Because of economic and environmental constraints, alternatives to chemical management of Canada thistle (Cirsium arvense) are frequently sought, but adequate nonchemical suppression of this invasive species remains elusive. Combining biological control with other tactics may be an effective approach to suppress Canada thistle, but more information is needed about how environmental conditions affect interspecific interactions. We investigated effects of a biocontrol agent (Hadroplontus litura, a stem-mining weevil) and a potential plant competitor (common sunflower, Helianthus annuus, native annual) on Canada thistle under two soil nutrient regimes in outdoor microcosms. Larval mining damage was relatively light, and weevils negatively impacted only main shoot height and flower number. All measures of Canada thistle performance were reduced when plants were grown with common sunflower or in reduced nutrients, although effects of the latter on root biomass were not significant. Effects of common sunflower and soil nutrients on Canada thistle were generally additive, though a marginally insignificant interaction indicated a trend for greatest flower number with high nutrients and absence of common sunflower. Effects of weevils and common sunflower on Canada thistle were also additive rather than interactive. Although larval damage ratings were significantly greater on plants grown in high-nutrient soil, under our experimental conditions weevils and soil nutrients did not have a significant interactive effect on Canada thistle plants. Our results indicate that H. litura is a relatively weak biological control agent, but when combined with competitive desirable vegetation, some level of Canada thistle suppression may be possible, especially if soil nutrient levels are not highly enriched from agricultural runoff. Assessing the true ecological impacts of Canada thistle infestations may be an important direction for future research.

Nomenclature: Stem-mining weevil, Hadroplontus litura Fabricius, Canada thistle, Cirsium arvense (L.) Scop., common sunflower, Helianthus annuus L.

Management Implications: Canada thistle is a problematic invasive plant in many temperate regions of the northern hemisphere, and management continues to be a challenge. Research suggests that integrating multiple control tactics enhances suppression of Canada thistle, but that effects often depend on environmental context. In our microcosm study, weevil herbivory, common sunflower competition, and reduced levels of soil nutrients had substantial negative effects on many measures of Canada thistle growth and reproductive capacity. Overall, the latter two factors exerted the strongest negative effects; however, weevil damage was generally light. Canada thistle grown in high-nutrient soils had greater main-stem biomass, numbers of side shoots, and numbers of flowers, even though larval stem-mining damage was greater for these plants. Common sunflower proved to be a good competitor against Canada thistle, and sowing this type of fast-growing annual forb, which occupies a similar soil niche to Canada thistle, alongside perennial native plants (e.g., competitive grasses) may enhance restoration outcomes.