The turnover of milkweed pollinia on bumble bees, and implications for outcrossing.

Bumble bees (Bombus spp.) picked up a pollen packet (pollinium) of milkweed (Asclepias syriaca L.) every 2-5 h, usually more rapidly on their feet than on their mouthparts. Pollinia were retained an average of slightly over one day on the mouthparts and one-quarter day on the feet. This long retention period enhances the possibility of outcrossing in this largely or completely self-incompatible species. Although many more pollinia were carried on the feet, the longer retention of those on the mouthparts resulted in their collective outcrossing potential exceeding that of pollinia on the feet. Once pollinia became attached to the bees (via their attachment mechanisms, the corpuscula), others often attached to the translator arms (connecting corpusculum with pollinia) of the first pollinia. Long strings of pollinia and corpuscula often resulted, but they frequently were shed down to a single corpusculum, which was usually retained for long periods (estimated retention time=12-43 days). During the middle and latter part of the flowering period about two-thirds of the bees' feet carried only a single corpusculum. Feet in this condition picked up extremely few pollinia and thus were largely unavailable as sites for pollen transfer. Having several single corpuscula on the feet greatly lowered the number of pollinia carried by a bee over the season. Single corpuscula occurred much less frequently on the mouthparts and were shed over 25 times as rapidly as those on the feet. Many more bumble bees moved between clones than did other possible diurnal pollinators, largely a result of being several times more abundant than all other visitors combined. This factor, plus their heavy pollinia load, suggests that they were the most important diurnal pollinators of these clones.