California State University, Northridge Host-Specificity and its Effect on Mate Choice in a Plant-Eating Beetle A thesis submitted in partial fulfillment of the requirements For the degree of Master of Science in Biology By Katherine Gould May 2014 Copyright by Katherine Gould 2014 ii The thesis of Katherine Gould is approved: _________________________________________ ___________________ Dr. David Gray Date _________________________________________ ___________________ Dr. James Hogue Date _________________________________________ ___________________ Dr. Paul Wilson, Committee Chair Date California State University, Northridge iii Dedication For Samantha and Jocelyn iv Acknowledgements First and most importantly, I would like to thank my advisor, Dr. Paul Wilson, for his advice, answers, encouragement, and occasional well-aimed prodding. Also instrumental in helping me through this process were my committee members, Dr. Dave Gray and Dr. Jim Hogue, who were always available to answer questions and assist in whatever way I needed. Thank you to Cindy Hitchcock, whose wonderful drawings of beetle mating illustrate this work, and whose beautiful watercolor of a mating pair of beetles concludes this work. I could not have completed all the lab work necessary without the loyal and consistent help of my lab assistants. My "Minions" fed, watered, counted, and observed thousands of beetles over the summer of 2013. Thank you, thank you, thank you to Victoria Amran, Elias Atri, Jamie Carrafa, Dona Cherian, Amanda Fitzpatrick, Liz Hamel, Amaya Mendez-Molina, Alexus Merino, Joshua Muñoz, Lara Parsekhian, Lela Remington, Joyce Theilig, and Dominique Zatarain. Thanks also to my lab mates, Nickte Mendez, Lena Coleman-Ayala, and Dani Amorosa, who were an invaluable resource and sounding board. To my parents, Phil and Mary Ann Andrews, and my brother, Scott Andrews, who supported my decision to go back to school at age 38, thank you, thank you, thank you. And to my wonderful daughters, Samantha and Jocelyn Gould, thank you for not freaking out too much about beetles in the refrigerator, a car filled with the smell of Eriodictyon, and a mom who talks way too much about insects. v vi Table of Contents Signature Page ................................................................................................................... iii Dedication .......................................................................................................................... iv Acknowledgements ..............................................................................................................v List of Tables and Figures................................................................................................. vii Abstract ............................................................................................................................ viii Introduction ..........................................................................................................................1 Materials and Methods .........................................................................................................8 Study Organisms ......................................................................................................8 Experiments ...........................................................................................................14 Statistics .................................................................................................................19 Results ................................................................................................................................25 Discussion ..........................................................................................................................43 References ..........................................................................................................................53 vii List of Tables and Figures Figure 1: Trirhabda eriodictyonis ........................................................................................8 Figure 2: Distinguishing the sexes .....................................................................................10 Figure 3: Trirhabda eriodictyonis mating..........................................................................11 Figure 4: Leaves of Trirhabda eriodictyonis’ host plants .................................................12 Figure 5: Study site ............................................................................................................14 Figure 6: Choice feeding trials, larvae ...............................................................................25 Figure 7: Larval growth and development .........................................................................26 Figure 8: Time to pupation ................................................................................................28 Figure 9: Lifetime survival, host-plant switch as larvae ....................................................29 Figure 10: Choice feeding trials, adults .............................................................................30 Figure 11: Adult survival, by sex .......................................................................................31 Figure 12: Adult survival, by treatment .............................................................................32 Figure 13: Average date of death .......................................................................................33 Figure 14: Mating preferences, without plant switch ........................................................34 Figure 15: Mating behavior ...............................................................................................35 Figure 16: Mating preferences, with plant switch .............................................................36 Figure 17: Potential fecundity ............................................................................................37 Figure 18: Realized fecundity, by female treatment ..........................................................39 Figure 19: Realized fecundity, by mating treatment ..........................................................41 Figure 20: Realized fecundity, by mating treatment ..........................................................42 viii Abstract Host-Specificity and its Effect on Mate Choice in a Plant-Eating Beetle A thesis by Katherine Gould Master of Science in Biology The beetle Trirhabda eriodictyonis lives on two shrubs with different plant defenses: Eriodictyon crassifolium has hairy leaves; E. trichocalyx has sticky leaves. The relationship between these plants and the leaf-eating beetles that depend on them has been unstudied until now. In choice tests, larvae and adults showed unexpected feeding preferences, with larvae from E. crassifolium showing no preference and those from E. trichocalyx preferring E. crassifolium. Adults all strongly preferred eating E. trichocalyx. Larvae and adults that I switched from E. trichocalyx to E. crassifolium died younger than beetles that I continued to feed the original host species. Mating trials showed that the only difference in preference involved males from E. trichocalyx, which were far more attractive to females on E. crassifolium than males on the same host. Finally, females laid more eggs if they ate E. trichocalyx than E. crassifolium, even if they had started life on the latter. It is clear that E. trichocalyx provides benefit to both males and females and these beetle populations are not differentiating based on host plants. Neither the differentiation hypothesis nor the preference-performance hypothesis are validated by this plant-insect interaction. Instead, it appears that the best explanation of this relationship is phylogenetic conservatism. The plant defenses, which appear dramatically different to humans, are unimportant to the beetles. ix Introduction Chrysomelid beetles lead a simple life. Birth to death, they live on and eat their host plant. Some beetles spend their whole life on a single individual plant. While great for the beetles, this lifestyle is problematic for the plants they feed on, and the conflict between plants and herbivores shapes both organisms. Plants have adapted to defend themselves against attackers, and herbivores have evolved ways around those defenses. Relationships between plants and insect herbivores have been widely studied, yet new studies continue to reveal surprises and unexpected interactions. I investigated an unstudied relationship of two plant species with one insect herbivore, a specialized chrysomelid beetle, to determine how well this relationship is explained by prevailing theories of plant-insect interactions. To understand the relationship between plants and their herbivorous adversaries, we must first appreciate the wealth of different plant defenses against herbivores that have evolved over eons. Some plants, many annuals for example, don’t grow when the insects that would eat them are around (Feeny 1976). Plants that grow all year often produce poisonous or bad-tasting chemicals, or grow spines, hairs, or tough tissues that make stems and leaves hard to eat (Feeny 1976, Rhoades 1979, Bottrell et al. 1998, Jolivet 1998, Lucas et al. 2000). Some plants release chemicals that attract predators to eat the herbivores (Jolivet 1998). Plants frequently respond in real time to herbivory by increasing the energy they put toward defenses, making more chemicals or hairs to slow or harm their attackers (Jolivet 1998, Agrawal et