Within-plant foraging behavior of bees and its relationship to nectar distribution in anchusa strigosa

The behavior of bees foraging for nectar on Anchusa strigosa was studied. The flowers of this species are irregularly distributed in three-dimensional space, and the corolla is violet in young flowers and blue in mature ones. Blue flowers produced nectar at higher rates and received more visits per unit time than violet flowers, which indicates that the bees were able to associate reward with flower color. Before the beginning of bee activity, blue flowers contained more nectar than violet flowers, but during foraging activity they contained lower amounts of nectar. This reversal is inconsistent with predictions of optimal foraging theory. Bees tended to commence foraging at bottom flowers and worked predominantly upwards. Nectar standing crop was uncorrelated with flower height before foraging activity and either uncorrelated or positively correlated with flower height during foraging activity. We relate the positive correlations to the observed “movement rules” (bottom flowers were depleted more frequently than upper ones) and suggest that in both our study and some previous studies these rules reduced, rather than increased, the probability of finding nectar-rich flowers.