THE ROLES OF PHENOLOGY AND REWARD STRUCTURE IN THE POLLINATION BIOLOGY OF WILD SUNFLOWER (HELIANTHUS ANNUUS L., ASTERACEAE)

ABSTRACT

A seven-year study of Helianthus annuus L., the common sunflower, in its native range in Texas has shown that the phenology and reward production schedules of individual florets within a head are of paramount importance in determining the foraging behavior of both specialist and generalist bees that serve as pollinators. Both pollen and nectar are produced only during the male phase of the protandrous disk florets, but pollen is produced bimodally whereas sugar accumulation is constant. Acceptance of heads by native bees was positively correlated with the number of male-phase florets per head, although the number of approaches was not. Native bees responded to increases in nectar amounts, as determined by experiments using bagged heads in which nectar accumulated, by increasing time per head and time spent foraging per floret. Comparisons across an array of visitors showed that large bees were more effective pollinators than small bees, but within the large bee category, there were no differences between specialists and generalists or between males and females. Within the Asteraceae, rewards can influence pollinator behavior directly, whereas in many other advanced groups of angiosperms with floral morphologies that restrict pollinator movement, the number of ways that rewards can directly affect pollinator behavior may be limited.