Cross- scale assessment of the competitive effects of an invasive weevil on a native floral herbivore

Background/Question/Methods

Mechanisms underlying invasive species impacts remain incompletely understood in general, and the potential competitive effects of invasive insect herbivores in particular have been relatively under-studied.  We combined a manipulative field experiment with a large scale survey to examine the potential competitive effects of an invasive biocontrol weevil, Rhinocyllus conicus, on the predominant native floral herbivore, Paracanta culta, associated with a native, non-target thistle host plant, Cirsium canescens.

Results/Conclusions

The field experiment demonstrated strong, asymmetric, competition with R. conicus out-competing P. culta within thistle flower heads. In addition, weevil priority access to floral resources increased the magnitude of its competitive suppression of  P. culta. Evidence for competitive suppression with increasing weevil priority included decreases in both the numbers and the total biomass of native flies, plus decreases in individual P. culta mass and thus potential fitness. The results suggest that competitive displacement of native flies to lower quality resources within thistle flower heads, when R. conicus has priority access, is a dominant mechanism underlying the competitive effects.  However, large-scale field surveys across the distributional range of the shared host plant demonstrated that densities of the two herbivores were not significantly negatively associated, either across individual plants or across sites.   Thus, despite the unequivocal evidence that R. conicus competitively suppresses native flies within individual flower heads, we did not find evidence that such effects scale up to influence larger scale distributional patterns.  We propose two potential hypotheses for the absence of negative associations between insects across larger scales: (1) insufficient time may have elapsed for competitive displacement to occur since the large scale surveys were carried out only 5 - 10 years post-weevil invasion, or (2) spatial shifts by the flies to resources unexploited by weevils (e.g., un-attacked plants or later-developing flower heads) may minimize competitive displacement over larger scales.  Distinguishing between these alternatives remains an important area of future research.