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

The effect of overall patch quality

To test whether bees would discriminate between patches of plants on the basis of their average pollen quality, two further outdoor plots were set up as follows. Plants were categorized as high- or low-pollen viability. One plot consisted of 14 plants of high-pollen viability and two of intermediate quality (high-viability plot), while the other contained 14 poor quality plants with the same two intermediate genotypes (low-viability plot). The plants were arranged in a 4 X 4 grid with the two intermediate plants occupying central positions along one diagonal of the grid. The two plots were 50 m apart.

Data were collected on the number of bees (Bombus spp. and Apis mellifera) that visited each plot and the numbers of flowers that the bees visited in the plot during 2 h of observation on each of 21 d spread throughout a 40-d period. Visits that were made to the intermediate-viability plants were recorded separately from visits to the other plants in the array. After 30 d, the positions of the two plots were swapped. We tested for heterogeneity of visitation patterns before and after the swap using rank sum tests before pooling the results. Over the course of the period of observation, it became necessary to substitute plants that had begun to stop flowering with fresh ones from the greenhouse of approximately the same pollen viability, thus maintaining the difference between plots in overall plot pollen quality. Contrasts between plots were made each day of the average number of visits made by bees per hour and the average number of flowers visited by each bee.

RESULTS

Variability of pollen production

Pollen production in greenhouse-grown Mimulus guttatus was variable in both number and quality and varied significantly with genotype and with the time at which the measurements were made (Table 1). Approximately equal amounts of variability in number was accounted by these two factors and between them they explained [greater than]70% of the variance. In the month between measurements, average pollen production dropped by a third (from a mean of 35 300 grains to 22 800 per flower). The rate of decline was variable among plants (as shown by the interaction). However, at any one time, pollen production was consistent within plants. This meant that only a small number of flowers were needed to be sampled in order to accurately characterize the pollen production of a plant.

Eighty-five percent of the variance in the proportion of pollen that was viable occured among genotypes; over time genotypes remained very consistent in this proportion despite the reduction in pollen number that occurred between samples. In fact, genotypes that were grown on by clonal propagation maintained their phenotype for viable pollen ratio and could be reliably predicted to exhibit very similar ratios from one trial to the next (A. Robertson, unpublished data).

Nectar production

The anthrone carbohydrate assay confirmed that M. guttatus produces little nectar (enclosed flowers averaged 50 [micro]g of sucrose equivalents, while the standing crops of nectar in flowers on outdoor plots was 20 [micro]g). A flying bumble bee consumes between 0.27 and 0.54 calories per minute (depending on temperature) (Heinrich 1979) and would thus be required to visit three to seven flowers per minute to satisfy this requirement. Although we cannot rule out the possibility that nectar was an important goal of foragers, most bees were observed to be actively gathering pollen and would spend longer than this on flowers that contained fresh pollen. Thus, the primary motivation for visits appeared to be for pollen gathering.