Nectar Production and Pollination of Asclepias exaltata

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Abstract
Nectar production and pollination of the herbaceous perennial milkweed Asclepias exaltata were studied in three populations in southwestern Virginia [USA]. Individual flowers were available to pollinators, chiefly large bees and wasps, an average of 6.2 days. Volumes, concentrations, and amounts of sucrose in nectar decreased as flowers aged. Most nectar were secreted overnight, and standing stocks increased in concentration from 20-30% in the morning to 40-60% in the afternoon. There were significant differences among plants in all components of nectar production, with sucrose amounts varying up to four-fold. The average number of pollinaria removed per flower was significantly greater in 1981 (1.38 .+-. 1.25; .hivin.x .+-. s) than in 1982 (0.78 .+-. 0.09), as was the average number of insertions per flower (0.66 .+-. 0.93 versus 0.39 .+-. 0.66). Pollination efficiency, the ratio of insertions to removals, was nearly the same in 1981 (0.43 .+-. 0.66) as in 1982 (0.36 .+-. 0.59). Rates of pollination were positively correlated with nectar concentrations. In addition, plants producing concentrated nectar matured more fruits and had higher percentage fruit-set than did plants with dilute nectar. These data from natural populations agree with general results of laboratory trials suggesting that bees prefer concentrated nectar and that bees can respond to differences in nectar quality among plants.