The relative impact of floral visitors on the reproductive success of common milkweed (Asclepias syriaca)

The relative impact of different species of floral visitors on the reproductive success of plants is determined by the abundance of floral visitations and many behavioral and morphological characteristics of the visitors. We investigated the relative impact of ten species of floral visitors on the reproductive success of A. syriaca in northern Pennsylvania. We quantified the abundance of each visiting species, as well as their efficiency in both removing and depositing pollen. Honeybees were by far the most common flower visitors (66% of the total visits), while bumblebees (9%) and blow flies (9%) were the next most common visitors. Hornets (4%) and monarchs (1%) were relatively rare visitors. Bumblebees removed the most pollinaria per visit (1.45), followed by hornets (1.21), honeybees (1.08), and monarchs (.45). Honeybees had nearly 3 times more pollinaria per individual on their bodies than bumblebees and 2 times more than hornets. Each individual hornet delivered 22 times more pollinaria per visit than each individual honeybee. Honeybees and hornets were both quite valuable pollinators of A. syriaca even though their visitation rates were quite different. Honeybees were very common visitors but the average delivery rate of pollinaria was very low. Hornets, on the other hand, were rare visitors but the average pollinaria delivery rate was very high. Our results suggest that data on floral visitation is just the beginning in understanding the relative importance of pollinators to plant reproductive success.