Flower-visitors, nectar production, and inflorescence size of Asclepias syriaca

Abstract

Spatial and temporal variation in insect visitation rates are analyzed with respect to nectar production and inflorescence size of A. syriaca L., the common milkweed. Nectar sugar production peaked in the afternoon and evening but this trend was not paralleled by visitations of native insects (either vectors or thieves). The main pollinia vectors were noctuid moths at night and honey bees (Apis mellifera) in the afternoon. Vector visitation rates were highest at the beginning (native species) or middle (total vectors) of the blooming season, declining markedly toward the end, probably as a result of decreasing frequencies of A. syriaca flowers. Large inflorescences attracted more vectors and more thieves and a higher proportion of all visits were made by vectors but the species composition of visitors did not usually change with inflorescence size. Honey bees may provide major new selection pressures on floral display, nectar production patterns and flowering phenology in the common milkweed.