Flower nectar is primarily comprised of sugars, which provide energy for the potential pollinators. But the floral nectar of some plant species also includes small quantities of substances known to be toxic, such as caffeine and nicotine... Caffeine is found at concentration levels of 11-17.5 milligrams per liter, mostly in citrus flowers. In the nectar of grapefruit flowers, however, caffeine is present in much higher concentrations, reaching 94.2 milligrams per liter...
The results showed that bees clearly prefer nectar containing nicotine and caffeine over the “clean” nectar. The preferred nicotine concentration was 1 milligram per liter, similar to that found in nature. Given a choice of higher levels of nicotine versus “clean” nectar, the bees preferred the latter.
The presumption is that natural selection has favored those plants that satisfy the desires of their pollinators. Further studies are underway to ascertain whether the bees are addicted to caffeine and nicotine.