Invasion success of the exotic weevil (Rhinocyllus conicus) at high elevations in the Rocky Mountains of Colorado

Background/Question/Methods

Native Colorado thistles are at risk of decline due to increased use by Rhinocyllus conicus, a weevil introduced to North America to biologically control nonnative musk thistle (Carduus nutans).  The distribution of Rhinocyllus conicus in North America has rapidly expanded since its introduction in 1969, and has invaded over one third of native Cirsium species. Here we examine the weevil’s relationship with high-elevation native thistles in Rocky Mountain National Park and on Niwot Ridge, Colorado.  Transects along an elevational gradient were sampled in Rocky Mountain National Park and presence/absence of R. conicus was recorded for every native thistle found.  We used a logistic regression to examine relationships between environmental variables and weevil presence/absence.  A controlled introduction experiment was conducted at Niwot Ridge to determine if weevils were able to complete their reproductive cycle at high elevation.  Mating pairs of R. conicus were introduced to enclosures containing native thistles and reproductive success was quantitatively measured.  Weevil enclosures were checked the following spring to determine if any individuals survived the winter. 

Results/Conclusions

Results of Rocky Mountain National Park survey indicate that R. conicus presence on native thistles is strongly associated with elevation, such that as elevation increases, the probability of finding R. conicus decreases.  Results of the Niwot Ridge experiment indicate that R. conicus is capable of reproducing at treeline, but is unable to overwinter at this elevation.  Our results show that although high-elevation thistles are acceptable hosts for R. conicus, climate conditions have not permitted invasion.  This is the first experimental study of exotic weevils at high elevation in Colorado.  Understanding that this exotic weevil can reproduce at treeline has serious implications for the last remaining mountain thistles, especially in the framework of climate change.  With increasing temperatures, the climatic limitations at treeline may soon diminish, leaving alpine thistles vulnerable to infestation by R. conicus.