By Mary Free, Extension Master Gardener
updated November 2022
As bees feed on flower nectar, their electrostatically-charged and branched body hairs attract and trap pollen grains. They carry the grains from flower to flower making them very effective pollinators. (About one-third of agricultural crops world-wide depend directly or indirectly on bees for pollination.) Both male and female bees feed on flower nectar, which provides carbohydrates and energy. However, only the female bees gather and carry pollen back to the nest or hive. Pollen provides protein that is essential for developing larvae.
Bees are attracted to flowers that are “bright white, yellow or blue or with contrasting ultraviolet patterns and with a mild, pleasant fragrance.” In some flowers, like asters and wild roses, the nectar and pollen are easily accessible. Other flowers hide their nectar and pollen and provide visual clues (such as ultraviolet patterns, signals) to guide bees to their reward. These nectar guides may be prominent when nectar is abundant and fade as nectar is depleted.
Other flowers, like tomato, potato, eggplant, blueberry and cranberry, hold their pollen tightly and require sonication aka “buzz pollination” to release it. When a bee (bumble, sweat or carpenter) grabs a flower anther and rapidly moves its flight muscles, it causes sonic vibrations that dislodge the pollen. Azaelas require sonication to release pollen from their poricidal anthers. Larger bees, like Bombus impatiens (common eastern bumble bee), that buzz the entire flower are more effective pollinators than small anther-buzzing bees. Though efficient in collecting large amounts of pollen, Andrena cornelli (azalea miner bee) tends to do so without touching the stigmas, making it an ineffectual pollinator.
Photo © Mary Free.
Flowers like milkweed and orchids have “pinch-trap” mechanisms where pollen grains are contained in agglutinated sacs called “pollinia.” They attach and dislodge from a bee if it inserts its leg in the mechanisms while feeding. Unfortunately, sometimes a bee’s tarsus is detached as well.
Some females, like honey bees and bumble bees, have “pollen baskets” (corbiculae) on the outside of their hind legs. They use their front and middle legs to rub the pollen collected on their body hairs toward their hind legs. Then they press the “raked” pollen between the two large segments of their rear legs. They deposit the resulting pollen pellets into the corbiculae. This enables them to carry more pollen back to their nests at one time.
Other female bees have specialized, tuft-like hairs called scopa that transport the pollen. Sweat bees and carpenter bees have scopa on their hind legs. Mining bees have scopa on their hind legs and the sides of their thorax. Leafcutter bees and mason bees have scopa beneath their abdomens.
Sometimes bees transport more than pollen back to their nests. The Japanese hornfaced (mason) bee in the picture above has pollen mites on its back. Pollen mites use insects to travel from flower to flower to find pollen to eat. Mites also travel on the backs of bees to the bee nest where they stay to feed on pollen intended for bee larvae. As a mason bee moves through its nest, pollen mites can cover it to such an extent as to hinder its flight.
To learn more about bees, visit these sites: EOL Observer Cards at http://fieldguides.eol.org/observer/Observer-Bees-ebook-v5DL.pdf; the Pollinator Partnership at http://www.pollinator.org/beeissues.htm; and Celebrating Wildflowers: Bee Pollination at http://www.fs.fed.us/wildflowers/pollinators/animals/bees.shtml.