Testing the marginal value theorem on Bombus appositus and Bombus nevadensis

Abstract

Previous studies on bumblebee foraging behavior found that bumblebees often exhibit optimal foraging behavior when collecting nectar and pollen. A popular theory is that bumblebee foraging can be represented by the marginal value theorem (MVT). This summer, I observed the foraging behavior of Bombus appositus and Bombus nevadensis workers in Gothic, Colorado, to analyze whether bumblebee foraging decisions on naturally occurring Delphinium barbeyi patches are consistent with the MVT. Specifically, I investigated 1) Do B. appositus and B. nevadensis workers spend more time foraging in denser patches of D. barbeyi, 2) do B. appositus and B. nevadensis workers visit proportionally fewer flowers in denser patches of D. barbeyi, and 3) are these trends less apparent at the end of the summer when all patches are D. barbeyi are more sparse. After following bumblebees throughout their foraging bouts and tracking their foraging decisions with a voice recorder, I found that patch flower density explains a significant amount of variation in the patch residence time and proportion of flowers visited by B. appositus workers. I found that patch flower density explains a significant amount of variation in the patch residence time, but not the proportion of flowers visited by B. nevadensis workers. Finally, I found that foraging behavior is not significantly different later in the summer when all D. barbeyi patches are more sparse.

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