Mason bees, one of the earliest spring bees in Minnesota, are out pollinating in full force. In contrast to most other solitary bees, mason bees spend the winter as adults in cocoons. This allows them to emerge earlier in spring than many other bees that overwinter as larvae and still need to pupate before emerging as an adult.
Mason bees get their common name from the habit many have of lining and sealing their nests with mud. In spring, a single solitary female mason bee will start building a nest in an empty cavity like a hollow stem, beetle burrow, or hole in the ground. Starting at the back of the hole, she will place a ball of pollen, lay an egg, and then seal off the chamber with mud or chewed leaves. She will continue this way until the entire tunnel is full.
There are 30 species of mason bees (genus Osmia) in the eastern United States and Canada. Of those, 15 species are likely to be found in bee blocks in Minnesota. Female mason bees use the hairs on the undersides of their abdomens to collect pollen from willows, maples, plums, Virginia waterleaf, wild geranium, and other spring wildflowers. They will then bring the pollen back to the nest to provision cells for their offspring.
Mason bees are commonly used to pollinate agricultural crops like apples, cherries, and plums due to their early spring activity and the ease of providing artificial nests.
For more information on mason bees, look for "Bees: An Identification and Native Plant Forage Guide" by Heather Holm or "The Bees in Your Backyard" by Joseph Wilson and Olivia Messinger Carril.