Assessing variation in floral scent and pollinators of the keystone plant species Cirsium pitcheri on its inflorescent heads with weevil Larinus planus predation

Floral scent or volatile organic compounds (VOCs), can influence and be influenced by
interactions with both pollinators and predators. Cirsium pitcheri, also known as Pitcher’s
thistle–a rare dune plant species endemic to the Great Lakes–is under threat by the
predation of an invasive seed weevil, Larinus carlinae (Curculionidae). Although we
know that L. carlinae infestation in C. pitcheri causes a decline in seed production, little
is understood about how infestation affects other floral traits.We collected C. pitcheri
floral scent samples and observed insect interactions at Whitefish Dunes State Park,
Wisconsin in high-quality dune habitats and found that both scent composition and
pollinator interactions varied according to the presence of L. carlinae. Our analysis of the
floral scent samples found VOCs present in C. pitcheri which are known to influence
plant-insect interactions such as benzaldehyde and cinnamaldehyde. Two compounds,
alpha-copaene and cinnamyl isovalerate, varied depending on predation presence.
alpha-Copaene was observed higher in predated heads while cinnamyl isovalerate was
found to be higher in non-predated flowering heads. Though pollinator visitation was
similar between predated and non-predated flowering heads, non-predated heads had a
higher pollinator richness. Overall, non-predated flowering heads have a more varied
floral scent composition and higher pollinator richness than predated flowering heads
which may inhibit reproductive success. Considering the rapid growth of L. carlinae
populations in the Great Lakes Region, further research is needed to understand the
broader implications of the function of floral scent and how that may affect pollinator
interactions.