financial support in the form of a Teaching Assistantship.

I would like to thank Lucas Majure for providing helpful advice and assistance in plant species identification. Keith Bradley of the Institute for Regional Conservation and

Julieta Brambila with USDA-APHIS provided locality information for Opuntia and

Consolea populations in the Florida Keys. Anastasia Woodard was indispensable by assisting with sampling throughout the summer of 2008. Travis Marsico also helped in sampling, provided helpful advice and comments, and identified unknown moth

iii specimens through genetic analysis. JoVonn Hill and Terry Schiefer of the Mississippi

Entomological Museum helped in the identification of collected insects. Joe MacGown taught me how to the use the Leica Z16 stereomicroscope, the Leica Application Suite with Montage Module software, and Adobe Photoshop CS4 (which I was able to use through the Mississippi Entomological Museum) to create the photo in Figure 1c.

I would be severely remiss if I did not acknowledge Archbold Biological Station,

Florida State Forests, Florida State Parks, Lee County Department of Parks and

Recreation, Pinellas County Parks and Recreation, National Park Service, Sarasota

County Parks and Recreation, St. Lucie County Environmental Resources Department,

United States Forest Service, and United States Fish and Wildlife Service for access to sites and for allowing the collection of plant and insect material. Without access to these sites this project would not have been possible.

Finally, I would like to thank my friends in the Department of Biological Sciences and Department of Entomology and Plant Pathology at Mississippi State University, with a specific thanks to Leah Chinchilla, as well as friends in the Bulldawg Cycling Club. I would also like to thank Sreepriya Pramod, who was a supportive friend and who allowed me to use her computer for the null model analyses of Chapter IV.

TABLE OF CONTENTS

DEDICATION...... ii

ACKNOWLEDGEMENTS...... iii

LIST OF TABLES...... vii

LIST OF FIGURES ...... viii

CHAPTER

I. CACTOBLASTIS CACTORUM, A BIOLOGICAL CONTROL AGENT NOW INVASIVE IN NORTH AMERICA...... 1

Introduction...... 1 Literature Cited...... 6

II. THEORETICAL PREDICTIONS OF THE INFLUENCE OF HOST SPECIES RICHNESS AND QUALITY ON THE NET REPRODUCTIVE RATE OF CACTOBLASTIS CACTORUM...... 9

Abstract...... 9 Introduction...... 10 Materials And Methods ...... 13 The model of C. cactorum population dynamics...... 13 Simulation of the Effects of Host Species Richness and Quality on Ro of C. cactorum ...... 14 Results...... 16 Discussion...... 18 Caveats...... 21 Conclusions...... 22 Literature Cited...... 23

v III. HOST IDENTITY AND DIVERSITY AFFECT THE PREVALENCE OF CACTOBLASTIS CACTORUM...... 28

Abstract...... 28 Introduction...... 29 Materials And Methods ...... 33 Study System ...... 33 Study Sites and Data Collection ...... 35 Statistical Analysis...... 36 Results...... 38 Discussion...... 45 Importance of Species Identity ...... 45 Diversity and Prevalence ...... 47 Alternative Factors Influencing C. cactorum Prevalence...... 48 Consequences of Spillover and the Amplification Effect...... 50 Literature Cited...... 52

IV. CACTOBLASTIS CACTORUM AND PATTERNS OF NATIVE INSECT SPECIES CO-OCCURRENCE IN FLORIDA ...... 57

Abstract...... 57 Introduction...... 58 Materials And Methods ...... 60 Study System ...... 60 Study Sites and Data Collection ...... 61 Null Model Analyses ...... 63 Partition Test...... 65 Results...... 66 Discussion...... 71 Literature Cited...... 74

APPENDIX

A. SELECTED PARTS OF A CACTUS...... 78

B. KNOWN HOST SPECIES OF CACTOBLASTIS CACTORUM AND THE LOCATIONS WHERE INFESTED HOSTS HAVE BEEN FOUND...... 80

Literature Cited...... 85

C. CACTUS-ASSOCIATED INSECTS ENCOUNTERED IN FLORIDA...... 88

LIST OF TABLES

1. Parameters of the probability density functions used in the simulations of the effects of host species richness and quality on Ro of C. cactorum ...... 15

2. Analysis of co-occurrence patterns among native insect assemblages...... 70

3. Known host species of Cactoblastis cactorum and the locations where infested hosts have been found ...... 81

4. Cactus-associated insects encountered in Florida...... 89

vii

LIST OF FIGURES

1. (a) A typical C. cactorum egg stick, (b) evidence of internally-feeding larvae with brown frass exuded from the cladode, and (c) a late- instar C. cactorum larva...... 3

2. Figures (a), (b), (c), (d), and (e) represent PDFs A, B, C, D, and E, respectively. Figures (f), (g), (h), (i), and (j) display Pr(Ro>1) calculated using host quality values derived from PDFs 1, 2, 3, 4, and 5, respectively. Blue symbols represent single-species assemblages, orange symbols represent two-species assemblages, and black symbols represent three-species assemblages. Figures (k), (l), (m), (n), and (o) display the difference () in Pr(Ro>1) values for assemblages of different species richness values calculated using host quality values derived from PDFs 1, 2, 3, 4, and 5, respectively. Yellow symbols represent differences between the Pr(Ro>1) value of S = 2 and S = 1 assemblages. Purple symbols represent the difference between the Pr(Ro>1) value of S = 3 and S = 2 assemblages. Green symbols represent the difference between the Pr(Ro>1) value of S = 3 and S = 1...... 17

3. Examples of (a) O. pusilla, (b) O. stricta, and the three varieties of O. humifusa: (c) O. humifusa var. ammophila, (d) O. humifusa var. austrina, and (e) O. humifusa var. humifusa...... 34

4. Common native host plant species found in Florida include (a) O. stricta and O. pusilla as well as (b) O. humifusa var. ammophila, var. austrina, and var. humifusa...... 41

5. Distribution of C. cactorum in Florida ...... 42

viii 6. Host plant species: S: O. stricta; A: O. humifusa var. ammophila; H, O. humifusa var. humifusa, and P: O. pusilla. (a) Prevalence of C. cactorum infestation by host species (mean ± SEM). Different lowercase letters denote statistically significant differences in the proportion of plants infested by C. cactorum. (b) Effect of host species presence on the prevalence of C. cactorum infestation (mean ± SEM). Statistical differences in the proportion of plants at sites with and without a host species are denoted as * (p<0.05) ...... 43

7. (a) Prevalence of C. cactorum by species richness. Different lowercase letters denote statistically significant differences in the proportion of plants infested by C. cactorum. (b) The effect of species richness on the prevalence of C. cactorum. Statistical differences in C. cactorum prevalence between host monocultures and polycultures for a particular host species are denoted as * (p<0.05)...... 44

8. Examples of (a) different larval instars of M. prodenialis, (b) the characteristic white coating of Dactylopius sp. and (c) C. vittiger nymphs and an adult ...... 68

9. Common cactophagous insect species found in Florida include (a) C. cactorum and M. prodenialis as well as (b) C. vittiger and Dactylopius sp...... 69

10. Selected parts of a cactus, including (a) the spine and glochids which grow from the areole and (b) the leaves growing on a cladode...... 79

CHAPTER I

CACTOBLASTIS CACTORUM, A BIOLOGICAL CONTROL AGENT NOW

INVASIVE IN NORTH AMERICA

Introduction

One of the most frequently cited success stories of invasive plant control involves the use of the South American Cactus Moth, Cactoblastis cactorum (Berg) (Lepidoptera:

Pyralidae), as a biological control agent (e.g., Begon et al. 1996, Morin 1999). After C. cactorum was able to eradicate an estimated 90% of the 60 million acres of invasive

Opuntia populations in Australia in the 1930s (Dodd 1940), it was subsequently introduced to at least 10 countries for biological control (Dodd 1940, Pettey 1948, Garcia

Tuduri et al. 1971, Simmonds & Bennett 1966, Zimmermann et al. 2005). Beginning in

1957, C. cactorum was introduced into the Caribbean country of Nevis to control weedy native Opuntia (Simmonds & Bennett 1966). By 1980, C. cactorum had spread throughout the Caribbean Islands to the U.S. Virgin Islands, St. Kitts, Puerto Rico, and

Cuba (Simmonds & Bennett 1966, Garcia Tuduri et al. 1971, Zimmermann et al. 2005).

In October 1989, C. cactorum was detected attacking O. stricta in the Florida Keys

(Dickel 1991) and has since spread as far north as Charleston, South Carolina along the

Atlantic Coast (High et al. 2002) and as far west as Jefferson Parish, Louisiana along the

Gulf of Mexico (Payne 2009).

1 Cactoblastis cactorum feeds on species of prickly pear cacti (Mann 1969) within the subfamily Opuntioideae of the Cactaceae family (Anderson 2001). This moth lays its eggs in linear chains of approximately 35 to 90 eggs, commonly referred to as egg sticks

(Dodd 1940, Mann 1969, Robertson 1987), that are affixed to the spines or areoles of cactus cladodes (Dodd 1940, Robertson 1987; Fig. 1a; Appendix A). Once hatched, first- instar larvae penetrate the cuticle and epidermis of a plant cladode and feed internally for the duration of their larval life cycle (Dodd 1940, Mann 1969). Evidence of feeding by

C. cactorum includes entry holes, frass (green to brown excrement from feeding), and hollowed or translucent cladodes (Dodd 1940; Fig. 1b). Late-instar larvae of C. cactorum have an orange body, transverse black bands or spots on the abdomen, and a black head and pronotum (Neunzig 1997; Fig. 1c). Upon completion of the larval phase, C. cactorum typically pupate at the base of the plant in the leaf litter, beneath dead cladodes or rocks, or in the soil (Dodd 1940). Adults live approximately seven to nine days and are not known to feed (Dodd 1940, Pettey 1948). During that time, adults may disperse up to 24 kilometers from their natal site (Dodd 1940).

Cactoblastis cactorum is known to feed on at least 46 species of Opuntioideae.

This includes all of the 12 described Opuntia species within its native range, which includes parts of Argentina, Brazil, Paraguay, and Uruguay (Mann 1969), as well as all known native and introduced prickly pear species in Florida (Hight et al. 2002,

Zimmermann et al. 2004; Appendix B). Based on the known host range, many of the 105 recognized species of Opuntia in North America, including the 20 species endemic to the

United States (Anderson 2001), may be susceptible to herbivory by C. cactorum.

2 Modeling of the invasive moth's fundamental niche, based on a limited portion of the moth's range, suggests that a potential distribution of the moth includes large portions of the southern United States as well as Mexico and parts of Central America (Soberon et al.

2001). Given the effectiveness with which it reduced populations of target Opuntia species, the possible range expansion of C. cactorum to the western United States and south into Mexico represents a serious threat to the prickly pear cacti in these regions

(Zimmerman et al. 2004).

(a) (b)

(c)

Figure 1. (a) A typical C. cactorum egg stick, (b) evidence of internally-feeding larvae with brown frass exuded from the cladode, and (c) a late-instar C. cactorum larva.

3 While the threat of C. cactorum to prickly pear species in North and Central

America appears obvious, ways to successfully mitigate the threat appear less clear.

Monitoring for the early detection of C. cactorum is being conducted through the use of pheromone-baited traps in various locations, including Alabama, Mississippi, and South

Carolina in the southeastern United States as well as in western states such as Arizona,

California, and Texas (Rose 2009, R. Brown personal communication). In an effort to create a barrier to any further westward range expansion, management has included host plant removal from locations along the known western leading edge of C. cactorum’s range. Host plants have been manually removed from areas in Alabama including Bon

Secour National Wildlife Refuge, Fort Morgan, and Dauphin Island as well as from

Pensacola Beach, Florida (Rose 2009). Other control strategies have included the release of sterile adult females and partially sterile adult males (Hight et al. 2005, Rose 2009).

The goal of sterile insect release is to reduce the net reproductive rate of C. cactorum through wasted mating attempts of wild males with sterile females and the production of sterile male offspring from the mating of wild females with partially sterile males (Hight et al. 2005).

Unfortunately, it appears that existing control strategies have not been effective at preventing the spread of the moth along the Gulf Coast of the United States. In 2008 C. cactorum was found infesting Opuntia species on Petit Bois Island, Mississippi (C.

Brooks & G. Ervin, personal communication) and in 2009 was found infesting O. stricta species in Jefferson Parish, Louisiana (Payne 2009, T. Marsico, personal communication). Both of these locations are west of the original sites of sterile insect

4 release and cactus removal in Florida and Alabama (Floyd et al. 2006, Rose 2009). In addition, management activities along the Gulf Coast of the United States are ineffective in the prevention of potential long-distance dispersal events of C. cactorum from the

Caribbean to Mexico. Various workers have suggested that hurricanes may provide a means of long-distance dispersal for C. cactorum and/or prickly pear cacti (Zimmermann et al. 2005, G. Ervin, R. Brown, and L. Majure, personal communication). Dispersal via hurricanes may have been responsible for the population of C. cactorum that was detected on the island Isla Mujeres off of the coast of the Mexican Yucatán Peninsula in

2006 (NAPPO 2006).

It is pertinent to increase our understanding of the driving forces behind C. cactorum population persistence and range expansion so as to make informative management decisions and to mitigate the threat of the moth to prickly pear cacti in

North America. In particular, little is known about the relative prevalence of C. cactorum among host species and the effects of host diversity on prevalence. In addition, little is known about the community structure of native cactus-feeding insects and how the presence of C. cactorum influences patterns of native insect co-occurrence. I explored these two aspects of the ecology of C. cactorum to increase our understanding of the factors that influence the population dynamics of C. cactorum as well as its impact on native species.

5 Literature Cited

Anderson, E.F. (2001). The Cactus Family. Timber Press, Inc., Portland.

Begon, M., Harper, J.L., & Townsend, C.R. (1996). Ecology: individuals, populations, and communities. Third Edition. Blackwell Science, Inc., Malden, MA.

Bloem, S., Hight, S.D., Carpenter, J.E. & Bloem, K.A. (2005). Development of the most effective trap to monitor the presence of the cactus moth Cactoblastis cactorum (Lepidoptera: Pyralidae). Fla. Entomol. 88, 300-306.

Dickel, T.S. (1991). Cactoblastis cactorum in Florida (Lepidoptera: Pyralidae: Phycitinae). Trop. Lepid., 2, 117-118.

Dodd, A.P. (1940). The Biological Campaign Against Prickly Pear. Commonwealth Prickly Pear Board, Brisbane.

Floyd, J., Hight, S., Carpenter, J., Dorn, S., Heath, R., & Epsky, N. (2006). Cactoblastis cactorum Activities Report for July 2006. [WWW document]. URL http://www.aphis.usda.gov/plant_health/plant_pest_info/cactoblastis/updates/July 2006.pdf.

Garcia Tuduri, J.C., Martorell, L.F., & Medina Guad, S. (1971). Geographical distribution and host plants of the cactus moth, Cactoblastis cactorum (Berg) in Puerto Rico and the United States Virgin Islands. J. Agr. U. Puerto Rico 55, 130- 134.

Hight, S.D., Carpenter, J.E., Bloem, K.A., Bloem, S., Pemberton, R.W. & Stiling, P. (2002). Expanding geographical range of Cactoblastis cactorum (Lepidoptera: Pyralidae) in North America. Fla. Entomol. 85, 527-529.

Hight, S.D., Carpenter, J.E., Bloem, S. & Bloem, K.A. (2005). Developing a sterile insect release program for Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae): effective overflooding ratios and release-recapture field studies. Environ. Entomol., 34, 850-856.

Mann, J. (1969). Cactus-Feeding Insects and Mites. United States National Museum Bulletin 256. Smithsonian Institution Press, Washington, D.C.

Morin, P.J. (1999). Community Ecology. Blackwell Science, Inc., Malden, MA.

6 [NAPPO] North American Plant Protection Organization. (2006). Detection of an outbreak of cactus moth (Cactoblastis cactorum) in Isla Mujeres, Quintana Roo, Mexico [WWW document]. URL http://www.pestalert.org/oprDetail.cfm?oprID=216.

Neunzig, H.H. (1997). Pyraloidea, Pyralidae, Phycitinae (Part). In R.B. Dominick, et al. [ed.] The Moths of America North of Mexico, Fascicle 15.4. The Wedge Entomological Research Foundation, Washington, D.C.

Payne, J.H. (2009). Detections of the South American cactus moth (Cactoblastis cactorum) in Jefferson Parish, Louisiana. [WWW document]. URL http://www.aphis.usda.gov/plant_health/plant_pest_info/cactoblastis/downloads/DA -2009-25.pdf.

Pettey, F.W. (1948). The Biological Control of Prickly Pears in South Africa. Department of Agriculture Science Bulletin 271. Government Printer, Pretoria.

Robertson, H.G. (1987). Oviposition site selection in Cactoblastis cactorum (Lepidoptera): constraints and compromises. Oecologia, 73, 601-608.

Rose, R.I. (2009). USDA 2009 Strategic Plan for Control of the Cactus Moth, Cactoblastis cactorum. [WWW document]. URL http://www.aphis.usda.gov/plant_health/plant_pest_info/cactoblastis/downloads/st rategic_plan_2009.pdf.

Simmonds, F.J., & Bennett, F.D. (1966). Biological control of Opuntia spp. by Cactoblastis cactorum in the Leeward Islands (West Indies). Entomophaga, 11, 183-189.

Soberon, J., Golubov, J. & Sarukhán, J. (2001). The importance of Opuntia in Mexico and routes of invasion and impact of Cactoblastis cactorum (Lepidoptera: Pyralidae). Fla. Entomol. 84, 486-492.

Stiling, P. (2002). Potential non-target effects of a biological control agent, prickly pear moth, Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae), in North America, and possible management actions. Biol. Invasions 4, 273-281.

Zimmermann, H., Bloem, S. & Klein, H. (2004). Biology, History, Threat, Surveillance and Control of the Cactus Moth, Cactoblastis cactorum. Joint FAO/IAEA Programme of Nuclear Techniques in Food and Agriculture, Austria.

7 Zimmermann, H.G., Pérez Sandi y Cuen, M. & Bello Rivera, A. (2005). The Status of Cactoblastis cactorum (Lepidoptera: Pyralidae) in the Caribbean and the Likelihood of its Spread to Mexico. Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture and the Plant Health General Directorate, Mexico.

CHAPTER II

THEORETICAL PREDICTIONS OF THE INFLUENCE OF HOST SPECIES

RICHNESS AND QUALITY ON THE NET REPRODUCTIVE

RATE OF CACTOBLASTIS CACTORUM

Abstract

A variety of studies have suggested that the host preference, development time, survivorship, and fecundity of Cactoblastis cactorum vary among host species. The population growth rate of C. cactorum therefore may vary among host species and assemblages varying in species richness. I used a theoretical model to determine the conditions under which C. cactorum populations would experience positive population growth. I simulated host assemblages by sampling from one of five different theoretical host species pools to determine how host species richness and the mean and variance of host quality influenced the probability of positive C. cactorum population growth at different C. cactorum death rates. At low death rates, increased host species richness led to a decrease in the probability of positive C. cactorum population growth, but at high death rates, increased host species richness led to an increase, or no difference, in the probability of positive C. cactorum population growth. Increased variance and mean of host quality also led to an increase in the probability of positive C. cactorum population growth. Simulations of the C. cactorum model presented here suggest that host species

9 richness, the mean and variance of quality among host species, and the death rate of C. cactorum interact to determine the probability of C. cactorum positive population growth.

Introduction

The South American Cactus Moth, Cactoblastis cactorum (Berg) (Lepidoptera:

Pyralidae), is recognized as a serious threat to its prickly pear cacti host species in North

America, where it is has unintentionally become established. Cactoblastis cactorum was successfully used as a biological control agent to control invasive Opuntia in countries like Australia (Dodd 1940) and was introduced into the Caribbean country of Nevis to control weedy native Opuntia (Simmonds & Bennett 1966). Eventually the moth spread throughout the Caribbean (Garcia Tuduri et al. 1971, Zimmermann et al. 2005,

Pemberton & Liu 2007) and was detected in the Florida Keys in 1989 (Dickel 1991).

Since then, C. cactorum has come to inhabit many areas throughout Florida (Johnson &

Stiling 1998, Hight et al. 2002, Baker & Stiling 2008, Simonsen et al. 2008) and coastal areas of Georgia, South Carolina, Alabama, Mississippi, and Louisiana (Hight et al.

2002, Simonsen et al. 2008, Payne 2009).

A variety of techniques have been employed in an effort to both monitor C. cactorum populations and stop its range expansion, including host plant removal from coastal habitats, deployment of pheromone-baited traps (Rose 2009), and the release of sterile adult females and partially sterile adult males (Hight et al. 2005). However, it appears that existing control strategies have not been effective at preventing the spread of the moth along the Gulf Coast of the United States. In 2008 C. cactorum was found

10 infesting Opuntia species on Petit Bois Island, Mississippi (C. Brooks & G. Ervin, personal communication) and in 2009 was found infesting O. stricta in Jefferson Parish,

Louisiana (Payne 2009, T. Marsico, personal communication). Both of these locations are west of the original sites of sterile insect release and cactus removal in Florida and

Alabama (Floyd et al. 2006, Rose 2009).

It is important to understand the mechanisms driving C. cactorum population dynamics in the southeastern United States so as to make informative management decisions. Specifically, host species diversity and identity may play particularly important roles in determining C. cactorum population growth. Cactoblastis cactorum is known to feed on at least 46 species of Opuntioideae. This includes all of the 12 described Opuntia species within its native range, which includes parts of Argentina,

Brazil, Paraguay, and Uruguay (Mann 1969), as well as all known native and introduced prickly pear cactus species in Florida (Hight et al. 2002, Zimmermann et al. 2004;

Appendix B). While evidence is mixed about whether ovipositing females discriminate among host species (Robertson 1987, Johnson & Stiling 1996, Mafokoane et al. 2007,

Tate et al. 2009), host species have been found to differ significantly in quality as measured by C. cactorum survivorship and/or fecundity (Pettey 1948, Robertson 1987,

Robertson 1989, Johnson & Stiling 1996, Mafokoane et al. 2007). In addition, evidence suggests that the overall rate of C. cactorum infestation in a host assemblage varies depending upon whether a putative reservoir species is present (Dodd 1940, Stiling &

Moon 2001).

11 In order to determine how the quality (the rate of reproduction per C. cactorum individual per host species i) and species richness of hosts within a community interact to determine the direction and magnitude of population growth, I modified the macroparasite-host model of Anderson and May (1978) to determine the influence of host species quality and richness on the net reproductive rate, Ro, of C. cactorum. While the original model was developed with macroparasites in mind, C. cactorum meets the criteria specified by Anderson and May (1978) in their ecological definition of a

“parasite”: “utilization of the host as a habitat, nutritional dependence, and causing

‘harm’ to its host.” Anderson & May (1978) also stipulated that the death of the host was not a requirement for successful development of the parasite. These characteristics are common to herbivores such as C. cactorum. Cactoblastis cactorum feeds internally within prickly pear cladodes for the duration of the larval phase, the only phase of the life cycle during which the insect is known to feed (Dodd 1940, Pettey 1948, Mann 1969). In addition, the dramatic decline of prickly pear cactus populations in Australia (Dodd 1940) and the Caribbean (Zimmermann et al. 2005, Pemberton & Liu 2007) support the notion that feeding by C. cactorum exerts a fitness cost on its hosts.

By calculating Ro under host assemblage conditions varying in quality and species richness, I was particularly interested in answering two questions. First, how does the species richness of a host assemblage affect the probability of positive population growth

[Pr(Ro>1)] of C. cactorum? I predicted that increased species richness would lead to an increase in Pr(Ro>1) because the likelihood of adding a high quality host species to the assemblage would increase with increased species richness. Second, how does host

12 species quality affect Pr(Ro>1) of C. cactorum? I predicted that Ro would be particularly sensitive to characteristics of the host species pool, such as the mean and variance of host quality. An increased understanding of the conditions that lead to positive population growth in simulated populations of C. cactorum may generate better predictions regarding population growth in real C. cactorum populations.

Materials And Methods

The model of C. cactorum population dynamics

I modified Anderson and May’s (1978) deterministic model of macroparasite-host population dynamics to determine the probability of positive C. cactorum population growth under different scenarios of host species richness and quality within assemblages.

For any host species i in the community, the efficiency with which new herbivores are produced is

H H' = i , ...... (1) i + i Hi where i is a constant which determines the efficiency with which host species i produces new C. cactorum individuals and Hi is the population size of host species i. Given the per individual C. cactorum rate of reproduction, , the per individual C. cactorum rate of

' reproduction on host species i is Hi (hereafter referred to as “host quality”). The composite host quality for the C. cactorum population is the mean quality of the host species present in an assemblage because host-herbivore contact is assumed to be

13 frequency-dependent (Brooks & Zhang, in review). The change in the population size of

C. cactorum is given by

S ' Hi dP = P i μ ,...... (2) dt S

where P is the population size of C. cactorum, S is the species richness of the host assemblage, and μ is the death rate of C. cactorum.

The net reproductive rate of C. cactorum is then

S ' Hi R = i ...... (3) o μ S

dP The conditions under which a C. cactorum population will grow > 0 are given by dt

S ' Hi i >1...... (4) μS

Simulation of the Effects of Host Species Richness and Quality on Ro of C. cactorum

In order to evaluate the effect of host quality on C. cactorum dynamics, I simulated community assembly with host quality determined by either exponential or truncated Gaussian distributions with varying means and variance (Table 1; Fig. 2a, b, c, d, and e). I created five probability density functions (PDF), each representing the

' distribution of host quality, Hi , in a pool of potential host species. In each iteration of the simulation model, the overall rate of herbivore production by the host assemblage was

14 calculated by drawing one, two, or three samples (representing communities of hosts with a species richness of 1, 2, or 3, respectively) from the theoretical species pool (the PDF) and calculating Ro. Each scenario was simulated 1000 times to determine the proportion of times the value of Ro was greater than one. The calculated Pr(Ro>1) values were then plotted against the death rate of C. cactorum to determine the relationship between the C. cactorum death rate and Pr(Ro>1) of C. cactorum. Separate plots were created for values calculated using each PDF and separate functions were plotted within each plot for each of the three species richness values.

Table 1. Parameters of the probability density functions used in the simulations of the effects of host species richness and quality on Ro of C. cactorum.

Distribution Mean Variance PDF A Truncated Gaussian 0.2 0.04 PDF B Truncated Gaussian 0.2 0.025 PDF C Truncated Gaussian 0.05 0.025 PDF D Exponential 0.2 0.04 PDF E Exponential 0.05 0.025

15 Results

The effect of host species richness on the probability of herbivore growth

[Pr(Ro>1)] varied depending upon the magnitude of the death rate of C. cactorum (Fig.

2f, g, h, i, and j). At low C. cactorum death rates, Pr(Ro>1) decreased as host species richness increased. However, for each theoretical species pool there was a threshold value of the C. cactorum death rate above which increased host species richness led to either an increase or no change in Pr(Ro>1). Above this threshold value the difference in

Pr(Ro>1) between host assemblages of higher species richness compared to lower species richness was positive until converging to a difference of zero at high C. cactorum death rates (Fig. 2k, l, m, n, and o).

The mean of host quality also influenced the shape of the relationship between the death rate of C. cactorum and Pr(Ro>1). PDFs A, B, and D (Fig. 2a, b, and d, respectively) represent distributions of mean host quality equal to 0.2. PDFs C and E

(Fig. 2c and e, respectively) represent distributions of mean host quality equal to 0.05.

There was a more immediate decline in Pr(Ro>1) with increasing rates of C. cactorum death for values of Pr(Ro>1) derived from PDFs C and E (Fig. 2h and j, respectively), which had a lower mean than that of PDFs A, B, and D (Fig. 2f, g, and i, respectively).

For all assemblages drawn from PDFs C and E, regardless of species richness, Pr(Ro>1) declined to 0 by a death rate of 0.5. In contrast, Pr(Ro>1) never reached a value of zero for two- and three-species assemblages sampled from PDFs A and D (Fig 2f and i, respectively) and for C. cactorum death rates between 0 and 1.

16 (a) (f) (k)

>1) >1) o o Pr(R Frequency 1 0 1 0.0 0.5 1.0

0.0000.0 0.010 0.020 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0

(b) (g) (l) >1) Pr(R >1)

o o

Pr(R Pr(R

1 0 1 0.0 0.5 1.0 0.000.0 0.04 0.08 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0

>1)

>1) o o

Pr(R Pr(R 1 0 1 0.0 0.5 1.0 0.000.0 0.04 0.08 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0

(d) (i) (n)

>1)

>1) o o Pr(R

Pr(R

Frequency Frequency 1 0 1 0.0 0.5 1.0 0.000.0 0.02 0.04 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0

(e) (j) (o)

>1) >1) o o Pr(R Pr(R Frequency 1 0 1 0.0 0.5 1.0 0.000.0 0.10 0.20 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0 Distribution of Host Quality C. cactorum Death Rate C. Cactorum Death Rate

Figure 2. Figures (a), (b), (c), (d), and (e) represent PDFs A, B, C, D, and E, respectively. Figures (f), (g), (h), (i), and (j) display Pr(Ro>1) calculated using host quality values derived from PDFs 1, 2, 3, 4, and 5, respectively. Blue symbols represent single-species assemblages, orange symbols represent two-species assemblages, and black symbols represent three- species assemblages. Figures (k), (l), (m), (n), and (o) display the difference () in Pr(Ro>1) values for assemblages of different species richness values calculated using host quality values derived from PDFs 1, 2, 3, 4, and 5, respectively. Yellow symbols represent differences between the Pr(Ro>1) value of S = 2 and S = 1 assemblages. Purple symbols represent the difference between the Pr(Ro>1) value of S = 3 and S = 2 assemblages. Green symbols represent the difference between the Pr(Ro>1) value of S = 3 and S = 1.

The variance in host quality also influenced the shape of the relationship between

Pr(Ro>1) and the C. cactorum death rate. PDFs A and B have equivalent means but differ in variance (Fig. 2a and b, respectively). The lower variance of host quality in PDF

B led to distinct threshold values of the C. cactorum death rate above which Pr(Ro>1) decreased rapidly to zero (Fig. 2g). In contrast, the higher variance of host quality in

PDF A led to more gradual declines in Pr(Ro>1) with increasing C. cactorum death rates

(Fig. 2f). The magnitude of the difference in Pr(Ro>1) between host assemblages of higher species richness compared to lower species richness was also larger for Pr(Ro>1) values calculated using host quality drawn from PDF B (Fig. 2l) compared to Pr(Ro>1) values calculated using host quality drawn from PDF A (Fig. 2k) .

Discussion

Models determining the net reproductive rate, Ro, of a particular organism have been commonly used to predict and understand patterns of spread, outbreak potential, and the efficacy of control measures for organisms such as diseases and invasive species (e.g.,

Neubert & Caswell 2000, Arino et al. 2006, Hastings et al. 2006, Colizza et al. 2007,

Fraser et al. 2009). Specifically, positive population growth is often an important requirement for the successful establishment of an invading species (Mack et al. 2000).

Also, the consideration of factors influencing the population growth rate of an invading species is often useful in modeling the species’ rate of spread (e.g., Skellam 1951, Taylor

& Hastings 2005, Johnson et al. 2006). A determination of the factors that influence Ro

18 may also help to identify the demographic weaknesses of an invasive species that can be targeted for control (e.g. Shea & Kelly 1998).

Simulations of the C. cactorum model here suggest that the interaction of host quality, host species richness, and the C. cactorum death rate in the determination the probability of positive C. cactorum population growth is complex. While the mean and variance in quality in the pool of potential host species as well as the species richness of the host assemblage are important, their effect on Pr(Ro>1) is ultimately determined by the magnitude of the C. cactorum death rate. At very low death rates, the species richness of the host assemblage is more important in determining Pr(Ro>1) than host quality. However, at intermediate death rates, the mean and variance of quality in the pool of potential host species determines the magnitude of Pr(Ro>1) as well as the direction of the diversity effect (i.e., whether Pr(Ro>1) increases or decreases with species richness).

A variety of other studies have demonstrated the importance of variance, or heterogeneity, among hosts in determining the growth rate of a particular consumer. For instance, recent disease outbreaks reinforce the notion that individual heterogeneity in transmission rates is important in determining the magnitude of disease outbreaks

(Woolhouse et al. 1997). The presence of superspreaders (infected individuals that are responsible for the majority of secondary infections) is deemed to have been an important determinant in the spread of HIV among metropolitan areas in the 1970s to 1980s

(Klovdahl 1985, Doherty et al. 2005) and of the magnitude of the SARS outbreak in 2003

(Donnelly et al. 2003, Shen et al. 2004). Simulation and empirical experiments of insect

19 herbivore populations have also demonstrated that population dynamics are often more strongly governed by the variance in host quality as opposed to the mean of host quality

(Underwood 2004, Helms & Hunter 2005, Underwood 2009).

When the mean of host quality was held constant, a higher variance in host quality led to more a more gradual decrease in Pr(Ro>1) with increased C. cactorum death rates. This suggests that C. cactorum populations may be more likely to persist and grow in areas where there is a higher likelihood of the inclusion of an above average species, due to high variability in quality among speciesk within the pool of potential host species. However, the importance of the mean and/or variance of host quality has not yet been determined in actual host assemblages. Qualitatively, heterogeneity in C. cactorum infestation rates among host species helps to explain the known geographic distribution of C. cactorum within the southeastern United States. Cactoblastis cactorum has been able to expand its range from the initial location of detection in southern Florida to encompass primarily coastal areas in Florida, Georgia, South Carolina, Alabama, and

Mississippi, presumably due to the presence of the relatively common host species, O. stricta (Johnson & Stiling 1998, Zimmermann et al. 2000, Hight et al. 2002, Solis et al.

2004, Baker & Stiling 2008, personal observations).

Studies of the population dynamics of multi-host pathogens have also supported the idea that species diversity within host assemblages can lead to either a decrease or increase in pathogen prevalence because host species vary in their ability to create new infections (Keesing et al. 2006). In addition, a review of the effect of diversity on herbivory found that densities of polyphagous species frequently were higher in plant

20 polycultures compared to plant monocultures while densities of monophagous species were more frequently lower in polycultures, but the high variability among studies made it difficult to draw conclusions (Andow 1991). Other studies have found that disease prevalence is not determined so much by host species diversity, but by the magnitude of the overall community competence of disease transmission (the average of individual host reservoir competence, controlling for relative abundances among species) (Allan et al. 2009). I found the mean of host quality in the pool of potential host species and host assemblage species richness to be important in complementary ways. The mean of host quality in the pool of potential host species dictated the rate at which Pr(Ro>1) approached 0 as the death rate of C. cactorum increased. In contrast, increased species richness served to either increase or decrease Pr(Ro>1), depending upon the C. cactorum death rate.

Caveats

Modeling the impact of host heterogeneity on the population dynamics of a multi- species pathogen in a host community will depend in particular on the form of the transmission term. Using compartmental models, Dobson (2004) and Rudolf and

Antonovics (2005) showed that when host selection occurs in a density-dependent manner, an increase in host diversity is predicted to increase consumer population growth. Alternatively, when host selection is frequency-dependent, the contact rate between the consumer and its hosts is fixed. In this case, increased host diversity can lead to a reduction (Dobson 2004, Rudolf & Antonovics 2005) or increase in consumer

21 population growth, depending on the characteristics of the host assemblage and the manner in which species are added or lost as community composition changes over time and/or space (Brooks & Zhang, in review). By averaging quality to determine the overall rate of herbivore production, I assume that host-herbivore contact is frequency- dependent.

As well, I implicitly assumed that C. cactorum shows no preference among host species. If C. cactorum does exhibit strong preferences among species within a host assemblage, the quality of an individual species will also depend upon the context of the host assemblage within which it is found. In other words, if preference is important in the determination of sites for oviposition, the proportion of plants infested of a particular host species will be determined, in part, by the preference of C. cactorum for that particular host species relative to other host species within the assemblage. I also implicitly assumed that all host species within an assemblage have the same relative abundance, which is rarely the case in real communities. These models also do not consider asymmetric between-species transmission rates of C. cactorum, which may be important in determining the population dynamics of C. cactorum.

Conclusions

Simulations of the C. cactorum model presented here suggest that species richness, the average and variance of quality in the pool of potential host species, and the death rate of C. cactorum interact to determine the probability of positive C. cactorum population growth. In particular, this model predicts that the effect of species richness on

22 Pr(Ro>1) is dependent upon the death rate of C. cactorum as well as the mean and variance in host quality. It will be important to test the predictions of this model to determine if it realistically predicts the population dynamics of C. cactorum within its current range in the United States.

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CHAPTER III

HOST IDENTITY AND DIVERSITY AFFECT THE PREVALENCE OF

CACTOBLASTIS CACTORUM1

Abstract

Biodiversity plays an important role in the structure of food webs and the mediation of trophic interactions. Consumers can be particularly impacted by the diversity of species that serve as their hosts. I examined the association between

Cactoblastis cactorum and its prickly pear cacti hosts in Florida in order to assess the role of diversity within the host assemblage in the spread of this invasive moth. The data suggest that the dynamics of the invasion are influenced by both a sampling effect as well as diversity per se. Cactoblastis cactorum prevalence was highest on Opuntia stricta and

O. humifusa var. ammophila compared to other host species. In addition, C. cactorum prevalence was significantly higher when O. stricta was present in the community.

Species evenness and density were not predictive of infestation. Instead, multi-species host assemblages were significantly more infested than single species host assemblages, independent of the sampling effect.

1 This chapter has been submitted for publication.

28 Introduction

During the past few decades there has been increased interest in the role of biodiversity in a variety of ecosystem- and community-level processes (e.g. Tilman et al.

1997, Worm & Duffy 2003), including the profound impacts that feedbacks among consumers and their resources can have on ecosystem structure and function (Pace et al.

1999, Worm et al. 2002). The population growth rate of a consumer can be particularly impacted, either positively or negatively, by the diversity of species on which it feeds

(i.e., its resources or hosts). An increased diversity of hosts can decrease the prevalence or impact of a particular consumer through the reduction of the absolute or relative abundance of high quality hosts. This “dilution effect” contrasts with the “amplification effect,” which occurs when increased host diversity leads to increased consumer prevalence or impact (Keesing et al. 2006).

The direction and magnitude of this diversity effect will depend, in part, upon the mode of host selection by the consumer (Dobson 2004, Rudolf & Antonovics 2005,

Keesing et al. 2006). When host selection occurs in a density-dependent manner, an increase in host diversity is predicted to increase consumer population growth (Dobson

2004, Rudolf & Antonovics 2005). Alternatively, when host selection is frequency- dependent, the contact rate between the consumer and its hosts is fixed. In this case, increased host diversity can lead to a reduction (Dobson 2004, Rudolf & Antonovics

2005) or increase in consumer population growth, depending on the characteristics of the host assemblage and the manner in which species are added or lost as community composition changes over time and/or space (Brooks & Zhang, in review).

29 The degree of heterogeneity in “quality” among hosts is one of the critical factors regulating the magnitude of the diversity effect (Tilman et al. 1997, LoGiudice et al.

2003, Brooks & Zhang, in review). This is due to a statistical effect often referred to as the “sampling effect,” which is the increased likelihood of the inclusion of a highly influential species in a more diverse community (Tilman et al. 1997). A variety of empirical studies have shown that the identity of the host species is important in determining the direction of the diversity effect; i.e. whether dilution (LoGiudice et al.

2003, Emery & Gross 2007) or amplification occurs (Power & Mitchell 2004, Russell et al. 2007).

The dilution effect has been the most frequently observed effect in empirical disease systems such as Lyme disease (Schmidt & Ostfeld 2001, LoGiudice et al. 2003) and West Nile Virus (Ezenwa et al. 2006) as well as in parasite systems such as Ribeiroia ondatrae (Johnson et al. 2008). However, the predominance of the dilution effect in plant-herbivore systems is less clear. A review of the effect of diversity on herbivory found that densities of polyphagous species frequently were higher in plant polycultures compared to plant monocultures while densities of monophagous species were more frequently lower in polycultures, but the high variability among studies made it difficult to draw conclusions (Andow 1991).

I use the interactions between the South American Cactus Moth, Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae), and its host species, prickly pear cacti

(subfamily Opuntioideae), in Florida as a study system through which to explore the effect of diversity on plant-herbivore interactions. This exotic moth is known to feed on

30 at least 46 species of Opuntioideae, including all of the 12 described Opuntia species within its native range (Mann 1969) as well as all known native and introduced prickly pear species in Florida (Hight et al. 2002, Zimmermann et al. 2004, Appendix B). This moth has been widely used as a biological control agent for invasive prickly pear cacti in countries such as Australia (Dodd 1940) and South Africa (Pettey 1948) as well as to control weedy native prickly pear cacti in the Caribbean country of Nevis (Simmonds &

Bennett 1966). Cactoblastis cactorum was first detected in the Florida Keys in 1989

(Dickel 1991) and has since spread as far north as Charleston, South Carolina along the

Atlantic Coast (High et al. 2002) and as far west as Jefferson Parish along the Gulf of

Mexico (Payne 2009). The recent detection of C. cactorum on the island Isla Mujeres off of the coast of the Mexican Yucatán Peninsula (NAPPO 2006) as well as the continued westward expansion of its range along the Gulf Coast raises concern about the potential invasion of the southwestern United States and Mexico, where Opuntioideae cacti are highly diverse (Hernández et al. 2008). It is difficult to predict the dynamics of C. cactorum and its potential impact on community structure without understanding the role that diversity plays in the population growth and impact of this insect in areas where it is already present.

To explore the potential influence of host diversity on the invasion of C. cactorum, I was particularly interested in addressing the following questions. First, does host species identity affect the presence and prevalence of C. cactorum in Florida?

Studies have implicated the native O. stricta as a preferred host species in Florida (Baker

& Stiling 2008) and in the Caribbean (Pemberton & Liu 2007). In particular, O. stricta

31 (Haw.) Haw. was the main species targeted for biological control in Australia (Dodd

1940). Also, the C. cactorum population that ultimately invaded Florida from the

Caribbean was derived from ancestral populations in Australia (Pettey 1948, Simmonds

& Bennett 1966) and therefore has had a relatively long history (60+ years) of association with O. stricta, allowing for potential adaptation to this host species to have occurred by the time C. cactorum colonized Florida. I therefore hypothesized that I would observe the highest prevalence of C. cactorum on O. stricta.

Second, does host diversity affect the presence and prevalence of C. cactorum? A number of studies have suggested that ovipositing females do not discriminate among host species (Robertson 1987, Johnson & Stiling 1996, but see Mafokoane et al. 2007), even though host species differ significantly in quality as measured by survivorship or fecundity (Pettey 1948, Robertson 1987, Robertson 1989, Johnson & Stiling 1996,

Mafokoane et al. 2007). I therefore hypothesized that I would see a dilution effect in more diverse host assemblages because the failure to discriminate between host species would suggest a failure to discriminate between low- and high-quality hosts. I would therefore expect to see a lower prevalence of moth infestation in more diverse host communities containing a mix of low- and high-quality hosts because contacts would be wasted on low-quality hosts.

32 Materials And Methods

Study System

Cactoblastis cactorum is a multivoltine pyralid moth that lays its eggs in linear chains of approximately 35 to 90 eggs, commonly referred to as egg sticks (Dodd 1940,

Mann 1969, Robertson 1987), that are affixed to the spines or areoles of cactus cladodes

(Dodd 1940, Robertson 1987). Once hatched, first-instar larvae penetrate the cuticle and epidermis of a plant cladode and feed internally for the duration of their larval life cycle

(Dodd 1940, Mann 1969). Upon completion of the larval phase, C. cactorum typically pupate at the base of the plant in the leaf litter, beneath dead cladodes or rocks, or in the soil (Dodd 1940). Adults live approximately seven to nine days and are not known to feed (Dodd 1940, Pettey 1948). During that time, adults may disperse up to 24 kilometers from their natal site (Dodd 1940).

Cactoblastis cactorum feeds on a wide array of Opuntia species, including all known native and introduced species found in Florida. The native species of Opuntia in

Florida include O. pusilla (Haw.) Haw. (Fig. 3a), O. stricta (Haw.) Haw. (Fig. 3b), and the three varieties of O. humifusa: var. ammophila (Small) L.D. Benson (Fig. 3c), var. austrina (Small) Dress (Fig. 3d), and var. humifusa (Raf.) Raf. (Fig. 3e). Additional native prickly pear cacti species in Florida include O. cubensis Britton & Rose, O. triacantha (Willdenow) Sweet, Consolea corallicola Small and the non-native O. ficus- indica (L.) Mill., O. engelmannii Salm-Dyck ex Engelm., and Nopalea Salm-Dyck species (Benson 1982, Pinkava 2003).

33 (a)

(b)

(c)

(d)

(e)

Figure 3. Examples of (a) O. pusilla, (b) O. stricta, and the three varieties of O. humifusa: (c) O. humifusa var. ammophila, (d) O. humifusa var. austrina, and (e) O. humifusa var. humifusa.

34 Study Sites and Data Collection

I surveyed 165 sites throughout Florida from March to September 2008 for the presence of C. cactorum. Sampling was restricted to Florida because the region west of

Pensacola is actively managed by the United States Department of Agriculture (USDA) for C. cactorum through host plant removal from public and private land. Sites were chosen to maximize the geographic coverage of Florida. At each site I established a linear transect through a patch of host plants. Each host plant along the transect was identified to species and inspected for the presence of C. cactorum larvae. Height was measured for every fifth plant of each species detected along the transect. Because host plant density can vary considerably among species and sites, the length and width of transects was varied to allow a thorough sampling of the host plant patch, and thus surveys were time- rather than space-constrained. Some surveys took as little as approximately 15 minutes at sites where few plants or insects were found but some lasted up to approximately two hours at sites where host plants and cactus-feeding insects were relatively abundant. A small number of host plant patches were so small that plants were only counted within one 5 m radius circle. For 99 of the transects I measured host plant density by counting the number of plants per species within up to three non-overlapping

78.5 m2 circles (5 m radius).

I examined all cladodes on plants along each transect for evidence of C. cactorum infestation, including frass (green to brown excrement from feeding), hollowed or translucent cladodes, entry holes, or egg sticks. Because similar signs of damage are also caused by the native cactus moth, Melitara prodenialis Walker, cladodes showing

35 evidence of possible infestation were dissected to confirm the presence and identity of any cactophagous moth larvae. Late-instar larvae of C. cactorum have an orange body, dorsolateral black bands or transverse spots of the abdomen, and a black head and pronotum. In contrast, late-instar larvae of M. prodenialis have a dark blue body, a brown head capsule, and a dark brown to black pronotum (Neunzig 1997). The brown to blue pigmentation in M. prodenialis and orange pigmentation of C. cactorum is not present in all instars. However, most instars of C. cactorum can be identified by a black head and pronotum whereas most M. prodenialis instars have a dark brown to black pronotum and brown head. In addition, many instars of C. cactorum can be identified by the presence of two subventral (SV) setae on the seventh and eight abdominal segments compared to the presence of three SV setae on the seventh and eighth abdominal segments of many instars of M. prodenialis (Neunzig 1997, personal observations). The identity of ambiguous samples were confirmed by sequencing the mitochondrial cytochrome oxidase I (COI – 1508 bp) gene and aligning ambiguous samples with known reference samples using Sequencher (Genecodes Corporation, Ann Arbor, MI).

Identification of ambiguous samples through sequencing revealed that approximately

97% of samples (n = 94) were identified correctly based on morphology alone.

Statistical Analysis

For site-level data, I restricted my analysis to host species that were found in at least 15 of the sites to allow for adequate sample sizes for comparisons. All analyses were conducted using the R Statistical Language (R Development Core Team 2008). I

36 define the prevalence of C. cactorum to be the proportion of plants at a site that were infested at the time of sampling. A comparison of C. cactorum prevalence among host species was conducted using a one-way ANOVA with post-hoc comparisons (Benjamini

& Hochberg 1995). I also explored the effect of host species presence on the prevalence of C. cactorum through a one-way ANOVA with post-hoc multiple comparison tests. To determine if a diversity effect operates in this system, I examined both the influence of evenness as well as richness on C. cactorum prevalence. I calculated evenness using

Simpson’s evenness measure (Simpson 1949, Smith & Wilson 1996) for all sites with a richness value greater than one. Linear regression was used to determine the relationship between C. cactorum prevalence and host species evenness. To determine the influence of species richness on C. cactorum prevalence I divided sites into two categories: sites with monocultures (assemblages containing only one host species) and sites with polycultures (assemblages containing more than one host species). I performed a one- way ANOVA with post-hoc multiple comparison tests to determine differences in prevalence for each host species in monoculture versus polyculture assemblages. I also examined plant height and density as additional factors that could explain observed patterns of C. cactorum infestation. I determined height differences among species through a one-way ANOVA. As well, I used a generalized linear model (binomial family) to explore the difference between the height of plants infested with C. cactorum and those not infested. Finally, I performed a one-way ANOVA to determine the relationship between species richness and plant density. I also used linear regression to

37 explore the relationship between density and C. cactorum prevalence. Maps were generated using ArcGIS Version 9.2 (ESRI, Redlands, CA).

Results

Ten taxa within the subfamily Opuntioideae were found at sites surveyed throughout Florida, including one unidentified species. These included the native O. stricta and O. pusilla (Fig. 4a) as well as O. humifusa var. ammophila, var. austrina, and var. humifusa (Fig. 4b). Vouchered herbarium specimens of many of these species have been added to the University of Florida Herbarium by L. Majure (L. Majure, personal communication). Of these taxa, six were found infested with C. cactorum in at least one site surveyed: Nopalea sp., O. ficus-indica, O. humifusa var. ammophila, O. humifusa var. humifusa, O. pusilla, and O. stricta. Opuntia humifusa var. austrina, O. engelmannii, O. triacantha, and the unidentified species were not found infested with C. cactorum. In the

Florida Keys, I found C. corallicola and two additional unidentified species, none of which were infested with C. cactorum, but these plants were not located within transects and therefore not included in any analyses. Cactoblastis cactorum was found at 24% of sites surveyed (n=165) and was primarily found at sites dominated by O. stricta and/or O. humifusa var. ammophila along the coast of Florida (within 5 kilometers of the Atlantic or Gulf Coasts or in the Florida Keys; mean prevalence ±1 SE = 3.54 ± 0.79%; Fig. 5).

Aside from 3 sites where C. cactorum was found in O. humifusa var. humifusa monocultures, all other sites where C. cactorum was detected either contained O. stricta

(29 sites), O. humifusa var. ammophila (10 sites), or both (3 sites).

38 Statistical analyses of host species-level differences in C. cactorum prevalence were restricted to the four host taxa that were found in at least 15 sites and were infested in at least one site: O. humifusa var. ammophila and var. humifusa, O. pusilla, and O. stricta. I found significant differences in C. cactorum prevalence among host species

(ANOVA: F[3,169] = 5.04, p = 0.0023, Fig. 6a). Average C. cactorum prevalence was greatest for O. stricta in comparison to O. humifusa var. humifusa (ANOVA with pairwise comparison tests: p = 0.0017) but was not significantly greater than O. humifusa var. ammophila or O. pusilla (ANOVA with pairwise comparison tests: p = 0.42 and p =

0.070, respectively). As well, there were no significant differences between O. humifusa var. ammophila and O. humifusa var. humifusa (ANOVA with pairwise comparison tests: p = 0.40), between O. humifusa var. ammophila and O. pusilla (ANOVA with pairwise comparison tests: p = 0.42), or between O. humifusa var. humifusa and O. pusilla

(ANOVA with pairwise comparison tests: p = 0.88).

The prevalence of C. cactorum also depended greatly on the presence of a particular host species within the host assemblage (ANOVA: F[7,652] = 3.22, p = 0.0023,

Fig. 6b). The presence of O. stricta at a site was associated with a significantly higher prevalence of C. cactorum (ANOVA with pairwise multiple comparison tests: p = 0.0069,

Fig. 6b). Mean prevalence was also higher in the presence of O. humifusa var. ammophila and O. pusilla, but was not statistically different than sites without either of the two species (ANOVA with pairwise multiple comparison tests: p = 0.33 and p= 0.35, respectively). In contrast, the presence of O. humifusa var. humifusa at a site was associated with a lower mean prevalence of C. cactorum but this also was not significant

39 (ANOVA with pairwise multiple comparison tests: p = 0.063). Opuntia. stricta was present at only 11 of the 70 sites (16%) containing O. humifusa var. humifusa, and O. humifusa var. humifusa was never found co-occurring with O. humifusa var. ammophila.

I did not find a relationship between species evenness and C. cactorum prevalence

2 (linear regression: F[1,26] = 0.12, r = 0.0044, p = 0.73). However, I did find that the prevalence of C. cactorum was higher in polycultures compared to monocultures

(ANOVA: F[7,165] = 4.85, p = 0.00054, Fig. 7a). While the mean prevalence was higher in polycultures for all species on an individual species basis, the relationship was only statistically significant for O. stricta (ANOVA with pairwise comparison tests: p =

0.0017, Fig 7b).

Average plant height was significantly higher at sites where C. cactorum was found (mean ±1 SE = 52.70 ± 4.49 cm; generalized linear model, binomial family: p <

0.0005). Plant height also differed significantly among host species (ANOVA: F[4,832] =

74.93, p < 0.0005). Average plant height was higher for O. stricta (mean ±1 SE = 40.86

± 2.06 cm) compared to O. humifusa var. humifusa (mean ±1 SE = 18.67 ± 0.51 cm), O. humifusa var. austrina (mean ±1 SE = 30.04 ± 1.84 cm), and O. pusilla (mean ±1 SE =

5.04 ± 0.32 cm; ANOVA with pairwise comparison tests: p < 0.0005, p = 0.00082, and p

< 0.0005, respectively), but was not significantly different than the average plant height for O. humifusa var. ammophila (mean ±1 SE = 40.23 ± 1.80 cm; ANOVA with pairwise comparison tests: p = 0.80). Species richness was not an adequate predictor of plant density (ANOVA: F[2,96] = 0.59, p = 0.56) and I found no relationship between C.

2 cactorum prevalence and density (linear regression: F[1,97] = 0.020, r < 0.0005, p = 0.89).

40 a

Opuntia stricta Opuntia pusilla

Opuntia humifusa var. ammophila Opuntia humifusa var. austrina Opuntia humifusa var. humifusa

050 100 200 300 400 500 Kilometers

Figure 4. Common native host plant species found in Florida include (a) O. stricta and O. pusilla as well as (b) O. humifusa var. ammophila, var. austrina, and var. humifusa.

41 Cactoblastis cactorum present Cactoblastis cactorum absent

050 100 200 300 400 500 Kilometers

Figure 5. Distribution of C. cactorum in Florida.

42 (a) (b) Host Species Absent a Host Species Present

* ab

ab Proportion of Plants Infested Proportion of Plants Infested

0.00 0.02SAHP 0.04 0.06 0.08 0.10 0.12 0.14 0.00 0.02SAHP 0.04 0.06 0.08 0.10 0.12 0.14 Host Species Host Species

Figure 6. Host plant species: S: O. stricta; A: O. humifusa var. ammophila; H, O. humifusa var. humifusa, and P: O. pusilla. (a) Prevalence of C. cactorum infestation by host species (mean ± SEM). Different lowercase letters denote statistically significant differences in the proportion of plants infested by C. cactorum. (b) Effect of host species presence on the prevalence of C. cactorum infestation (mean ± SEM). Statistical differences in the proportion of plants at sites with and without a host species are denoted as * (p<0.05).

43 (a) (b) Species Richness = 1 *** Species Richness > 1

b Proportion of Plants Infested Proportion of Plants Infested a

0.00S = 0.05 1 S 0.10 > 1 0.15 0.200.00 0.25 0.05SAHP 0.10 0.15 0.20 0.25 Species Richness Host Species

Figure 7. (a) Prevalence of C. cactorum by species richness. Different lowercase letters denote statistically significant differences in the proportion of plants infested by C. cactorum. (b) The effect of species richness on the prevalence of C. cactorum. Statistical differences in C. cactorum prevalence between host monocultures and polycultures for a particular host species are denoted as * (p<0.05).

44 Discussion

Many studies have found that a particularly productive species in a community, rather than diversity per se, is often responsible for ecosystem properties such as production (Tilman et al. 1997, Paine 2002) or resistance to invasion (Emery & Gross

2007). This “sampling effect” can confound observed diversity effects (Aarssen 1997,

Huston 1997, Tilman et al 1997). While variation among sites in terms of C. cactorum prevalence is due, at least in part, to the presence or absence of particularly important host species, I also observed an amplification effect. The average prevalence of C. cactorum was significantly greater in polycultures compared to monocultures, a trend particularly pronounced for sites containing O. stricta. That is, even though O. stricta appears to be the most important host species in Florida, the presence of additional species in a host assemblage containing O. stricta leads to an even higher prevalence of

C. cactorum beyond that observed in sites containing only O. stricta.

Importance of Species Identity

I observed the highest prevalence of C. cactorum infestation on O. stricta compared to other species. This is in agreement with other studies that have quantitatively compared rates of infestation between host species. Baker and Stiling

(2008) found that O. stricta was more frequently infested with C. cactorum in comparison to O. humifusa in Florida. Pemberton and Liu (2007) found that O. stricta was the most frequently infested species in the Caribbean.

45 I also identified a significant degree of heterogeneity in C. cactorum infestation among the three varieties of O. humifusa. No study exploring C. cactorum population dynamics to our knowledge has explicitly accounted for differences between the three varieties of O. humifusa (var. ammophila, var. austrina, and var. humifusa). While C. cactorum has previously been detected on O. humifusa var. austrina along the Lake

Wales Ridge at Archbold Biological Station (Baker & Stiling 2008), I did not find this variety to be infested at the nine sites surveyed, in contrast to the other two varieties of O. humifusa. I also found that O. humifusa var. ammophila experienced much higher levels of infestation by C. cactorum in comparison with other host taxa, including the other varieties of O. humifusa. In fact, there was no significant difference in the average prevalence of C. cactorum between populations of O. humifusa var. ammophila and O. stricta.

The prevalence of C. cactorum was also significantly higher in host assemblages that included O. stricta, suggesting that it is an important reservoir species and that other, less competent/preferred host species are much more likely to be infested when found co- occurring with O. stricta. For instance, C. corallicola, a species endemic to the Florida

Keys and facing extinction due to habitat loss and infestation by C. cactorum, was attacked more frequently when growing in close association with infested O. stricta

(Stiling & Moon 2001). In Australia, O. tomentosa Salm-Dyck was also often infested by

C. cactorum when found associated with O. stricta (Dodd 1940). This spillover effect from high- to low-competence species has been demonstrated for other insect herbivores

(e.g. Russell et al. 2007) as well as in disease systems (e.g. Power & Mitchell 2004). A

46 spillover effect may be most likely to occur if a host species that is highly competent at maintaining populations of the consumer is also relatively common and widespread, like

O. stricta in coastal areas of Florida.

Diversity and Prevalence

In contrast to our predictions, host assemblages containing two or more species experienced higher levels of infestation by C. cactorum in comparison to host species monocultures. This increase in average prevalence with increased richness can, in part, be attributed to the sampling effect (Aarssen 1997, Huston 1997, Tilman et al. 1997) because assemblages with greater species richness were more likely to contain O. stricta,

O. humifusa var. ammophila, or both. As well, the failure to find a relationship between changes in species evenness and changes in average prevalence of C. cactorum suggests that highly competent host species like O. stricta and O. humifusa var. ammophila may drive the infestation of other species even when present at low relative abundances in a community.

The higher average prevalence of C. cactorum observed in host species polycultures can also be explained by a diversity effect per se. For example, C. cactorum prevalence was significantly higher in polycultures containing O. stricta compared to monocultures of O. stricta. This indicates that, even though O. stricta is likely to be the most important species in terms of the C. cactorum invasion, alternative host species in the presence of O. stricta may play an important role in amplifying the prevalence of C. cactorum and contributing to the invasiveness of this insect. These alternative hosts do

47 not necessarily act as demographic “sinks” as might be expected if a dilution effect was taking place, but instead act in a complimentary manner to maintain high prevalence levels of C. cactorum. However, it is important to note that because of low host species richness in Florida, few sites contained more than a single host species and no more than three species were present at any of the sites sampled. As a result, extrapolation of these results to more diverse host assemblages would be difficult without further study.

Alternative Factors Influencing C. cactorum Prevalence

Patterns of C. cactorum prevalence could also be influenced by factors such as plant height, photosynthetic rate, or by site characteristics such as plant density. Because larvae rarely leave the host plant upon which they hatch (Dodd 1940, Pettey 1948), plants that are infested likely reflect plants that have been selected for oviposition by adult females. There is evidence to suggest that adult females choose host plants through the use of CO2 gradients that are detectable within approximately three meters from the ground (Stange et al. 1995). Adult females have been observed to fly close to the ground where CO2 gradients are detectable and to tap the plant surface with their CO2 sensors prior to oviposition (Stange et al. 1995). While egg sticks has been observed on short, young plants of treelike prickly pear cacti, they have rarely been observed at heights greater than two meters (Mann 1969, Monro 1975). Cactoblastis cactorum may also utilize volatile organic compounds (VOCs) to detect host plants because receptor neurons have been found that are sensitive to a number of VOCs emitted by O. stricta (Pophof et al. 2005).

The “apparency” of a particular host plant thus may depend upon its detectability through the emission of VOCs and CO2 gradients. Apparency is defined as a plant’s

“susceptibility to discovery” and depends on plant characteristics such as genotype and size as well as the characteristics of the surrounding community (Feeny 1976). I found that the average height of infested host plants was significantly higher than the average height of host plants not infested, but was still less than one meter and therefore within the range of a detectable CO2 gradient. Myers et al. (1981) also found that green plants were selected more frequently than yellow plants and that plants in full sun were chosen more frequently than plants in partial to full shade, suggesting that the detection of photosynthetic activity via CO2 gradients is important in host selection. Therefore, larger plants may be more apparent than smaller plants because they fix CO2 over a wider area and may emit larger volumes of VOCs.

Plant height can therefore be used to explain the differing severity of C. cactorum prevalence among host species. In contrast to the generally decumbent O. humifusa var. humifusa, O. humifusa var. ammophila can grow as an erect shrub up to two meters high, often with a distinct trunk (Benson 1982, Pinkava 2003) and a similar average height to

O. stricta. This lack of a statistical difference in average height between O. stricta and O. humifusa var. ammophila may help to explain why the prevalence of C. cactorum did not differ between the two species. While O. humifusa var. austrina is also upright and can form large bushes, it is on average shorter than O. stricta and O. humifusa var. ammophila. The greater average height of O. stricta and O. humifusa var. ammophila in

49 comparison to other native host species in Florida may make these species more

“apparent” as hosts to ovipositing females.

The prevalence of C. cactorum infestation could also be influenced by host plant density. If plant density were an important factor in determining the “apparency” of host plants, and therefore the severity of C. cactorum infestation, I would expect to see C. cactorum present more frequently at sites with higher plant densities. I also would expect to see higher plant densities in polyculture sites, where the prevalence of C. cactorum was significantly higher than in monocultures. However, I found no difference in plant density among sites with and without C. cactorum nor among monocultures and polycultures. Because host plants can range from a few centimeters to a over a meter in height and width, the abundance of resources for C. cactorum may be better be represented as a function of biomass or height as opposed to plant density.

Consequences of Spillover and the Amplification Effect

Increased levels of infestation of C. cactorum in more species-rich host assemblages in comparison to host monocultures suggests that there may be long-term demographic consequences for O. stricta and O. humifusa var. ammophila, the other host species that co-occur with them in a host assemblage, and for the native consumers that depend upon these plant species. Consequences of C. cactorum infestation may be manifested through apparent competition leading to changes in the relative abundance of host species or through the exclusion of particular species from a host assemblage (Holt

& Lawton 1994; e.g. Monro 1975). The introduction of C. cactorum to Kruger National

50 Park in South Africa led to the transition from populations of mature O. stricta plants before C. cactorum invasion to populations of smaller, more dense populations of plants after infestation (Hoffmann et al. 1998). Other studies have suggested that young, small plants are most at risk of infestation (Pettey 1948) and experience higher mortality rates than larger, more mature plants (Johnson & Stiling 1998). This potential reduction in biomass and flower and fruit production may have cascading effects for other consumers that utilize these plant species, including M. prodenialis, whose life history is similar to

C. cactorum, as well as for other more generalist consumers.

While C. cactorum has been hailed as a textbook example of the successes of biological control elsewhere in the world, it represents a serious threat to the native

Opuntioideae cacti in North and Central America. There are a number of rare, endangered cacti that are currently threatened, such as C. corallicola in the Florida Keys, or may be threatened in the future, like O. basilaris var. kernii and var. treleasei in

California, if C. cactorum continues its westward range expansion in North America. In addition, prickly pear cacti are economically important for food and livestock fodder in

Mexico (Zimmermann et al. 2004). Regions of the southwestern U.S. and Mexico have many more species of Opuntia than are present in the southeastern U.S (Benson 1982), where C. cactorum is currently present. If our results hold for these regions such as the southwestern United States and Mexico, C. cactorum prevalence could reach higher levels than observed in this study. It is therefore important to determine the long-term effects of C. cactorum infestation, as well as the factors influencing host selection, to allow for better predictions of the population dynamics of this exotic insect.

51 It will also be important to explore other factors potentially influencing patterns of infestation by C. cactorum such as weather events and the spatial distribution of host plants. Although here I assume that C. cactorum is primarily distributed along the coast because of the distribution of its primary host, O. stricta, and the most diverse host assemblages, weather events like hurricanes that exert their greatest impact on coastal areas may be important in the dispersal and distribution of C. cactorum (L. Majure, personal communication). In addition, an examination of the distribution of C. cactorum at varying spatial scales may reveal that factors such as density are important in determining regional population dynamics.

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CHAPTER IV

CACTOBLASTIS CACTORUM AND PATTERNS OF NATIVE INSECT SPECIES CO-

OCCURRENCE IN FLORIDA

Abstract

A number of studies have documented the negative impact of the invasive species, Cactoblastis cactorum, to populations of its prickly pear cactus host plants.

However, little is known about the potential impact of C. cactorum to populations of native cactus-feeding insects. I explored patterns of co-occurrence among Chelinidea vittiger, Dactylopius sp., and Melitara prodenialis to determine the potential impact of C. cactorum to the structure of native cactus-feeding insect assemblages. I used null models of species co-occurrence to determine if native insect species, in the absence of C. cactorum, co-occurred at random, were significantly segregated, or were significantly aggregated among sites and among plants. I used a partition test to determine if co- occurrence patterns of native insect species among sites and among plants differed depending upon the presence of C. cactorum. I was unable to distinguish co-occurrence patterns of native insect species in the absence of C. cactorum from patterns that could be generated by chance. The structure of native insect assemblages in the absence of C. cactorum was also not significantly different than the structure seen among native insects in the presence C. cactorum.

57 Introduction

The South American Cactus Moth, Cactoblastis cactorum (Berg) (Lepidoptera:

Pyralidae), now an invasive insect in the southeastern United States, gained initial fame as a biological control agent by eradicating millions of acres of invasive Opuntia populations in Australia in the 1930s (Dodd 1940). However, the subsequent introduction of C. cactorum into the island of Nevis in the 1950s (Simmonds & Bennett

1966) led to its spread throughout the Caribbean and eventual detection in the Florida

Keys in 1989 (Dickel 1991). Cactoblastis cactorum has since spread as far north as

Charleston, South Carolina along the Atlantic Coast (High et al. 2002) and as far west as

Jefferson Parish, Louisiana along the Gulf of Mexico (Payne 2009).

Since its initial detection in the Florida Keys in 1989, an increasing amount of attention has been focused on documenting the impacts of C. cactorum to native species in Florida. Cactoblastis cactorum causes considerable plant mortality, particularly among young plants (Johnson & Stiling 1998) and the endangered Consolea corallicola

Small, a species endemic to the Florida Keys (Stiling & Moon 2001). The invasion of host populations by C. cactorum may also lead to population-level changes in plant abundance or size. For example, prior to the arrival of C. cactorum, Opuntia stricta

(Haw.) Haw. at Florida Atlantic University in Boca Raton, Florida measured up to 6 feet tall. However, by 1993 plants could only be found with a height of one meter or less

(Pierce 1995).

While considerable effort has been expended documenting the impacts of C. cactorum to its host species in Florida, little is known about its potential impact on

58 populations of native insects that also utilize prickly pear cacti in Florida as food. Three species, in particular, are common specialist consumers on cacti in Florida: the native cactus moth, Melitara prodenialis Walker (Lepidoptera: Pyralidae), the cochineal

Dactylopius sp. (Hemiptera: Dactylopiidae), and the cactus bug Chelinidea vittiger

McAtee (Hemiptera: Coreidae) (Mann 1969). Much of what is known about these native cactus-feeding insects in Florida before the invasion of C. cactorum is due in large part to the efforts of Australian entomologists who traveled throughout North and South

America in the 1920s and 1930s searching for insects that could be used as biological control agents against invasive Opuntia in Australia (Dodd 1940, Mann 1969).

Chelinidea vittiger, Dactylopius sp., and M. prodenialis were all introduced along with C. cactorum to Australia for the biological control of exotic Opuntia. However, only C. cactorum was able to establish populations and persist (Dodd 1940). While M. prodenialis was considered at one point to be “the most destructive Opuntia insect in the

United States” (Hamlin 1925), attempts to establish populations in Australia failed, possibly due to a high prevalence of disease. Chelinidea vittiger and D. confusus were able to establish relatively large populations in Australia until C. cactorum destroyed many of their host plants, after which the two species were never seen again (Dodd

1940). These accounts suggest, as an initial hypothesis, that C. cactorum is a superior competitor for resources and may exclude C. vittiger, Dactylopius sp., and/or M. prodenialis through increased rates of host plant mortality or through reductions in host plant biomass.

59 To identify potential impacts of C. cactorum to native cactus-feeding consumers, I was interested exploring two aspects of the structure of cactus-feeding insect assemblages. First, I was interested in determining whether native insect species exhibited non-random patterns of co-occurrence in the absence of C. cactorum. Second, I was interested in determining if the structure of native insect assemblages differed depending upon the presence or absence of C. cactorum. I used a partition test to detect differences in co-occurrence patterns and hypothesized that the presence of C. cactorum would lead to significantly different patterns of species co-occurrence in comparison to patterns seen among insect assemblages in which C. cactorum was absent.

Materials And Methods

Study System

Two moth species known to feed on prickly pear cacti in Florida include the exotic C. cactorum and the native M. prodenialis (Fig. 8a). Both are pyralid moths that deposit eggs in linear chains that are affixed to the spines or areoles of cactus cladodes

(Hubbard 1894, Dodd 1940, Robertson 1987). In the case of M. prodenialis, egg masses are occasionally attached to cladode leaves as well (Hubbard 1894, personal observations). Once hatched, first-instar larvae of both species penetrate the cuticle and epidermis of a plant cladode and feed internally for the duration of their larval life cycle

(Dodd 1940).

60 Two other native insect species commonly found on prickly pear cacti in Florida include Dactylopius sp. (Fig. 8b) and C. vittiger (Fig. 8c). Chelinidea vittiger is a coreid bug that is native to and widespread throughout North America. Damage to the plant from feeding may cause the plant to take on a sickly yellow appearance and growth may be impeded (Hunter et al. 1912). Dactylopius sp. is a scale insect that is covered by secreted waxy materials with a white fuzzy appearance and is most often found feeding on young cladodes and fruit (Dodd 1940). Larvae may disperse passively via wind currents, but soon after hatching females select a position for feeding, insert the proboscis into the plant, and remain stationary for the duration of the life. The females then begin to secrete a waxy covering for protection that gives them their white, fluffy appearance

(Dodd 1940, Mann 1969).

Study Sites and Data Collection

From March to September 2008 I surveyed 165 sites throughout Florida for the presence of C. cactorum, C. vittiger, Dactylopius sp., and M. prodenialis. Sampling was restricted to Florida because the region west of Pensacola is actively managed by the

United States Department of Agriculture (USDA) for C. cactorum through host plant removal from public and private land. Sites were chosen to maximize the geographic coverage of Florida. At each site I established a linear transect through a patch of host plants. Each host plant along the transect was identified to species and inspected for the presence of C. cactorum and/or M. prodenialis larvae. Additionally, the presence of C. vittiger and Dactylopius sp. was documented for every fifth plant of each host species

61 detected along each transect. The presence of C. vittiger was confirmed by the presence of eggs, nymphs, and/or adults and the presence of Dactylopius sp. was confirmed when the insects were found feeding on the plant, identified by the characteristic white fluffy coating. Because host plant density can vary considerably among species and sites, the length and width of transects was varied to allow a thorough sampling of the host plant patch, and thus surveys were time- rather than space-constrained. Some surveys took as little as approximately 15 minutes at sites where few plants or insects were found but some lasted up to approximately two hours at sites where host plants and cactus-feeding insects were relatively abundant. A small number of host plant patches were so small that plants were only counted within one 5 m radius circle.

I examined all cladodes on plants along each transect for evidence of C. cactorum and M. prodenialis infestation. Because both species produce similar signs of damage, cladodes showing evidence of possible infestation were dissected to confirm the presence and identity of any cactophagous moth larvae. Late-instar larvae of C. cactorum have an orange body, dorsolateral black bands or transverse spots of the abdomen, and a black head and pronotum (Neunzig 1997). In contrast, late-instar larvae of M. prodenialis have a dark blue or blue-brown body, a brown head capsule, and a dark brown to black pronotum (Neunzig 1997). The brown to blue pigmentation in M. prodenialis and orange pigmentation of C. cactorum is not present in all instars. However, most instars of C. cactorum can be identified by a black head and pronotum whereas most M. prodenialis instars can be identified by a dark brown to black pronotum and brown head. In addition, many instars of C. cactorum can be identified by the presence of two subventral (SV)

62 setae on the seventh and eight abdominal segments compared to the presence of three SV setae on the seventh and eighth abdominal segments of many instars of M. prodenialis

(Neunzig 1997, personal observations). The identity of ambiguous samples were confirmed by sequencing the mitochondrial cytochrome oxidase I (COI – 1508 bp) gene and aligning ambiguous samples with known reference samples using Sequencher

(Genecodes Corporation, Ann Arbor, MI). Identification of ambiguous samples through sequencing revealed that approximately 97% of samples (n = 94) were identified correctly based on morphology alone.

Null Model Analyses

Null model analyses were conducted using EcoSim Version 7 software (Gotelli &

Entsminger 2004). Patterns of species co-occurrence were analyzed among sites and among plants where C. cactorum was absent using presence-absence matrices. For the site matrix there were three rows, each corresponding to one of the three focal native insect species (C. vittiger, Dactylopius sp., or M. prodenialis), each column was a different site (n = 126), and the individual cells of each matrix represented the presence

(“1”) or absence (“0”) of a species at a site. For the plant matrix there were also three rows, each corresponding to one of the three native insect species, each column was a different plant (n = 785), and the individual cells of each matrix represented the presence or absence of a species on a plant.

I used the C-score index to quantify patterns of species co-occurrence within each of the presence-absence matrices (Stone & Roberts 1990). This metric represents the

63 average number of checkerboard units of each pair of species in a community. A

01 10 checkerboard unit is a 2 x 2 set of cells that takes on either the form or , 10 01

(where “1” represents the presence and “0” represents the absence of a particular species). Checkerboard units are calculated using all possible 2 x 2 sets of cells such that the ordering of rows and columns does not influence the C-score. A C-score that is significantly larger than expected by chance indicates that there is significant segregation of species in communities. Alternatively, a C-score that is significantly smaller than expected by chance indicates that species co-occur (aggregate) more frequently than would be expected in a randomly assembled community (Stone & Roberts 1990, Gotelli

2000).

For each presence-absence matrix, I created a corresponding null distribution through randomizations of the data so as to allow the assignment of a probability value to an observed C-score. Because plants and sites varied in a number of ways (i.e., host species identity and host assemblage composition, in addition to size) I used the “fixed- fixed” null model in which the observed count of each species (the row sum of the presence-absence matrix) and the observed species richness of each site/plant (the column sum of the presence-absence matrix) are preserved during the randomization process (Connor & Simberloff 1979, Gotelli 2000). Therefore random matrices were created in which the relative differences among native insects (i.e., the relative frequency of detection across sites) as well as the relative suitability of each site/plant for colonization were preserved. While the “fixed-fixed” null model is unable to detect

64 significant patterns of co-occurrence if the presence-absence matrix is highly nested or highly segregated (Ulrich & Gotelli 2007) it has a low probability of Type I and Type II errors when used in conjunction with the C-score index (Gotelli 2000, Ulrich & Gotelli

2007).

Random matrices were generated using the “Sequential Swap” algorithm. This

10 01 algorithm transposes submatrices of the form to the form and thus 01 10 maintains fixed row and column sums (Stone & Roberts 1990). To generate a null distribution, 30,000 transpositions are first performed to randomize the presence-absence matrix. Then, with each subsequent transposition a new matrix is created and retained for use in generating a null distribution. I retained 5000 matrices to generate a null distribution from which to calculate the significance of an observed C-score (Gotelli &

Entsminger 2001).

Partition Test

I used a partition test to determine if the structure of native insect assemblages at sites with C. cactorum differed from the structure of the native insect assemblages at sites without C. cactorum. I also used a partition test to determine if the structure of native insect assemblages on plants with C. cactorum differed from the structure of native insect assemblages on plants without C. cactorum. For site data, a presence-absence matrix was constructed with four rows, three of which corresponded to one of the three focal native insect species (C. vittiger, Dactylopius sp., or M. prodenialis) and one row in which

65 individual cells designated whether C. cactorum was present at the site (“C. cactorum

present”) or absent (“C. cactorum absent”). Each column of the matrix was a different

site (n = 165) and the individual cells represented the presence (“1”) or absence (“0”) of a

species at a site. A similar matrix was created with plant data, but each column

represented an individual plant (n = 852). To perform the partition test on a matrix, the

labels “C. cactorum present” and “C. cactorum absent” were shuffled and C-scores for

each of the new “C. cactorum present” and “C. cactorum absent” matrices were

calculated. This procedure was repeated 5000 times to generate null distributions. The

observed C-score variances for the “C. cactorum present” and “C. cactorum absent” sites

and plants were then compared to the null distributions created through the random

partitioning. A variance value significantly larger than expected by chance would

indicate that the patterns of species co-occurrence in the two different matrices were

statistically different (Stone et al. 2000, Sanders et al. 2003).

Results

The most common of the four native insect species were C. vittiger and

Dactylopius sp., present at 32% (n = 52) and 35% (n = 57) of sites, respectively (Fig. 9b).

Cactoblastis cactorum was present at 39 sites (24%) (Fig. 9a) and M. prodenialis, the least encountered of the focal insect species, was present at only 18 sites (11%, Fig. 9a).

Sites without C. cactorum had, on average, 0.78 native species compared to an average of

0.74 native species present at each site with C. cactorum. The low values of native insect species richness can be explained by the fact that 47% of sites (n = 165) did not have any

66 native insect species present. Plants without C. cactorum had, on average, 0.26 native species compared to an average of 0.18 native species present on each plant where C. cactorum was also present. Thus, 78% of plants (n = 852) did not have any native insect species present.

Using the “fixed-fixed” null model analysis of co-occurrence, I was unable to

distinguish species co-occurrence patterns at sites without C. cactorum from patterns that

could be generated by chance. Similarly, patterns of species co-occurrence were not

significantly different than what would be expected by chance on plants without C.

cactorum (Table 2). Finally the structure of native insect assemblages was not

significantly different between sites with and without C. cactorum. The structure of

native insect assemblages was also not significantly different between plants with and

without C. cactorum (Table 2).

67 (a)

(b)

(c)

Figure 8. Examples of (a) different larval instars of M. prodenialis, (b) the characteristic white coating of Dactylopius sp. and (c) C. vittiger nymphs and an adult.

68 a

Cactoblastis cactorum Melitara prodenialis

Dactylopius sp. Chelinidea vittiger

050 100 200 300 400 500 Kilometers

Figure 9. Common cactophagous insect species found in Florida include (a) C. cactorum and M. prodenialis as well as (b) C. vittiger and Dactylopius sp.

Table 2. Analysis of co-occurrence patterns among native insect assemblages.

70 Standardized

Mean of simulation Variance of Lower Upper Effect Size Scale Observed C-score C-scores simulated C-scores Tail Tail (SES) Test for co-occurrence patterns among native insect assemblages where C. cactorum was absent. Site 298 297.8 3.9 0.74 0.39 0.1 Plant 2632.33 2624.66 734.32 0.63 0.41 0.28

Partition test for differences between assemblages with and without C. cactorum. Site 164 167.5 152.59 0.4 0.61 -0.28 Plant 1333.67 1281.5 2314.27 0.86 0.14 1.08

The columns “Lower Tail” and “Upper Tail” include p-values for the probability that the observed C-score was significantly less than or greater than predicted, respectively.

Discussion

I was unable to distinguish patterns of co-occurrence among native cactus-feeding insects at plants and sites without C. cactorum from patterns generated by random community assembly. Additionally, the presence of C. cactorum did not significantly influence the co-occurrence patterns of native cactus-feeding insects. While competition is considered common among phytophagous insects (Kaplan & Denno 2007), the detection of competition in insect assemblages using presence-absence data has been mixed. While Gotelli and McCabe (2002) and Horner-Devine et al. (2007) found broad support for the notion that species co-occur less frequently than expected by chance,

Gotelli and McCabe (2002) were unable to demonstrate in a meta-analysis of presence- absence data that invertebrate co-occurrence patterns are significantly nonrandom. In ant communities, for example, significant patterns of species co-occurrence have been detected under some circumstances (e.g., Sanders et al. 2003, Blüthgen et al. 2004,

Sanders et al. 2007) but not others (Sanders et al. 2007).

Competition is difficult to detect using only co-occurrence data, even if two strongly competing species are surveyed (Hastings 1987). As well, significant co- occurrence patterns are more difficult to detect if there has been insufficient sampling

(Horner-Devine et al. 2007). I surveyed my sites and plants only once for C. cactorum and the three native insects. Therefore, the list of species per site/plant may be incomplete. A number of plants and sites exhibited damage by either C. vittiger and/or the cactus moths. However, because individuals of those species were not found, they were considered “absent” from the site or plant. Repeated sampling at these sites may

71 therefore help to resolve issues of undercounted species and may reveal different patterns of species co-occurrence than those reported here.

Without repeated sampling, it also would be difficult to detect any form of indirect interaction, such as apparent or exploitative competition, that may occur between species. In a meta-analysis, Kaplan and Denno (2007) found that the majority of interspecific insect herbivore interactions were mediated through indirect interactions. It is possible that C. cactorum may impact populations of native insect species through exploitatitve or apparent competition, such that the presence of C. cactorum in a community leads to an increased host death rate or reduction in host biomass. This could then lead to the local extinction of both the native insects species and C. cactorum from a community, thus providing a plausible explanation for the absence of the cactus-feeding insects from so many sites and plants. The fact that populations of insects such as C. vittiger and D. confusus declined to apparent extinction in Australia after colonization of sites by C. cactorum (Dodd 1940) suggests that this may be a potentially important way in which C. cactorum interacts with other consumers in Florida. It is also possible that some interactions among cactus-feeding insects are of a facilitative nature. Carlton and

Kring (1994) suggested that feeding by C. vittiger on prickly pear cacti in Arkansas might predispose plants to colonization by M. prodenialis. The weevil Gerstaeckeria hubbardi (LeConte) was also observed to colonize prickly pear plants damaged by M. prodenialis (Hunter et al.1912). Detailed observations of the colonization and extinction patterns of a variety of cactus-feeding insects thus might help to identify the mechanisms structuring cactus-feeding insect assemblages and the potential impact of C. cactorum.

72 Finally, the inclusion of other cactus-associated organisms may also help to clarify the structure of native insect assemblages and the impact of C. cactorum to community structure. For example, ants have been identified as important predators of C. cactorum during the egg and/or larval stage in Australia (Dodd 1940) and South Africa

(Pettey 1948, Robertson 1988, Robertson 1989) as well as in the native range of C. cactorum (Lobos & Ochoa de Cornelli 1997). Ants are also important defenders of many plant species because they reduce rates of herbivory and their presence is associated with higher plant fitness (Rosumek et al. 2009). Ant presence on O. stricta along the Gulf

Coast of Mexico was associated with a significantly lower level of damage due to an unidentified pyralid moth and a significantly higher rate of fruit production (Oliveira et al. 1999). As well, documentation of the distribution of other organisms, such as fungi putatively vectored by C. cactorum (Lachance et al. 2000, Martin & Dale 2001), may also help to determine the potential impacts of C. cactorum on the co-occurrence patterns of native insect species.

While a variety of studies have documented the ability of an invasive species to negatively impact native species and/or native communities (e.g. Bertness 1984, Byers

2000, Sax et al. 2007), I was unable to demonstrate a different in co-occurrence patterns among native cactus-feeding insects in the presence of C. cactorum. There are many other organisms that utilize prickly pear cacti in Florida that were not included in this analysis. The inclusion of other species and repeated sampling may help to clarify the ways in which native cactus-feeding insect species are structured within a community and the potential impact of C. cactorum to populations of these species.

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Sanders , N.J. & Crutsinger, G.M. (2007). An ant mosaic revisited: dominant ant species disassemble arboreal ant communities but co-occur randomly. Biotropica, 39, 422-427.

76 Sax, D.F., Stachowicz, J.J., Brown, J.H., Bruno, J.F., Dawson, M.N., Gaines, S.D., Grosberg, R.K., Hastings, A., Holt, R.D., Mayfield, M.M., O’Connor, M.I. & Rice, W.R. (2007). Ecological and evolutionary insights from species invasions. Trends Eco. Evol., 22, 465-471.

Simmonds, F.J., & Bennett, F.D. (1966). Biological control of Opuntia spp. by Cactoblastis cactorum in the Leeward Islands (West Indies). Entomophaga, 11, 183-189.

Stiling, P., & Moon, D.C. (2001). Protecting rare Florida cacti from attack by the exotic cactus moth, Cactoblastis cactorum (Lepidoptera: Pyralidae). Fla. Entomol., 84, 506-509.

Stone, L., Dayan, T. & Simberloff, D. (2000). On desert rodents, favored states, and unresolved issues: scaling up and down regional assemblages and local communities. Am. Nat., 156, 322-328.

Stone, L. & Roberts, A. (1990). The checkerboard score and species distributions. Oecologia, 85, 74-79.

Ulrich, W. (2004). Species co-occurrences and neutral models: reassessing J.M. Diamond’s assembly rules. Oikos, 107, 603-609.

Ulrich, W. & Gotelli, N.J. (2007). Disentangling community patterns of nestedness and species co-occurrence. Oikos, 116, 2053-2061.

APPENDIX A

SELECTED PARTS OF A CACTUS

78 (a) (b)

Figure 10. Selected parts of a cactus, including (a) the spine and glochids which grow from the areole and (b) the leaves growing on a cladode.

APPENDIX B

KNOWN HOST SPECIES OF CACTOBLASTIS CACTORUM AND THE LOCATIONS

WHERE INFESTED HOSTS HAVE BEEN FOUND.

80 Table 3. Known host species of Cactoblastis cactorum and the locations where infested hosts have been found.

Species Location of Infestation Austrocylindropuntia subulata Australia (Dodd 1940), South Africa (Moran (Muehlenpf.) Backeb. & Zimmermann 1991)

Consolea corallicola Small Florida (Stiling & Moon 2001) Consolea moniliformis A. Berger Puerto Rico (Garcia Tuduri et al. 1971) Consolea rubescens Lem. Puerto Rico (Garcia Tuduri et al. 1971), Nevis, St. Kitts (Zimmermann et al. 2004) Consolea spinosissima Lem. Jamaica (Zimmermann et al. 2005) Cylindropuntia caribaea (Britton & Dominican Republic (Zimmermann et al. Rose) F.M. Knuth 2005) Cylindropuntia fulgida (Engelm.) South Africa (Zimmermann et al. 2004) F.M. Knuth Cylindropuntia imbricata (Haw.) South Africa (Zimmermann et al. 2004) F.M. Knuth Nopalea Salm-Dyck Florida (Sauby et al., submitted) Opuntia anacantha var. retrorsa Argentina (Mann 1969) (Speg.) R. Kiesling

Opuntia anacantha var. utikilio Argentina (Dodd 1940, Mann 1969) (Speg.) R. Kiesling Opuntia antillana Britton & Rose Puerto Rico (Garcia Tuduri et al. 1971), Dominican Republic (Zimmermann et al. 2005) Opuntia aurantiaca Lindl. Australia, Argentina (Dodd 1940), South Africa (Pettey 1948)

Opuntia canterae Arechavaleta South America (Dodd 1940, Mann 1969)

Table 3 (Continued)

Species Location of Infestation Opuntia cochenillifera (L.) Mill. Antigua, Dominican Republic, Cayman Islands, Cuba, Puerto Rico, Jamaica, Montserrat, St. Kitts (Zimmermann et al. 2005), Nevis (Pemberton & Liu 2007) Opuntia dejecta Salm-Dyck Australia (Hosking et al. 1988) Opuntia delaetiana (F.A.C. Weber) Argentina (Dodd 1940, Mann 1969) Vaupel Opuntia discolor Britton and Rose Argentina (Mann 1969, Dodd 1940) Opuntia elatior Mill. Australia (Hosking et al. 1988) Opuntia engelmannii Salm-Dyck South Africa (Pettey 1948), Antigua, Nevis (Moran & Zimmermann 1984) Opuntia ficus-indica (L.) Mill. Argentina (Dodd 1940, Mann 1969), South Africa (Pettey 1948, Moran & Zimmermann 1984), Hawaii (Fullaway 1954, Moran & Zimmermann 1984), Australia (Moran & Zimmermann 1984), South Carolina (Hight et al. 2002), Florida (Hight et al. 2003), Domincan Republic, Cuba (Zimmermann et al. 2005) Opuntia humifusa (Raf.) Raf. Australia (Dodd 1940), South Carolina (Hight et al. 2002), Florida (Pierce 1995, Johnson & Stiling 1998, Baker & Stiling 2008) Opuntia humifusa var. austrina Florida (Sauby, et al., submitted) (Small) Dress

Opuntia humifusa var. ammophila Florida (Sauby, et al., submitted) (Small) L.D. Benson Opuntia jamaicensis Britton & Jamaica (Zimmermann et al. 2005) Harris

Table 3 (Continued)

Species Location of Infestation Opuntia leucotricha DC. Domincan Republic (Zimmermann et al. 2005)

Opuntia megacantha Salm-Dyck Hawaii (Fullaway 1954), South Africa (Monro 1975)

Opuntia monacantha Haw. Argentina (Dodd 1940), Mauritius (Greathead 1971)

Opuntia paraguayensis K. Schum. Australia (Hosking et al. 1988)

Opuntia pilifera F.A.C. Weber Dominican Republic (Zimmermann et al. 2005)

Opuntia pusilla (Haw.) Haw. South Carolina, Georgia (Hight et al. 2002), Florida (Sauby et al., submitted)

Opuntia quimilo K. Schum. Argentina (Dodd 1940, Mann 1969)

Opuntia repens Bello Puerto Rico (Garcia Tuduri et al. 1971, Zimmermann et al. 2005)

Opuntia robusta Wendl. South Africa (Annecke & Moran 1978)

Opuntia salmiana J. Parm. ex. South America (Dodd 1940, Mann 1969), Pfeiff. South Africa (Moran & Zimmermann 1991)

Opuntia sanguinea Proctor Jamaica (Zimmermann et al. 2005)

Opuntia spinulifera Salm-Dyck South Africa (Moran and Zimmermann 1991)

Opuntia streptacantha Lem. Australia (Dodd 1940)

Table 3 (Continued)

Species Location of Infestation Opuntia stricta (Haw.) Haw. Australia (Dodd 1940), Puerto Rico, U.S. Virgin Islands (Garcia Tuduri et al. 1971), Cayman Islands, New Caledonia (Moran & Zimmermann 1984), The Bahamas (Starmer et al. 1988), South Africa (Moran & Zimmermann 1991), Florida (Pierce 1995, Hight et al. 2002), Cuba (Zimmermann et al. 2000), Georgia (Hight et al. 2002), Antigua, Dominican Republic, Dominica, Guadeloupe, Jamaica, Montserrat, Nevis, Saint Lucia (Zimmermann et al. 2005), St. Kitts (Pemberton & Liu 2007)

Opuntia sulfurea G. Don. South America (Dodd 1940, Mann 1969)

Opuntia taylori Britton & Rose Dominican Republic (Zimmermann et al. 2005)

Opuntia tomentosa Salm-Dyck Australia (Dodd 1940)

Opuntia triacantha Sweet Antigua (Simmonds & Bennett 1966, Zimmermann et al. 2005), Florida (Zimmermann et al. 2004), Cayman Islands, Guadeloupe, Nevis (Zimmermann et al. 2005), Mauritius (Moran & Zimmermann 1984), Montserrat (Simmonds & Bennett 1966), Puerto Rico (Garcia Tuduri et al. 1971)

Opuntia tuna Mill. Jamaica (Zimmermann et al. 2005), Mauritius (Moran & Zimmermann 1984)

Opuntia x cubensis Britton and Rose Florida (Zimmermann et al. 2004)

84 Literature Cited

Annecke, D.P. & Moran, V.C. (1978). Critical review of biological pest control in South Africa. 2. The prickly pear, Opuntia ficus-indica (L.) Miller. J. Entomol. Soc. Sth. Afr., 41, 161-188.

Dodd, A.P. (1940). The Biological Campaign Against Prickly Pear. Commonwealth Prickly Pear Board, Brisbane.

Fullaway, D.T. (1954). Biological control of cactus in Hawaii. J. Econ. Entomol., 47, 696-700.

Greathead, D. J. (1971). A Review of Biological Control in the Ethiopian Region, Commonwealth Institute of Biological Control Technical Communication No. 5, Commonwealth Agricultural Bureaux, Slough, England.

Hight, S.D., Bloem, S., Bloem, K.A. & Carpenter, J.E. (2003). Cactoblastis cactorum (Lepidoptera: Pyralidae): observations of courtship and mating behaviors at two locations on the Gulf Coast of Florida. Fla. Entomol., 86, 400-408.

Hosking, J.R., McFadyen, R.E. & Murray, N.D. (1988). Distribution and biological control of cactus species in eastern Australia. Plant Prot. Quart., 3, 115-123.

Johnson, D.M. & Stiling, P.D. (1998). Distribution and dispersal of Cactoblastis cactorum (Lepidoptera: Pyralidae), an exotic Opuntia-feeding moth, in Florida. Fla. Entomol., 81, 12-22.

Mann, J. (1969). Cactus-Feeding Insects and Mites. United States National Museum Bulletin 256. Smithsonian Institution Press, Washington, D.C.

85 Moran, V.C. & Zimmermann, H.G. (1984). The biological control of cactus weeds: achievements and prospects. Biocontrol News Inform., 5, 297-320.

Moran, V.C. & Zimmermann, H.G. (1991). Biological control of cactus weeds of minor importance in South Africa. Agric. Ecosys. Environ., 37, 37-55.

Monro, J.M. (1975). Environmental variation and the efficiency of biological control – Cactoblastis in the Southern Hemisphere. Proc. Ecol. Soc. Australia, 9, 204-212.

Pemberton, R.W. & Liu, H. (2007). Control and persistence of native Opuntia on Nevis and St. Kitts 50 years after the introduction of Cactoblastis cactorum. Biol. Control, 41, 272-282.

Pettey, F.W. (1948). The Biological Control of Prickly Pears in South Africa. Department of Agriculture Science Bulletin 271. Government Printer, Pretoria.

Sauby, K.E., Marsico, T.D., Ervin, G.N. & Brooks, C.P. (Submitted). Host identity and diversity affect the prevalence of an invasive herbivore.

Simmonds, F.J., & Bennett, F.D. (1966). Biological control of Opuntia spp. by Cactoblastis cactorum in the Leeward Islands (West Indies). Entomophaga, 11, 183-189.

Starmer, W.T., Aberdeen, V. & Lachance, M.-A. (1988). The yeast community associated with decaying Opuntia stricta (Haworth) in Florida with regard to the moth, Cactoblastis cactorum (Berg). Fla. Science, 51, 7-11.

Stiling, P., & Moon, D.C. (2001). Protecting rare Florida cacti from attack by the exotic cactus moth, Cactoblastis cactorum (Lepidoptera: Pyralidae). Fla. Entomol., 84, 506-509.

86 Zimmermann, H.G., Pérez Sandi y Cuen, M. & Bello Rivera, A. (2005). The Status of Cactoblastis cactorum (Lepidoptera: Pyralidae) in the Caribbean and the Likelihood of its Spread to Mexico. Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture and the Plant Health General Directorate, Mexico.

APPENDIX C

CACTUS-ASSOCIATED INSECTS ENCOUNTERED IN FLORIDA.

Table 4. Cactus-associated insects encountered in Florida.

Order Family Insect Species Host Species

Coleoptera Curculionidae Gerstaeckeria hubbardi (LeConte) O. humifusa var. ammophila Melyridae Collops nigriceps Say O. humifusa var. austrina and var. humifusa Scarabaeidae Trichiotinus piger (Fabricius) O. humifusa var. austrina Hemiptera Alydidae

89 Alydus pilosulus Herrich-Schaeffer O. humifusa var. humifusa Dactylopiidae Dactylopius sp. O. stricta, O. pusilla, O. engelmannii, Nopalea sp. and O. humifusa var. ammophila, var. austrina, and var. humifusa Coreidae Chelinidea vittiger McAtee O. stricta, O. pusilla, O. engelmannii, Nopalea sp. and O. humifusa var. ammophila, var. austrina, and var. humifusa Narnia femorata Stal O. humifusa var. humifusa, O. stricta

Table 4 (Continued) Order Family Insect Species Host Species Hymenoptera Formicidae Camponotus ashmeadi Mayr O. stricta Camponotus floridanus (Buckley) O. stricta Camponotus planatus Roger O. stricta, O. humifusa var. humifusa Cardiocondyla emery Forel O. humifusa var. humifusa Dorymyrmex bureni (Trager) O. humifusa var. humifusa Forelius pruinosus (Roger) O. humifusa var. humifusa& var. austrina, O. stricta Monomorium minimum (Buckley) O. humifusa var. austrina & var. humifusa Pseudomyrmex gracilis (Fabricius) O. stricta

90 Solenopsis invicta Buren O. humifusa var. humifusa, O. stricta

Lepidoptera Pyralidae Cactoblastis cactorum (Berg) O. stricta, O. pusilla, Nopalea sp., O. ficus-indica, and O. humifusa var. ammophila and var. humifusa Melitara prodenialis Walker O. stricta, O. pusilla, and O. humifusa var. ammophila, var. austrina, and var. humifusa Insect specimens are vouchered in the Mississippi Entomological Museum and stored in the lab of Christopher Brooks in the Department of Biological Sciences at Mississippi State University. Only C. cactorum, C. vittiger, Dactylopius sp., and M. prodenialis were surveyed systematically. All other insects were collected opportunistically and host records may not sufficiently document their full host range. Terry Scheifer of the Mississippi Entomological Museum identified G. hubbardi, C. nigriceps, T. piger, A. pilosulus, and N. femorata. JoVonn Hill of the Mississippi Entomological Museum identified all Formicidae samples.