MULTI-TROPHIC LEVEL INTERACTIONS BETWEEN THE INVASIVE PLANT CENTAUREA STOEBE, INSECTS AND NATIVE PLANTS by Christina Rachel Herron-Sweet A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Land Resources and Environmental Sciences MONTANA STATE UNIVERSITY Bozeman, Montana May 2014 ©COPYRIGHT by Christina Rachel Herron-Sweet 2014 All Rights Reserved ii DEDICATION To my parents and grandparents, who instilled in me the value of education and have been my biggest supporters along the way. iii ACKNOWLEDGEMENTS Special thanks go to my two advisers Drs. Jane Mangold and Erik Lehnhoff for all their tremendous support, advice and feedback during my graduate program. My two other committee members Drs. Laura Burkle and Jeff Littlefield also deserve a huge thank you for the time and effort they put into helping me with various aspects of my project. This research would not have been possible without the dedicated crew of field and lab helpers: Torrin Daniels, Darcy Goodson, Daniel France, James Collins, Ann de Meij, Noelle Orloff, Krista Ehlert, and Hally Berg. The following individuals deserve recognition for their patience in teaching me pollinator identification, and for providing parasitoid identifications: Casey Delphia, Mike Simanonok, Justin Runyon, Charles Hart, Stacy Davis, Mike Ivie, Roger Burks, Jim Woolley, David Wahl, Steve Heydon, and Gary Gibson. Hilary Parkinson and Matt Lavin also offered their expertise in plant identification. Statistical advice and R code was generously offered by Megan Higgs, Sean McKenzie, Pamela Santibanez, Dan Bachen, Michael Lerch, Michael Simanonok, Zach Miller and Dave Roberts. Bryce Christiaens, Lyn Huyser, Gil Gale and Craig Campbell provided instrumental consultation on locating field sites, and the Circle H Ranch, Flying D Ranch and the United States Forest Service graciously allowed this research to take place on their property. Finally, I would like to express my heartfelt gratitude to my family and friends for their continual encouragement in my educational pursuits, especially to my mom for fielding both elated and distressed phone calls, and to Alex for his stable and positive influence. iv TABLE OF CONTENTS 1. LITERATURE REVIEW ................................................................................................1 Introduction ......................................................................................................................1 Centaurea stoebe: What’s the Problem?..........................................................................3 Plant-Pollinator Interactions in Invaded Systems ............................................................5 Biological Control Agents: Unintended Effects ............................................................10 Objectives ......................................................................................................................21 2. INDIRECT IMPACTS OF THE INVASIVE PLANT CENTAUREA STOEBE ON NATIVE PLANTS AND POLLINATORS IN MONTANA .................................24 Introduction ....................................................................................................................24 Methods..........................................................................................................................28 Study Species .........................................................................................................28 2012 Observational Study (Study Question 1) ......................................................29 Field Sites...................................................................................................29 Study Design ..............................................................................................29 Pollinator Observations ..............................................................................31 Data Analysis .............................................................................................31 2013 Manipulative Study (Study Question 2) .......................................................33 Field Site ....................................................................................................33 Study Species .............................................................................................34 Study Design ..............................................................................................34 Pollinator Observations ..............................................................................36 Reproductive Output of Heterotheca villosa .............................................37 Data Analysis .............................................................................................37 Results ............................................................................................................................39 2012 Observational Study (Study question 1) .......................................................39 Plant Community .......................................................................................39 Pollinator Community ................................................................................41 Pollinator Abundance, Richness, and Visitation. .......................................43 2012 Manipulative Study (Study Question 2) .......................................................46 Plant Community .......................................................................................46 Pollinator Community ................................................................................47 Pollinator Visitation Rates .........................................................................48 Reproductive Success of Heterotheca villosa............................................48 Discussion ......................................................................................................................50 3. ENEMY OF THE ENEMY: NATIVE PARASITOIDS FOUND ATTACKING THE BIOLOGICAL CONTROL AGENTS OF CENTAUREA STOEBE IN MONTANA .......................................................................56 v TABLE OF CONTENTS CONTINUED Introduction ....................................................................................................................56 Methods..........................................................................................................................60 Root Collection and Rearing ..................................................................................60 Seedhead Collection and Rearing ..........................................................................63 Plant Material Dissection .......................................................................................64 Identification ..........................................................................................................64 Percent Parasitism and Statistical Analysis ...........................................................65 Results ............................................................................................................................66 Biological Control Agents Present.........................................................................66 Parasitoids Present .................................................................................................68 Host-Parasitoid Associations and Parasitism Rates ...............................................70 Pronotalia carlinarum ...............................................................................71 Scambus brevicornis ..................................................................................73 Bracon sp. ..................................................................................................73 Discussion ......................................................................................................................74 4. CONCLUSIONS AND FUTURE DIRECTIONS ........................................................84 REFERENCES CITED ......................................................................................................89 APPENDICES .................................................................................................................103 APPENDIX A: Pollinator Observation Sites .......................................................104 APPENDIX B: Biological Control Agent and Parasitoid Collection Sites .........................................................106 vi LIST OF TABLES Table Page 1.1. Biological control agents of Centaurea stoebe and dates of first release in the United States. ................................................................11 1.2. Parasitoids found attacking Centaurea stoebe biological control agents in their introduced range described in the literature to date ........................................................................................20 2.1. Plant species found co-flowering with Centaurea stoebe at the 2012 field sites, their native/non-native status, the number of sites where they were present (n = 9), and the number of weeks it co-flowered with C. stoebe .............................................39 2.2. Flower species and mean floral density per m2 surrounding each pair of Heterotheca villosa plants at the 2013 study site ........................46 2.3. Pollinator groups observed visiting Heterotheca villosa and Centaurea stoebe by percent ...........................................................................47 3.1. Parasitoids found attacking Centaurea stoebe biological control agents in their introduced range