Learning of colored targets with vertical and horizontal components by bumblebees (Bombus terrestris L.)

Colorful tufts of bracts, which attract insect pollinators, terminate the vertical inflorescences of several plant species. These flag-like bracts contrast in color with the leaves below them, and are oriented perpendicular to the flowers on the inflorescence. We studied how color contrast and perpendicular orientation affect the feeding choices of bumblebees in laboratory experiments. We trained bees to feeders with perpendicular two-color displays, and subsequently recorded their choices among feeders that displayed only one of these cues. The bees preferred perpendicular displays that resembled the training model in the color of the horizontal component. We then evaluated the effects of the horizontal vs. vertical display component on foraging choices. After training bees to two-color, perpendicular displays, we allowed them to choose between displays with either the same horizontal or the same vertical component as the training model. Foragers mostly oriented to the horizontal displays to which they had been trained. We conclude that (a) bumblebees learn perpendicular, two-color displays; (b) horizontal display components influence foraging choices more than vertical components; (c) vertical visual cues might guide the approach to a feeder, serving as landmarks. We discuss possible implications of our findings for the role of extra-floral bracts in pollination.