With spring arriving and fruit trees starting to open their buds, it is time to think about pollinators. Among the first to visit my orchard are the mason bees (genus Osmia), often considered the best general pollinators, delivering much more pollen from flower to flower than honey bees.
Mason bees are extremely efficient pollinators because they store collected pollen on the undersides of their abdomens, not on their hind legs like most bee species. While foraging, they pick up pollen from the flowers’ anthers and transfer it to pollen-carrying hairs. They drag their pollen-dusted underside across the flower’s stigma, ensuring successful pollination.
There are about 70 species of mason bees in the Pacific Northwest. They are often metallic blue or green, making an artful picture when visiting an apple flower, like a beautiful polished blue stone against a flowery white backdrop.
Mason bees are solitary; every female builds her own nest in which to lay eggs. Unlike bumble bee queens, the female mason bee has no worker bees to help her. While some solitary bee species excavate their own nests, Osmia females use naturally occurring tubular cavities, such as hollow plant stems or abandoned tunnels that were dug out by wood-boring beetles. Like miniature masons, the bees gather mud and pebbles for constructing the interior of their nests and use pebbles to seal the nest entrance when construction is complete.
Female mason bees provision cells within the nest with a mass of pollen and nectar that is roughly the size of a small pea. They lay a single egg on each pollen ball, and then build a wall of mud, clay or chewed leaves around the egg and food for protection. The female bees can “choose” the sex of each egg they lay, which allows them to place the male eggs closest to the entrance of the nest. Male bees then emerge first from the nest and wait for females to surface in order to mate.
Mason bees may sound like the perfect pollinator for your garden. Many nurseries and websites sell “mason bee motels” or extol their virtues. But solitary bees like mason bees build their nests scattered throughout the landscape, not clumped together in tight groups. What effect does using bee motels have on the health of the species? It provides predators, parasites and parasitoids (parasites that eventually kill their hosts) easy access to a large group of mason bees. Probably worst of all, it encourages the spread of diseases such as fungus chalkbrood and Chaetodactylus mites, both of which harm bee larvae. The mites feed on pollen and nectar provisions, causing the larvae to starve. Hygiene and sanitation are crucial for the survival of mason bees. Unless your bee motels are kept scrupulously clean, don’t use them.
What to do instead? Provide scattered nesting sites throughout your landscape. Garden with plants that have hollow or pithy stems, like elderberries, blackberries, figs and roses. For mason bees, the best stems are at least 6 inches long with a hard, dry exterior. These bees will also nest in dead or dying trees if the wood is soft or has hollow portions.
Create a safe home in your garden where native plants flourish and the native wildlife will thrive. Mason bees make good, hardworking neighbors. Give them a warm welcome them this spring.
Pete Haggard (he/him) and Jane Monroe (she/her) are the coauthors of Rewilding: Native Gardening for the Pacific Northwest and North Coast, available now from The Press at Cal Poly Humboldt and in local bookstores and nurseries.
This article appears in A Wing and A Prayer.