Milkweeds (Asclepias spp.) are of conservation concern as the sole food source for Monarch butterfly (Danaus plexippus) larvae. While many are familiar with the relationship between Monarch butterflies and milkweed, bumble bees (Bombus spp.) are some of the only pollinators that can effectively pollinate common milkweed (Asclepias syriaca). In this study we asked several questions about pollination services to milkweed; 1) Do co-flowering plants surrounding milkweed act as magnet species, drawing in pollinators and increasing pollination services? 2) Does milkweed distance into prairie from the forest edge influence pollinia (pollen packet) export? 3) Does the land use history of milkweed habitat influence pollinator visitation and pollen export? We conducted this study in the Prairie Fork Conservation Area (PFCA, Callaway Co., MO) and South Farm Research Center (SFRC, Boone Co., MO) in 2019. At PFCA, we monitored the kinds and numbers of co-flowering species surrounding randomly selected plants of A. syriaca and of bumble bees visiting their flowers twice weekly from 6/5-7/16. We also tallied the number of pollinia removed and fruit set for each focal milkweed plant to estimate male and female reproductive success, respectively. At SFRC, we conducted similar studies with swamp milkweed (Asclepias incarnata) and common milkweed, with patches of the two species either surrounded by planted wildflowers or not.
Results/Conclusions
The bumblebee species we observed visiting milkweed were B. griseocollis, B. impatiens, B. auricomus, and B. bimaculatus. We found no correlations of either distance from forest edge or number of heterospecific coflowering plants with pollinia removal, suggesting no edge effects or magnet species effect. However, we found a significant positive relationship between number of milkweed flowers and total number of bee visits to milkweed flowers over the summer (df= 15, z= 2.772, P<0.005) and a trend for increasing fruit set with greater milkweed flower display (df=16, z=1.928, P=0.0539), suggesting density dependent pollination services. In the prairie, we only observed bumblebees on milkweeds, while at the farm bumblebees visited other wildflowers more often than the milkweeds. This contrast suggests that cultivated wildflower assemblages may compete with milkweed for pollinators. We are assessing seed germination rate and outcrossing rate of PFCA milkweed to determine the impact of flower density, the co-flowering community and pollination services on mating system parameters and fitness. Our work builds on previous studies of pollination services to A. syriaca chiefly by demonstrating density dependence in its interactions with pollinators.