Why bees pollinate plants that don’t need it
Cross pollination—the moving of pollen from the flowers of one plant to the flowers of another—is usually accomplished by wind or animals. There are a few other vectors, including water and gravity, but wind and animals are the main ones. Many animals move pollen—including bats, birds, and butterflies—but bees of one species or another do most of it.
Nevertheless, the bees don’t have any contracts or business deals with the plants. The foraging bees have a mission, and that mission is to go get something the colony needs and bring it back. The things the hive needs are pollen, nectar, water, and propolis, and there are feedback mechanisms and other communication systems that tell the foragers which of these they need to collect.
So given the mission to go forth and collect pollen, that’s what the forager does. She doesn’t give a rip about what the plant wants. Once she finds a satisfactory source of pollen, she collects it and brings it home. If the pollen comes from a wind-pollinated plant such as corn or alder, it makes no difference to the bee.
Plants that need cross pollination have evolved ways of attracting pollinators, including brightly colored petals, pleasing aromas, or plentiful nectar. (Technically speaking, the plants that had some of these characteristics were able to survive and reproduce more successfully than those which didn’t, so those genes increased in frequency in the gene pool.) In any case, the plants benefited from the insects and the insects benefited from the plants.
Plants that don’t need to attract pollinators usually don’t have showy flowers, nectar, or fragrance. They still need pollen in order to reproduce, but the wind takes care of moving it around. Some of these plants actually produce pollen that is toxic to bees, or that is not pleasing to them. Because producing pollen is energy-expensive for the plants, if they don’t need to share it with insects, it is best if it’s not attractive to them.
When you’re thinking about honey bees, the whole thing is made more complicated by the fact that honey bees are not native to North America, so the plants that are native here did not co-evolve with them. Honey bees are polylectic which just means they forage on many different species of plants. And for the most part, the wind-pollinated plants that produce “tasty” pollen just have to put up with them.