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UBC Theses and Dissertations

Patterns and impact of herbivory by a biological control insect on its target weed and a native nontarget plant Catton, Haley Autumn


Classical biological control (biocontrol) of weeds involves importing foreign, host- specific insects with the intent of reducing the fitness of invasive plants in their introduced range. When anticipated risks to nontarget species are low, insects capable of feeding and developing on some native nontarget plants have been given regulatory approval for release. In this thesis, I study patterns and impacts of herbivory by the root-feeding weevil Mogulones crucifer on its target weed Cynoglossum officinale and a native nontarget plant Hackelia micrantha. I released large numbers of M. crucifer into naturally-occurring patches of H. micrantha growing with or without C. officinale to simulate a ‘worst case’ scenario of high insect density and low target plant density, and subsequently recorded herbivory patterns and plant demographic parameters for two years on release and non-release sites. Additionally, I collected oviposition data from non-experimental sites 0-4 years after M. crucifer release. Compared to the target weed, H. micrantha use by M. crucifer was temporary, rare, mild, and limited to immediately around release points, suggesting that the nontarget plant is buffered from population-level effects by spatial, temporal and probabilistic refuges from biocontrol herbivory. M. crucifer did not persist 2 years after release in the absence of C. officinale, indicating that the insect is limited to ‘spillover’ nontarget use. A separate mark-release-recapture experiment indicated that M. crucifer has reduced host-finding behaviours for its novel host compared to its evolutionary host. Plant demographic data indicated that outbreak densities of M. crucifer appeared to impact C. officinale populations by increasing rosette mortality. While there was some evidence of impact to individual H. micrantha plants immediately adjacent to release points (i.e., plant death or dieback of flowering shoots), these effects did not translate to the population level. I synthesize this information to suggest why M. crucifer has been an effective biocontrol agent against C. officinale in Canada, and why H. micrantha is unlikely to incur population-level effects from the weevil. This study is a clear example of how nontarget use of individual plants can be noticeable yet not have population-level implications, and demonstrates the importance of post-release research in weed biocontrol.

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Attribution-NonCommercial-NoDerivs 2.5 Canada