Patterns of Nectar Production in Asclepias curassavica (Apocynaceae))

Abstract

Milkweeds are important nectar resources for insects in the New World. In addition, nectar is the germination medium for milkweed pollen. This study is the first controlled, greenhouse examination of patterns of nectar production in a milkweed species. We measured nectar volume, concentration, and mg of sugar in the pantropical, weedy milkweed Asclepias curassavica. Our results show that A. curassavica secretes nectar primarily during daylight hours and it continues at a constant daily rate for four to five days. Freshly secreted nectar is lower in sugar concentration than older nectar. This provides an opportunity for milkweed pollen to germinate throughout the day, but pollen germination could be inhibited at times when the sugar concentration increases. Nectar production in A. curassavica is adapted to attract diurnal insect pollinators over several days and to allow pollen germination to occur quickly. Significant differences in nectar production exist among plants and among inflorescences within plants. Nectar production increases in flowers when nectar is extracted using paper wicks that simulate removal by insects in nature. Removal-enhanced nectar production in milkweeds may allow plants to adjust resources to inflorescences receiving insect visitation. Significant inter-plant differences in nectar production and the unique milkweed flower provides a model system for examining the role of pollinator-mediated selection on nectar traits.