Bees may be grouped into three categories based on their foraging habits. Bees that prefer only a small number of flowering species are known as oligolectic. The advantage to the plant kingdom from this behavior is enormous, since it assures cross-pollination within a single species.
A few species of bee are known to pollinate one—and only one—species of flower. Bee-flower mutualisms of this type, known as monolectic, are rare but extremely important from an evolutionary perspective. Neither species will survive without the other, so a loss of one means the loss of both. Most bees, however, are opportunistic foragers that gather pollen from a vast number of species. These bees, known as polylectic, are valuable to farmers who often grow more than one crop at a time or more than one crop in sequence. Both honey bees and bumble bees are polylectic.
Even bees that are polylectic tend to visit only one type of flower per foraging trip, a trait known as “floral consistency.” Nature’s way of ensuring good pollination, floral consistency prevents a bee from going from a clover to a vinca to a cucumber to a bean, for example. Such random flower visits would not yield the pollination necessary to set seed and maintain plant populations from year to year.
Although polylectic bees are able to forage on many different plants, they still have preferences. Nectar-collecting bees such as honey bees and stingless bees prefer flowers that have high sugar content. Honey bees will readily visit apple, cherry, and plum, for example, but avoid pear unless there is nothing else to eat. On the other hand most wild bees—because they collect only pollen and not nectar—readily visit the low-sugar-producing flowers of pear and similar plants.