Foliage affects colour preference in bumblebees (Bombus impatiens): a test in a three-dimensional artificial environment

Red flowers are a defining character of the bird-pollination syndrome. Birds do not, however, innately prefer red, suggesting that rather than attracting birds, red flowers may serve to exclude other visitors (e.g., bumblebees). Bees are sometimes considered blind to red, but studies have in fact documented both blue and red preferences in various bee species. These mixed results may be an effect of overly simplistic lab settings. We hypothesized that bees might readily locate red flowers in a simple laboratory environment, but struggle to find the same flowers in a complex, foliated setting. We tested the effects of environmental complexity on visitation to red and blue artificial flowers and on the foraging rate of captive worker bumblebees (Bombus impatiens). Bees made significantly fewer visits to red flowers when foraging in a complex environment with artificial green foliage, suggesting that red becomes harder to locate in this context than in a simple, leafless environment. Bees also foraged more slowly, on average, in the complex environment, although the difference was apparent only among experienced bees. Our findings provide a possible explanation for previous laboratory tests finding no colour preference in bumblebees. This contextual colour-blindness of bees supports the hypothesis that red evolved as a mechanism for plants to avoid visitation by bees, favouring bird pollination instead.