Study from Newcastle university has revealed that bees are more likely to remember, and therefore re-visit flowers whose pollen contains caffeine. Obviously, regular visits by bees are beneficial because they improve the chance of plant pollination.
Unsurprisingly, the caffeine acts like a mild drug which lures the bees back. Some species of plants are natural caffeine producers, like Grapefruits, Mandarins and of course Coffee Plants.
The levels of caffeine in these plants is strictly limited, probably due to the fact that too much caffeine would lead to a bitter tasting nectar which would deter the bees from visiting again.
Source: Buzzing Bees — Supahome