Mason Bees
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Mason Bees: Tiny Garden Heroes

During a typical northwest spring we become aware of the season dawning by the gradual appearance of flowering fruit trees in our yards and along city streets. For those of us with trees that bear fruit, the budding on our plums, cherries and apples is a welcome nod to another year of harvesting and enjoying the food we grow. But in order for our fruit trees to bear their delicious gifts, the flowers we are so glad to welcome need to be pollinated. That is primarily the work of our friend and little hero, Osmia lignaria, the orchard mason bee.

In the cooler temperatures of our northwest spring, the orchard mason bee begins to emerge from the wood cavities where they have spent their last nine months or more growing from eggs to winged adults. They emerge much earlier than honeybees who are waiting for temperatures in the 60s to do their business. The mason bee males emerge before the females, chewing through their way through their cocoons and the plugs put in place by their mothers the spring before. The little females live only a month or so, devoting their time to laying eggs and gathering pollen to place with their eggs as food for development. Their short but productive life becomes our great benefit; as they gather pollen for their eggs they are also pollinating our trees.

Mason bees are gentle, non-aggressive bees. With their minds focused on reproduction and pollen gathering you will see them flying through your urban orchard from tree to tree and back to their cavities, intent on finishing their task. Smaller than a honeybee and shiny dark blue they resemble a fly in passing. The males also pollinate flowers, though less productively than the female. She can visit up to 75 flowers each trip out, gathering pollen on specialized abdominal hairs and then depositing the pollen into her chosen cavity! Laying one to two eggs per day and with each pollen pile needing about 20 trips, this efficient little pollinator deserves our complete respect for a job well done! 

Their nesting cavities are adapted from pre-existing ones that they clear of debris each spring. They do not create holes and so are not damaging to your property. They can be found in house siding, old trees or posts. You can also provide them with homes by placing manufactured mason bee houses in the garden. Be sure to use materials that are free of toxic elements and can be easily cleaned out. Also provide protection from predators like the long beaked flickers which love to eat them! A good method is to use specialized paper straws that can be composted each season once they have emerged. Another type of house can be made by using grooved layered wood that creates holes when put together. Both of these methods allow you to remove the cocoons in the fall and clean them with bleach and water or a fine sand rinse. This helps to minimize the mites that are attracted to their pollen piles and the fungal problems that can occur. Once cleaned, store in a container in a cool place until it is time for them to emerge again. An outbuilding can work but a refrigerator is more reliable for steady temperature control. Bring them back out into the garden, protected from predators, when the daytime temperatures reach 57 degrees F and your trees are in bloom.