Birdfoot Violet (Viola pedata) in a Minnesota USA Dry Bluff Prairie: Population Assessment of a Preferred Host Plant of the Threatened Western Regal Fritillary Butterfly (Argynnis idalia occidentalis)

A self-sustaining population of birdfoot violet (Viola pedata), a host plant for the threatened western subspecies of regal fritillary butterfly (Argynnis idalia occidentalis) caterpillar, was examined during a single year from April to June 2021 on a small, 3.1 ha dry bluff prairie hillslope within the Whitewater Wildlife Management Area in southeastern Minnesota USA. Assessments were conducted to determine if violet populations on small prairie remnants could support seed collecting to establish new populations nearby. Ten transects and five random plots were used to assess violet density and monitor violet growth, reproductive phenology, and seed production. Violet densities were high (>5 plants/m2), with greatest densities at middle elevations on the hillside in the middle of the prairie rather than near the edges. The total population of birdfoot violets on the hillside was extrapolated from density estimates based on 200, 1-m2 plots to be >62,000 plants. Seed set was low (less than one pod per plant) but nearly 400,000 total seeds were produced during the 2021 growing season. More than 3000 seeds (<1% of estimated seed production on the study hillslope) were collected for out-planting to establish a new violet population in nearby Whitewater State Park. Some small bluff prairies in southeastern Minnesota and elsewhere under certain conditions may sustain violet populations large enough to permit seed collecting to establish additional populations during restoration of native prairie communities. These ultimately should provide much needed habitat for regal fritillary butterflies to partially compensate for ongoing habitat losses.