There are roughly 4,000 species of bees (other than honey bees) in the U.S. Nearly all of these are solitary, meaning that only one female tends the nest. Some species of mason bees (Osmia spp.) have been used for pollination in Japan, Europe and in North America because they often nest in aggregations and they will nest in tubes that are provided for them. They are in the family Megachilidae, which includes the leafcutter bees. The blue orchard bee (Osmia lignaria) has been very useful for pollination of fruit trees in the U. S.
The mason bees get their name from the cement that the female bees make from mud. This cement is used to construct sturdy nests on twigs, in underground burrows or in hollow stems. Some species only nest in empty snail shells. The species commonly used for pollination of crops nest in hollow tubes. Artificial tubes such as straws or holes drilled into wood can be used. A single nest consists of a linear arrangement of cells separated by mud walls that will contain developing bees. The female mason bee provisions each nest cell with a mixture of pollen and nectar. Then she lays a single egg in the cell. Females typically provision about 20 cells during their lifetimes. The larva will spin a cocoon in about one month and will pupate inside the cell. In the fall adults emerge from pupation but remain dormant within the cell until the following spring. The dormant bees can be moved to orchards for spring pollination. Adults usually appear from April to June, corresponding with the time of fruit tree bloom. All female bees have control over laying either female or male eggs. The male eggs of the mason bee are usually laid in the outer cells, so they are the first to emerge in the spring. Females often initiate their nests in the same location from which they emerged. In favorable conditions, the number of new nests in an orchard can double or triple each year.
Unfortunately, mason bees cannot be produced in the huge numbers that are present in honey bee colonies. They also are only active for about six weeks in the spring, so are only available for pollination at that time. However, individual mason bees are more efficient at pollinating fruit trees than honey bees. Working with Osmia cornifrons in Japan, researchers found satisfactory pollination of apple flowers was achieved up to 50-100 meters from nest sites. It was calculated that about 80 bees were needed per hectare (2.2 acres). However, intensive spray programs in orchards can be devastating to these bees. In some cases, the bees are bred in a nesting aggregation outside of the orchards and an expendable proportion of this population is brought to the orchard each year for pollination.
Small fruit tree growers may want to consider purchasing the materials to start keeping blue orchard bees. The only requirement for keeping these mason bees is to provide suitable habitat and hope the bees take advantage of it. The nesting straws are usually about placed 2 meters abouve the ground and protected from rain. Nests can also be provided by drilling 6-inch deep holes that are 19/64 inches diameter in a block of wood. If the holes are made as grooves between two boards that fit together, the nests can be cleaned of debris each year. If you want to be sure to have som Mason bees, buy them from the commercial suppliers list. There is little danger of bee stings from these bees because they are solitary bees. Each nest belongs to only one female and she does not defend the nest by stinging. Social bees, such as honey bees have large numbers of individuals which will sting to defend the nest if it is disturbed.
- Greg Hunt, Purdue University
Bee Informed Partnership