Bumble Bee Selection Of Mimulus Guttatus Flowers: The Effects Of Pollen Quality And Reward Depletion

Ecology, Dec, 1999 by Alastair W. Robertson, Claire Mountjoy, Brian E. Faulkner, Matthew V. Roberts, Mark R. Macnair

At the start of the trial and at the end of each week, we recorded the number of open flowers on each plant and rearranged the plants randomly in the grid. At this time, four buds were harvested and preserved for pollen scoring. In one plot (91-1), we measured the production of nectar over 24 hr prior to placing the plants outside. On each genotype, five flowers that were just beginning to open were tagged. Twenty-four hours later, the accumulated nectar was removed using small pieces of filter paper and total carbohydrate measured following the method of McKenna and Thomson (1988).

Pairwise choice tests

In the spring of 1991, we ran a series of choice tests on captive bumble bees. Small colonies of Bombus pratorum were obtained from bumble bee rearers in The Netherlands. The colonies were connected to a flight chamber by a tube that could be opened or closed to control access by the workers. The bees were fed supplementary honey and pollen during the course of the trials. Prior to the start of the trials, the bees were trained to forage from vases of Mimulus flowers for 2 d to accustom them to variation in pollen quality amongst flowers.

To test for bee preferences, "vases" of two genotypes were arranged randomly in a checkerboard fashion in the flight cage, and workers were allowed to forage one at a time on the vases. Each vase contained 10 open but previously unvisited flowers of one or other of two genotypes of Mimulus. There were 10 vases in all, five of each genotype. The total number of the flowers visited of each genotype was recorded. The trial was terminated when a bee rested after visiting more than -50 flowers or by our intervention when -100 flowers had been visited. Many visits were, in fact, revisits to previous flowers so that even after 100 visits, there were many unvisited flowers in some of the vases. Six trials were performed each with a different pair of genotypes. For each trial, the preference of three to five bees was tested. All the flowers used in the trials were obtained from large plants that were producing many flowers in the greenhouse. When a genotype was chosen for a trial, five buds were collected and preserv ed as above for pollen counting later. At the same time the nectar standing crop was sampled from five fresh flowers. Pollen production as well as the proportion and number of viable pollen grains were also recorded.

The effect of pollinator density

In 1992, we established five plots in a polythene tunnel greenhouse from which wild bees were excluded. We reared colonies of Bombus terrestris from locally collected queens and placed them inside the tunnel with an exit tube that could be blocked if necessary. Two different sized colonies were used to provide different foraging bee densities. In the smaller colony, there were only one or two active foragers while the larger colony had up to 12 workers. We observed the visitation rates to the plants as described for the outdoor trials above. For each plot, foragers were monitored continuously for one 8-h period. The bees were allowed to forage on the plants for a day prior to measurements being made, except on plot 92-5, where the plants received no visits prior to observation. For this plot, we kept the data on the visits made in the first 4 h separate from the data gathered in the subsequent 4 h.