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UBC Theses and Dissertations

Cryptic host-associated and frequency-dependent patterns of host species selection of a candidate weed biological control agent in its native range Moffat, Chandra Elaine


At least nine species of European hawkweeds in the genus Pilosella Vaill. (Asteraceae) are invasive in western North America, where they are a detriment to natural and managed lands and have vast economic impacts. A promising candidate biological control (biocontrol) agent for multiple species of Pilosella is the gall inducing wasp Aulacidea pilosellae Kieffer (Hymenoptera: Cynipidae). Initial studies of this candidate agent revealed the potential for two biotypes of the insect that differed in physiological host range, voltinism, and reproductive mode. It was hypothesized that these differences were related to geographic separation of the populations and it was further observed, within one subset of the distribution, that different host species were being used at nearby sites, despite similar host species being present. The overarching goal of this thesis was to increase the understanding of the patterns of host species use by A. pilosellae in order to inform the biocontrol programme for invasive Pilosella hawkweeds. The specific objectives were to (i) test the hypothesis that A. pilosellae has definitive preferences for species of Pilosella within its ecological host range, (ii) further quantify the ecological host range of A. pilosellae by conducting systematic surveys across a broad geographic distribution, and (iii) test the hypothesis that differences between the purported biotypes were due to cryptic genetic differentiation, predicted to be based primarily on geographic location of populations and secondly on host-association. By utilizing an integrative approach of thorough and widespread field surveys and molecular methodologies, this thesis presents findings that substantially increase the understanding of patterns of host species use by A. pilosellae in its native range. Specifically, (i) the hypothesis of host preferences of A. pilosellae is rejected in favor of frequency-dependent host species selection, (ii) five species of Pilosella invasive in North America are confirmed as hosts of A. pilosellae in its native range and (iii) the hypothesis that cryptic genetic differentiation exists within the species A. pilosellae was supported. However, the prediction that differentiation was based primarily on geographic separation was rejected in favor of differentiation based on a combination of disjunct hostassociations, infection with the bacterial endosymbiont Wolbachia, and to a lesser extent, on geography.

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