Neighboring plants influence damage from a biocontrol insect on an unintended host

The non-native weevil, Larinus planus, is a biocontrol agent for the noxious weed, Canada thistle. However, this weevil is also known to attack native thistles, including the federally threatened Pitcher's thistle, Cirsium pitcheri. C. pitcheri is endemic to dunes of the western Great Lakes and seed predation from L. planus has the potential to cut its extinction time by half. To help prevent further damage to this vulnerable species, it is important to understand the factors that influence weevil host selection. The goal of this study was to quantify the spatial variability in weevil damage within a natural dune habitat and identify any biotic or abiotic associations that are correlated with weevil damage on C. pitcheri. This study was conducted at Whitefish Dunes State Park (WDSP) in Wisconsin. There, a population of over 300 thistles was mapped and I characterized the plant community within 1 m² surrounding each thistle. Throughout the summer, weevil presence, oviposition scars, and seed damage was noted on each plant. I found a significant negative relationship between the distance to the nearest Marram grass (Ammophila breviligulata) and weevil presence on a thistle. The subsequent summer, I selected 60 thistles surrounded by Marram grass and clipped the glasses within a 1 m radius surounding half of the thistles to experimentally determine whether the presence of grass neighbors influences weevil host selection. Clipping grass neighbors significantly reduced weevil oviposition and seed damage. Marram grass is not host to L. planus, therefore the nature of this association is curious. Weevils may be dispersing through this grass matrix to find and climb onto its host, weevils may be using grassy debris for protection from enemies, or the grass may be altering the microclimate in ways that are preferable to the insect. Future studies will test how Marram grass provides associational susceptibility to C. pitcheri and results should help inform management strategies.