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

Variation in overall patch quality

We were also interested to know whether bees could discriminate between patches of plants that differed in pollen quality. More bees were observed foraging on M. guttatus patches that had higher pollen quality and, once a bee arrived at that patch, the numbers of flowers visited on that foraging trip was higher than on trips made to the lower quality patch (Table 5). Both the length and frequency of the visits to the two patches was highly significantly different as shown by sign tests. This avoidance of the low-quality patch continued even when the positions of the two patches were reversed, suggesting that bees quickly re-learned the relative quality of the patches (Wilcoxon rank sum tests to test heterogeneity in proportion of bees seen at the high-viability plot before and after the plot positions were swapped, Z = -1.2687, P = 0.2046; proportion of flowers visited that were in the high-viability plot, Z = -0.4849, P = 0.6277).

The tendency to visit more flowers on the high-quality patch extended even to the two intermediate-quality plants embedded in the patches. Expressed as flowers visited per plant per hour, the rate of visitation to these plants was significantly greater in the high-quality patch than in the poor-quality patch (Table 5), which suggests that these plants were benefiting from the effect of the higher rate of bee-use of the high-quality patch. We had expected that bees would recognize the poorer quality of these plants in the high-quality patch and make fewer visits to them compared to other plants in the array but in fact, the proportion of flowers that were visited in the patch that belonged to the intermediate plants was slightly higher than we would have expected if the bees were foraging at random (expected, 2/16 plants = 12.5%; observed = 13.5%, Table 5). In the low-viability plot, the departure from random visitation was even greater (observed 23.0%). If we compare the number of visits to intermediate plan ts to the expected number using a binomial distribution and an expected value of 0.125, in both plots the intermediate plants appear to be over-visited (P [less than] 0.001), but the departure from expected is much higher in the case of the low-viability plot. The proportion of visits made to these plants were again unaffected by the swapping of the plot positions (Wilcoxon rank sum test, Z = -0.0412, P = 0.9671).

DISCUSSION

Bumble bee discrimination on the basis of pollen availability

Mimulus guttatus genotypes differ markedly with respect to pollen quantity and quality. We have shown that when given a choice of genotypes with previously unvisited flowers in an indoor choice test, pollen-collecting bumble bees can readily discriminate among them on the basis of the quantity of viable pollen. In outdoor plots, however, the preference for the better pollen producers of an array was variable. During the 1985 to 1987 a series of poor summers meant that very few bumble bees used the plots. The summers of 1990 and 1991, on the other hand, were more favorable, and bumble bees were more common. Bees made 2 visits/ h to the 1985-1987 plots while they averaged 4.4 visits/ h in 1990 and 1991 (range 1.3-6.4). The 1992 plots were an attempt to test the idea that such differences in relative bee densities could affect the ability of bees to assess pollen quantity and quality. We postulated that the scarcity of bees during the earlier seasons allowed those that were present to be able to discriminate b etween genotypes on the basis of pollen. In later seasons, when bees were numerous and foraging could persist for several days, bees may have been rapidly depleting the resource and preventing preferences from being shown. Bumble bees harvest Mimulus pollen quite efficiently. Bombus pratorum workers remove 50% of the total pollen production after one visit to virgin flowers (A. Robertson, unpublished data). In our outdoor plots, flowers were estimated to receive 1-2 visits/d on average in our 1990 and 1991 plots (Robertson and Macnair 1995) and given that the flowers last 3-5 d, it is likely that flowers received multiple visits by bumble bees. Under such a regime it seems plausible that many flowers had most or all of their pollen stripped, making it difficult for bees to discover or learn the position of consistently rewarding plants. The indoor trials of 1992 confirmed quite clearly that bees could only discriminate on the basis of pollen production when they were foraging at low densities. The reason was clear: the standing crops of pollen of open flowers only closely mirrored the production per flower when there were few bees removing pollen.