The effect of temperature and vegetative substrate on the short-distance dispersal behavior of an invasive weevil (Coleoptera: Curculionidae) to a protected dune thistle

The invasion of a seed-eating weevil, Larinus carlinae (Olivier, 1807) poses a threat to the persistence of native Pitcher's thistle (Cirsium pitcheri), a federally-threatened plant in sand dune communities of the western Great Lakes. At Whitefish Dunes (Sturgeon Bay, WI. USA), seed predation by weevils is higher for thistles surrounded by native marram grass (Ammophila breviligulata) than for those located in sandy habitats. In this study, we observed how the local abiotic and biotic environment affects the short-distance dispersal behavior of L. carlinae. Weevils were captured on site and individually released on the sand approximately 0.35 m from target thistle replicates to determine the extent to which weevils used vegetative substrate to reach their host. Weevils spent significantly more time walking or resting on grass substrate than on ground-level sand or debris, and weevils were more likely to reach the host plant directly from grass neighbors. Sand surface temperature negatively affected the proportional time dispersing weevils spent on sand relative to grass and debris. We found evidence for both refuge-mediated and dispersal-mediated effects of grass neighbors on weevil short-distance movement onto host plants. Grass neighbors may provide a refuge from extreme sand temperatures, and the grass matrix may serve as a conduit for L. carlinae movement in unstable sand dune habitats. However, adding cut grass stems to bare sand-surrounded thistles did not affect host preference in a subsequent choice experiment. These results highlight the importance of maintaining bare sand habitat in dune communities to maximize C. pitcheri reproductive success in the face of weevil invasion.