Contributions of diurnal and nocturnal insects to the pollination of common milkweed (Asclepias syriaca L.) in a pollen-limited system.

Milkweed (Asclepias syriaca L.: Asclepiadaceae) flowers exposed only to diurnal pollinators (primarily bumble bees, Bombus spp.) matured about eight times as many pods as did flowers exposed only to nocturnal visitors (moths). Rates of pollinaria removal and pollinia insertion were similarly higher during the day than at night. Moths deposited a much lower proportion of their pollinia in stigmatic chambers (one-fifth) than did diurnal pollinators. In spite of the poor service provided by nocturnal pollinators, flowers produced four times as much nectar at night as at day, and sugar production (sucrose equivalents) at night was twice that of the daytime.Supplemental hand pollinations doubled the production of mature pods, indicating normal pollinia limitation. Mature pod production of clones within a 5-km diameter varied between 0.1 and 3.5 per flowering stem, and numbers of pollinia vectors at these clones were closely correlated with numbers of mature pods produced.Pollinia-limited clones should thus attract both diurnal and nocturnal pollinators, if costs are not too extreme. Pollinia-limitation should decrease selection for pollinator specialization, especially if the relative importance of different pollinators varies over time, as in this study.