Multi-trophic level interactions between the invasive plant Centaurea stoebe, insects and native plants

Abstract

Centaurea stoebe is one of the most common and problematic invasive plants in the western United States and Canada. To fully comprehend C. stoebe's impact on native organisms and enhance methods for its control, we must come to a better understanding of how it integrates into the ecological networks of its introduced range. My research objectives addressed this need by assessing (1) how pollinator communities vary with C. stoebe density, (2) the impact of C. stoebe invasion on pollinator visitation to and reproduction of the native plant Heterotheca villosa, and (3) the extent to which C. stoebe biological control agents are attacked by native parasitoids. Objective 1 was completed during summer 2012 at nine C. stoebe infested sites in western Montana. Each site was visited once per week from June to August to document plant-pollinator interactions. Differences in pollinator abundance, richness, and community composition were found across varying C. stoebe densities, but differences depended on the time of season. Prior to C. stoebe blooming, plots without C. stoebe were visited by a higher abundance and richness of pollinators than plots with high C. stoebe density. The opposite pattern was observed during peak C. stoebe bloom. Objective 2 was completed during summer 2013 at a site near Bozeman, Montana. Potted C. stoebe plants were placed at densities of 1, 2, or 4 plants next to naturally occurring H. villosa plants to assess effects on pollinator visitation to and reproductive output of H. villosa. Centaurea stoebe, at the densities used, had no effect on the visitation to or reproduction of H. villosa. For objective 3, throughout 2012 and 2013, 19 and 45 sites were visited to make C. stoebe root and seedhead collections, respectively. Collections were then monitored for biological control agent and parasitoid emergence in the laboratory. Nine parasitoid species (four of which were not previously known to be associated with C. stoebe) emerged along with ten biological control agent species. Host-parasitoid associations were verified for three of the nine parasitoid species: Urophora affinis - Pronotalia carlinarum, Larinus spp. - Scambus brevicornis, and Agapeta zoegana - Bracon sp.