The Great Lakes Entomologist
Cirsium pitcheri (Torr. ex Eaton) Torr. & A. Gray (Pitcher’s thistle) is a threatened herbaceous plant endemic to sand dune ecosystems along Lakes Huron, Michigan, and Superior in North America. Habitat for this plant is limited to active dunes with moving sand. I observed floral visitors of C. pitcheri in Indiana Dunes National Lakeshore and Indiana Dunes State Park, and calculated frequency and density of visitor families. Additionally, I tested for relationships between visitor counts and previous growing season mean temperature and precipitation. Formicidae, Anthomyiidae, and Cecidomyiidae were the most frequent families. However, Apidae was the only family correlated with the number of subsequent C. pitcheri seedlings. Counts of mean visitors per plant were different between years, with 2013 being the lowest. These values were related to previous growing season precipitation, which was lowest in 2012 due to a widespread severe drought. There was clear depression of floral visitor frequency and density following the 2012 drought, but that was short-lived and subsequent years displayed recovery of visitor numbers. Many of the floral visitors of C. pitcheri are likely feeding on nectar, pollen, and vegetative structures, and providing minimal, if any, pollination benefit. However, families such as Apidae and Halictidae carry visible pollen loads between multiple individual plants. Pollinator augmentation with these families may benefit C. pitcheri reproduction, especially following years of drought.
Apidae, pollinators, Pitcher’s thistle, sand dunes
Biology | Botany | Entomology
Jordan M. Marshall (2017).
Insect visitors of Cirsium pitcheri, a threatened and endemic dune species, in relation to annual weather variation. The Great Lakes Entomologist.50, 86-92.