Foraging interactions between native and exotic bumblebees: enclosure experiments using native flowering plants.

To assess the impact of Bombus terrestris invasion on the foraging efficiency of native Japanese bumblebees, consumption and acquisition of floral resources during foraging on flowers of native Japanese plant species were investigated using enclosures with three treatments: one with only B. terrestris (exotic), one with both B. terrestris and native Japanese bumblebee species (mixed), and one with only Japanese species (native), but with the bumblebee density held constant. Changes in the body mass of queens and the nest mass of colonies for two days did not significantly differ among four combinations of the species and treatment, B. terrestris in the exotic and mixed treatments and Japanese species in the mixed and native treatments. Thus, it is not clear that B. terrestris has higher foraging efficiency than native species and that B. terrestris individuals more negatively affect the foraging efficiency of native species than individuals of the native species themselves. The nectar standing crop of Cirsium kamtschaticum was smaller in the exotic treatment than in the mixed and native treatments. However, this may have been an artifact of differences in the numbers of flowers in the various treatments.