Resource heterogeneity and patterns of movement in foraging bumblebees

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Abstract
Differences in the foraging behavior of B. terricola workers on white clover, Trifolium repens, were examined on previously unvisited (filled) and depleted flowers, and as a function of flower-head density. The number of florets visited per unit time was independent of flower head density from 20 to at least 290 heads/m2, in part because the bees utilized more florets per head at low flower head densities, and also because they approached but rejected more flower heads at high rather than at low flower head densities. Previously visited clover-heads were approached but often rejected, while unvisited heads were not rejected. The bees behaved markedly different while foraging in patches of flowers which were available to all foragers, than in those which had been screened and contained on the average 3.9 times more sugar; they tendent to move through depleted areas and to concentrate in rich areas. On successive flower-head visits in depleted areas they moved more forward than backward (82% vs. 18%), while in rich areas they tended to move almost as much backward as forward 47% vs. 53%). The distances of inter-head moves were approximately twice as long in depleted as in rich areas. The bees visited almost as many florets per unit time in the rich as in the depleted areas (32 vs. 35 per min). But in the rich patches they probed on the average into 11.6 florets per head in contrast to only 2.3 florets per depleted head. In an experiment with B. vagans workers foraging from Aconitum napellus inflorescences, the bees did not reject previously visited flowers, and they moved upward in successive flower visits on inverted as well as on unaltered inflorescences. On horizontal inflorescences they moved both right and left. The movements are not a direct response to nectar differences, nor to differences in average nectar distributions. The systematic foraging behavior on vertical inflorescences may thus be a mechanism of reducing the revisiting of just-emptied flowers.