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

Not surprisingly, bumble bees had little trouble in recognizing and discriminating between patches of plants that contained either high- or low-quality plants. Both the number of foragers and the numbers of flowers visited was very much lower in the low-quality patch compared to the high. This kind of discrimination suggests that bees learn to associate patches with reward value. The behavior shown on the intermediate plants in these patches was less expected. In the low-quality patch, flowers on the two plants of relatively higher quality were visited much more often than flowers in the rest of the plot, as expected. However, in the high-quality patch, these plants, which should have been perceived as being of lesser quality than rest of the plants in the patch, were visited in greater numbers than their proportion of the plot suggests they should have been. In fact, the intermediate plants were achieving higher rates of visitation per flower when surrounded by good-quality plants than when they were the be st plants in an otherwise low-quality patch. The failure of bees to discriminate against the poor producers in the high-quality plot may be due to pollen-stripping on the plants in the rest of the plot, or it could be that the bees fail to notice that there are two poor plants in amongst the otherwise high-quality patch. Such "mistakes" are suggested to account for the relatively high visitation to rewardless species when they grow in close association with highly rewarding species, the so-called "magnet effect" (Laverty and Plowright 1988, Laverty 1992, Dafni 1993).

Pollen limitation of the colony-development rate of social bees is at least as likely as nectar limitation (Fisher 1987, Sutcliffe and Plowright 1990, Plowright et al. 1993), so there should be selective pressure on foragers to develop efficient pollen-harvesting behaviors. Our results for bumble bees feeding on Mimulus show, for the first time, that bees not only monitor the quantity but also the quality of the pollen they harvest from flowers. However, it is not clear how bees determine either the quantity or quality of the pollen they are collecting. Harder (1990) suggested that the sensory apparatus on honey bee corbiculae that are thought to sense the size of the growing corbicular loads (Ford et al. 1981) may also be involved in assessing the rate of pollen harvest by bumble bees. If this is the sensory system involved then it must also be able to differentiate between the full and empty pollen grains that Mimulus produces. It is possible that some property of the empty grains (perhaps the mass, electr ostatic charge, or stickiness of the grains) means that they do not pack properly into the baskets. Alternatively, the odor of pollen may differ between full and empty grains. Pollen odor has been recently implicated in behavioral responses of honey bees to the presence of pollen including the ability to determine levels of available pollen in individual flowers (Dobson et al. 1996).