Alfalfa Leafcutting Bees

Idaho’s Treasure Valley is an irrigated high desert, making it a diverse agricultural region producing crops for humans and animals. Many of these crops require pollination, so there are plenty of managed bee colonies in the area. Most of the crops are pollinated by the honey bee, Apis mellifera, that American beekeepers house in familiar Langstroth bee hives. However, one crop, alfalfa, an important animal food crop and the principal hay crop for dairy cattle, is largely pollinated by another bee species. When alfalfa is grown to produce seed, the alfalfa leafcutting bee, Megachile rotundaata, may be brought into the fields to ensure adequate pollination for the production of seed. The alfalfa plant, a member of the legume family, produces ample amounts of nectar and pollen and is attractive to honey bees. However, honey bees don’t like foraging alfalfa due to the physical structure of alfalfa flowers. When a honey bee forager attempts to access alfalfa nectar or pollen, the flower slaps the bee’s face with considerable mechanical force.

The reluctance of honey bees to forage alfalfa makes the alfalfa leafcutting bee a favorable choice especially for alfalfa seed production where ample insect visits are necessary for pollination. Alfalfa leafcutting bees are not social bees like honey bees; they are solitary. Honey bees live in large colonies housed in wooden hives. Gregarious alfalfa leafcutting bees are solitary bees; large numbers of beekeeper-managed solitary bees live in close proximity in tubes bored into wood or plastic boards. The alfalfa leafcutting bee nesting tubes shown in today’s photo are cut into blocks of polystyrene. Adult leafcutting bees are emerging from some blocks. Other blocks with empty tubes are available for leafcutting bees to occupy. After mating, a female leafcutting bee cuts leaf material and carries it to an available tube where she deposits an egg along with pollen and nectar to feed the developing offspring. A large amount of chewed leaf matter is visible under the leafcutting bee hive blocks.

--Richard