Interactions of the invasive thistle Carduus nutans and its biocontrol agent Rhinocyllus conicus in heterogeneous environments

Sezen, Zeynep

Interactions of the invasive thistle Carduus nutans and its biocontrol agent Rhinocyllus conicus in heterogeneous environments

Open Access

Author:: Sezen, Zeynep
Graduate Program:: Ecology
Degree:: Doctor of Philosophy
Document Type:: Dissertation
Date of Defense:: November 27, 2006
Committee Members:: Katriona Shea, Committee Chair/Co-Chair
David A Mortensen, Committee Member
Andrew George Stephenson, Committee Member
Consuelo M De Moraes, Committee Member
Keywords:: host location
mark-release-resight
Carduus nutans
Rhinocyllus conicus
host choice
invasion management
Abstract:: How herbivores locate and select their host plants is of fundamental importance in a variety of basic and applied ecological fields, including spatial population dynamics, invasive species management and biological control. This thesis addresses these questions for Rhinocyllus conicus, a flower head weevil that is a specialist herbivore of thistles, in particular the musk thistle, Carduus nutans. Rhinocyllus conicus has been widely used as a biological control agent for C. nutans, but success has been very variable, and a better understanding of the ecological determinants of host location, choice and impact will be invaluable. In order to assess the ecological determinants of patch finding and patch choice by R. conicus, C. nutans patches of varying size and density were artificially created. With an individually mark – mass release – resight (IMMRR) technique, the dispersal abilities, host location, and between patch movement patterns of R. conicus were studied. Rhinocyllus conicus was able to find host plant patches from a distance of up to a kilometer, though it located taller and closer patches at the highest rate. Weevils were more likely to leave small patches, and more likely to disperse to large patches. There was no significant effect of patch size and plant density on the number of weevils resighted per plant. Plant level characteristics, such as flower head diameter, plant height and total number of heads on a plant, were found to be important determinants of R. conicus oviposition response; no patch level characteristics were significant predictors of attack. In a related study, the effect of management practices, such as availability of plant resources, mowing, and abundance of the biocontrol agent R. conicus and their interaction on the oviposition response were also investigated. While an abundance of the biocontrol agent led to an increase in the number of eggs per flower head, this positive effect was eliminated by mowing. Fertilizer addition did not have an effect on R. conicus attack rates. As before, oviposition rates were found to increase significantly with flower head diameter, and there was a small positive effect of plant height. However, while larger C. nutans flower heads provide more sites for oviposition, there is also more seed production in these heads, and the reduction in seed production by R. conicus is not sufficient for complete control of its host. In an effort to determine some of the sensory mechanisms underlying the weevils’ behavior, olfactometer studies were conducted. The behavioral response of R. conicus to C. nutans olfactory cues in the presence or absence of feeding weevil conspecifics, and also to two key life stages, bolting plants and rosettes, were investigated. Rhinocyllus conicus showed a preference for bolting C. nutans both in the presence and absence of weevil conspecifics. Colonization and establishment of the insect herbivore relative to the patch dynamics of its host strongly determine the success or failure of biological control efforts. The thesis concludes with a model that explores the thistle - herbivore dynamics in a metapopulation context. This framework will allow the empirical results to be extrapolated to larger spatial and temporal scales. The results of the baseline model show that the proportion of the landscape occupied by C. nutans decreases in the presence of R. conicus, though it predicts a lower proportion of attacked patches of C. nutans than actually occurs in central Pennsylvania. This model does not yet take into account the detailed results of the empirical work in this thesis; however it is a useful starting point for exploring these issues at larger scales. In conclusion this thesis shows for the first time that R. conicus moves further than previously known, locates its host based on characteristics of the host patch, but oviposits in response to plant characteristics. The weevil has a moderate negative impact on seed production, but, critically, it may miss isolated plants, which has significant implications for its efficacy as a biocontrol agent as the thistle spreads.

Interactions of the invasive thistle <i>Carduus nutans</i> and its biocontrol agent <i>Rhinocyllus conicus</i> in heterogeneous environments

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