Mason bee is a name now commonly used for species of bees in the genus Osmia, of the family Megachilidae. Mason bees are named for their habit of using mud or other “masonry” products in constructing their nests, which are made in naturally occurring gaps like between cracks in stones or other small, dark cavities.
They are important pollinators since they appear while the weather is still fairly cool before the honey bees are out. Unlike honey bees, Osmia species are solitary; every female remains fertile and makes her own nest. No worker bees for these species exist.
Osmia females typically nest in narrow gaps and naturally occurring tubular cavities. Commonly this means hollow twigs, but can be in abandoned nests of wood-boring beetles or carpenter bees, in snail shells, under bark, or in other small protected cavities. They do not excavate their own nests. The material used for the cell can be clay, mud, grit, or chewed plant tissue. A female might inspect several potential nests before settling in.
You can assist the bees by providing nests for them. Cardboard tubes may be lined with straws and reused from year to year. Wood blocks are available too. Sometimes it’s difficult to find replacement straws, so I make mine out of parchment paper cut into 2-by-6 inch strips and rolled around a dowel. If you fold the paper on the long edge about one-eighth of an inch in, it makes the rolling easier. Lining the nests with paper straws makes it easier to clean them in the fall. I have used both the cardboard tubes and the wood blocks for years.
Place your bee straws under an eave against a south or east wall with the opening facing east or south, preferable protected from rain. The bed should be about 5 feet off the ground. If you water your garden regularly the bees should have no trouble finding mud to seal up their nests. If you don’t normally water, dig a shallow hole 6 to 12 inches wide in bare earth near the nest and fill it with water. Rough up the sides of the hole and make sure there is loose soil available for the bees. Keep a milk jug filled with water nearby and sprinkle the hole with water every time you walk by if it hasn’t rained. (Note: I’ve never done this and the bees seem to have no trouble making mud.)
The bees begin to emerge from their cocoons in the spring when the daytime temperature reaches 57 degrees Fahrenheit. The bees look like large flies and make a humming sound as they hover around the nests. The males emerge first and some are known to actively extract females from their cocoons. They remain near the nesting site and wait for the females to emerge, which can be several days to weeks depending on the number of days of warm weather. The first thing the females do is mate. A female typically mates once, or maybe twice. She is absent from the nesting site for several days while she feeds and waits for her ovaries to fully mature.
Within a few days of mating, the female has selected a nest site and begun to visit flowers to gather pollen and nectar for her nests; many trips are needed to complete a pollen/nectar provision mass. Once a provision mass is complete, the bee backs into the hole and lays an egg on top of the mass. Then she creates a partition of “mud,” which doubles as the back of the next cell. The process continues until she has filled the cavity. Female eggs are laid in the back of the nest and male eggs toward the front.
Once a bee has finished with a nest, she plugs the entrance to the tube, and then may seek out another nest location. The males soon die, and within a few days the females begin provisioning their nests.
A female might inspect several potential nests before settling in. Once she has found a preferred nesting cavity, she flies outside of the hole and does an in-flight dance. She is orienting on major visual features to find her nest when she returns from foraging.
Orchard mason bees arrange their nests as a series of partitions, with one egg per partition. A female begins the process by collecting mud and building the back wall, if necessary, of the first partition. She then makes several back-and-forth trips to nearby flowers. Unlike honey bees, which visit flowers that are miles away, females visit flowers nearest the nest. One bee can visit 75 flowers per trip, and it takes 25 trips to create a complete pollen/nectar provision. The female works tirelessly during the day, only stopping once the sun has gone down. When the sun rises the next morning, she basks in its rays until warm enough to fly, then continues foraging.
Once the pollen provision is large enough, she backs into the hole and lays an egg directly upon it. She then collects more mud to seal off the partition. The new wall also doubles as the back wall of the next cell, and she continues until she has filled the nest hole with a series of offspring. O. lignara bees, like many insects, can select the gender of the egg they lay by fertilizing the egg, or not. Unfertilized eggs are males, while fertilized eggs are females. On average, she lays about three males and one to two females per cavity. Because females are larger than males and require more pollen reserves, cavity dimensions can play a significant role in the cavity selection process.
Once the female has finished the nest, she plugs the entrance with a mud wall, thicker than the partitions that precede it. She then seeks another location for a new nest. She works tirelessly until she dies. An O. lignara female lives for about four to eight weeks, and can fill an average of four 6-inch tubes in her lifetime, with about eight eggs per tube. Her work includes nearly 60,000 blossom visits per bee.
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