Colony condition and differences in individual preferences influence forage type collected by bees. Physiological bases for the changing preferences of individual foragers are just beginning to be examined. Recently, for honey bees octopamine is shown to influence age at onset of foraging and probability of dance for rewards. However, octopamine has not been causally linked with foraging preference in the field. We tested the hypothesis that changes in octoparnine may alter forage type (preference hypothesis). We treated identified foragers orally with octoparnine or its immediate precursor, tyramine, or sucrose syrup (control). Octopamine-treated foragers switched type of material collected, control bees did not. Tyramine group results were not different from the control group. In addition, sugar concentrations of nectar collected by foragers after octoparnine treatment were lower than before treatment, indicating change in preference. In contrast, before and after nectar concentrations for bees in the control group were similar. These results, taken together, support the preference hypothesis. (C) 2007 Elsevier Ltd. All rights reserved.