Mason Bees
Osmia spp.

Appearance

Mason bees are frequently metallic green or blue to blackish. They are 3/8 to 5/8 inch in length. Males are smaller than females and have light-colored hair on their face. Aside from color, you can tell a mason bees from a honeybee by the way they transport pollen: mason bees carry pollen on their abdomen while honey bees carry it on their hind legs.

Feeding

The favorite food for mason bee larvae is fruit tree pollen plus some of their nectar. Females collect this food, bring it to their nests, and knead it into a ball, mixing it with nectar and their own saliva.

Predators

Mason bee nests are often subject to predation by wasps. The wasps drill through the nests to lay their own eggs on developing bees—once the wasp eggs hatch, the larva eat the developing mason bee. Female mason bees protect their young from this predation by plugging holes in their nests with mud.

Reproduction and Life Cycle

Mason bees typically emerge in early spring and are active for roughly 6 to 8 weeks (from about mid-April through mid-June) often coinciding with the blossoming of various fruit trees. Males emerge first and wait around the nest for the female to emerge so that they can mate.

After mating, the males soon die and the females start looking for a suitable location to nest. They place a loaf of nectar and pollen in each cell, lay an egg on top of it, and wall-off the cell with mud. Come summer, each egg hatches into a larva and feeds on the pollen-nectar loaf. When the larva is fully grown, it begins a resting stage before spinning a cocoon and changing into a pupa. By the end of summer, it will transform into an adult.

Did You Know?

• Mason bees are extremely important for pollinating fruit trees like apples and cherries and are often referred to as orchard bees.
• Six mason bees can pollinate an entire fruit-tree; it would take 360 honey bees to pollinate the same tree.
• Mason bee males emerge a few days earlier than the female, a phenomenon called "protandry."
• Mason bees carry pollen on their abdomen instead of their legs.
• Mason bees do not produce honey, are not aggressive and do not sting.

Sources and Additional Information

• Common Maryland Bees – Maryland Department of Natural Resources
• How to Manage the Blue Orchard Bee – U.S. Department of Agriculture
• Blue Orchard Mason Bee – U.S. Forest Service
• Orchard Pollination: Solitary Mason Bees – Penn State Extensions