Undergraduate Honors Theses

Thesis Defended

Spring 2016

Document Type


Type of Thesis

Departmental Honors


Ecology and Evolutionary Biology

First Advisor

Dr. Tim Seastedt

Second Advisor

Dr. Pieter Johnston

Third Advisor

Dr. Gregory Carey


Invasive species are reducing native biodiversity in many distinct regions across the globe. While biological control agents have been effectively utilized to manage invasive plant species and preserve existing biodiversity, not all biological control agents are successful. The invasive thistle seed head weevil (Rhinocyllus conicus) was originally introduced to America as a biological control agent to manage musk thistle (Carduus nutans), but it has been found to attack several species of native thistle. This study examined the movements of the invasive thistle seed head weevil away from musk thistle into two different species of native thistle in the Colorado Front Range: wavyleaf thistle (Cirsium undulatum) and fringed thistle (Cirsium centaureae). Specifically, the proximity of invasive musk thistle to wavyleaf thistle was observed to see if it has any influence on thistle seed head weevil attack upon wavyleaf thistle. Additionally, fringed thistle was observed in three different habitat types in the Colorado Front Range (i.e. meadow, riparian, and tree line habitat) to see if the invasive thistle seed head weevil has been moving up in elevation in response to climate change. The results indicate that the proximity between musk thistle and wavyleaf thistle had no significant effect on thistle seed head weevil abundance on the native thistle. Moreover, the invasive thistle seed head weevil is still restricted to elevations below 3100 meters, meaning the tree line population of fringed thistle are protected from attack. Although both species of native thistle seem to have little risk of local extirpation due to the invasive seed head weevil, continued monitoring is advised to assess the indirect impacts of this invasive species through time and space.