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

When rewards are concealed, the ability to make foraging decisions on the basis of reward distribution may depend on the local density of foragers as they deplete the resource. Work on nectar-gathering bees has shown that where bees are numerous, the reward basis on which foraging descisions are made may be altered as the standing crops of nectar in highly rewarding plants is lowered by foraging activity. Heinrich (1976) showed that in meadows in Maine, favored species of plant became successively depleted of nectar following the heavy usage by bumble bees, and as a result, foragers switched their attention away from the high nectar producers to less rewarding species. If this phenomenon also occurs within a species at a site, it could limit the potential for pollinator-mediated selection on reward production (Zimmerman 1988).

Here, we report bumble bee discrimination on the basis of pollen quality among plants of the monkey flower Mimulus guttatus (Scrophulariaceae). This species produces little nectar but does produce large amounts of pollen of varying quality (Robertson et al. 1994) which is collected by bumble bees and honey bees. In this paper, we present the results of seven seasons of field and indoor trials on the foraging preferences of bumble bees. We investigated the variability of pollen production within and among genotypes and asked how this affected the foraging behavior of bumble bees at three scales: indoors, in two-genotype choice tests; outdoors within patches; and outdoors between patches that varied in pollen quality. Finally, we tested whether the ability of bumble bees to differentiate among plants is affected by the density of foragers.

METHODS

Study system--Mimulus guttatus

Mimulus gutattus is a hydrophilic annual or short-lived perennial, native to western North America, though it has become naturalized in many temperate areas worldwide. Vegetative reproduction can occur by layering in favorable conditions, but the primary mode of reproduction is through the production of numerous small seeds in dry dehiscent capsules. The large yellow flowers (length 30 mm, width 25 mm) are funnel-shaped and are borne on erect stems and contain two pairs of anthers partially concealed in the corolla tube. Conspicuous red dots at the mouth of the funnel act as "nectar guides" but direct visitors, not to the nectar (which is minimal), but to pollen that is shed by the sequentially dehiscing anthers.

The materials used for this study were chosen from greenhouse-raised stock grown from seeds collected within the natural range, from tailings at Copperopolis Mine in Calveras County, California, USA (37[degrees]58' N, 120[degrees]38' W) (for more details see Macnair et al. 1993). In this population the plants are pollinated by Bombus californicus and Apis mellifera (Macnair et al. 1989). In common with a number of other Californian populations (A. Robertson and M. Macnair, unpublished results), this population is polymorphic for pollen quality (Fig. 1). Some plants produce a mixture of normal pollen and pollen with no stainable contents. This variation has a strong heritable basis, although total pollen number per flower apparently does not (Robertson et al. 1994).