Bees are a well-studied group of pollinating insects that vary widely in the number of plants that they collect pollen from. We explored whether specialized and generalized pollen diets were associated with the evolution of three families of olfactory receptor genes, a set of proteins that are crucial components of the peripheral olfactory systems of insects. We found that pollen specialization in bees requires the diversification of existing genes while dietary generalization is associated with losses of odorant receptor genes.