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 Asclepias 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. We discuss the possibility that honey bees may provide major 'new' selection pressures on floral display, nectar production patterns, and flowering phenology in the common milkweed.

Selection on reproductive characters: floral morphology in Asclepias syriaca

“…Removed pollinia often do not get inserted into stigmatic chambers (Table 2; Piper & Waite, 1988;Nilsson et a!., 1992) and a prodigiously small fraction of flowers mature fruit following pollinia insertion (Willson & Bertin, 1979;Bookman, 1984).…”

“…Pollen grains are packaged into pollinia, each pollinium consisting of two pollen sacks joined by translator arms and a corpusculum; only the corpusculum is exposed to pollinators. Hoods accumulate copious nectar (Willson & Bertin, 1979;Wyatt et al, 1992), and the arrangement of the hoods around the gynostegium directs hairs of foraging pollinator limbs and other body parts toward a small groove in the corpusculum. The hairs become wedged in the groove, and the corpusculum and associated pollinia become dislodged as the pollinator leaves the flower (Robertson, 1886).…”

EVIDENCE FOR SELECTIVE FRUIT PRODUCTION IN ASCLEPIAS

“…Initiation and abortion of pods is slightly more costly, 1-5 times that of ovary production for the same nutrients, H 2 0 , or total dry weight, despite the withdrawal of a large proportion of nutrients prior to abortion. The cost entailed in providing surplus ovaries per umbel may be reduced by the related benefits of an increased floral display, which may increase pollinator attraction (Willson and Price, 1977, but not true in Lynch, 1977;Willson and Bertin, 1979) and pollen production (Willson and Rathcke, 1974;Willson and Price, 1977). Surplus ovaries may also function in satiation of herbivores and provide the flexibility to increase fruit set when resources are more available (Willson and Price, 1980; reviewed by Lloyd, 1980;Stephenson, 1981).…”

The Pollination Biology of North American Orchids: Volume 2

“…Apparently, P. unipuncta was sensitive to the total amount of nectar available and broke-off visits when this amount fell below some undetermined, minimum level. Willson and Bertin (1979) recorded similar behavior of this moth on Asclepias where it functioned as a common pollinator for only 1 week of an extended flowering period. Whigham and McWethy (1980) found the pollinators not only responded to the overall cycle of nectar production, but also were able to concentrate their attention on the portion of the inflorescence that produced the most nectar.…”