THE POLLINATION ECOLOGY OF ASTRAGALUS CIBARIUS AND ASTRAGALUS UTAHENSIS (LEGUMINOSAE)

Astragalus cibarius and A. utahensis are common perennial species of a widespread legume genus. The pollination of Astragalus has been briefly discussed in the literature, but little work has been done on species in the intermountain West. This study was conducted from 1970–1973 in Utah with mixed and single species populations. The flowers of both species were homogamous and papilionaceous, but the species were different as to color, size, and ultraviolet reflectance. Astragalus cibarius usually flowered 10 days ahead of A. utahensis, but both species flowered earlier than most other plants in the community. Bagging experiments indicated both species were strongly allogamous. Exclosure studies indicated both species relied on insects as pollen vectors. Of the 44 insect species which were observed visiting flowers, only 14 carried Astragalus pollen, and the pollinator fauna varied between study sites. Pollen quantities and distributions on Diptera and Coleoptera indicated a poor potential for pollination. Floral structure, pollen distribution and quantity, and behavior implied that large bees of the families Apidae and Anthophoridae were the primary pollinators. These bees visited only one species of Astragalus when the plants occurred in mixed populations; this constancy may have been related to relative flower abundance. Non-pollinating floral foragers affect other phases of Astragalus life history.