Native Mason Bees for Early Crops – Part II


In part one, we covered what a mason bee is and why they are important.  Now I’d like to talk about what mason bees need, and how to encourage and manage your own populations.

Mason bees are cavity nesting solitary bees, meaning they take advantage of holes, usually in wood, left behind by the activities of other insects or birds.  After mating, a female mason will collect and build a ball of pollen inside the cavity, lay an egg beside it, then seal off that cell with a wall of mud.  She will continue to lay in the same hole, building a new egg chamber every inch or so, until it is full; then she will cap it with a thicker plug of mud, and seek a new cavity to continue nest building.

So, for mason bees to stick around and reproduce, they must have flowers that provide pollen for larvae (and nectar to feed the adults!) that bloom during the March to May adult life cycle.  They must have available cavities to start nests in.  They need ready access to mud for wall and plug building.  And all of these needs must be sufficiently met in an area of only one to two hundred feet.

Food:  Mason bees prefer flowers large enough and open enough to crawl through, that provide both pollen and some nectar.  Pollen is held and carried on hairs on their bodies – one of the reasons they pollinate so efficiently. Some favored plants include commercial fruit trees and shrubs, hellebores, and Oregon Grape.  Note that some early blooming ornamentals (such as forsythia) do not provide resources for pollinators.  Volume of available bloom matters – a single small fruit tree might provide enough food for (and be sufficiently pollinated by) only a few mason bees.

Nests:  Mason bees will nest in cracks in wood or stone, woodpecker or beetle holes, or inside the stems of naturally pithy plants and trees.  To increase nest availability or population density, put out mason bee houses and nesting tubes.  Nesting tubes for mason bees should be a minimum of 6″ long; because they prefer to nest in spaces they just barely fit snugly into, preferred hole diameter is about 3/8″ to 5/16″ (approx. 8 mm).  Use disposable tubes or take-apart wood trays that are easy to open for cocoon harvest, to reduce the risk of building up mite or disease populations.  A house is any structure to hold and protect nest materials – wood houses, PVC tubes, etc.  Your mason bee house should shed water, provide a 2″ overhang for rain protection, and be deep enough and large enough to hold your nesting blocks or tubes,   The sides and back should be solid, leaving only the front open.  Mount your house 3 ft to 6 ft above ground, facing east or southeast to balance maximum morning sun exposure with some protection from intense afternoon sun later in the spring.  Don’t mount near bird feeders – many birds find mason bees to be a tasty snack.  Don’t move your house or re-arrange tubes during the nest building season.

Mud:  You’d think that spring in western Oregon would provide all the mud any bee could want; but many gardening practices actively try to eliminate open mud patches through cover crops and mulches.  To ensure a good mud supply near a nesting area, dig a shallow hole into native soil and mound the excavate around the rim.  If, by some weather miracle, the mud is drying out too fast, add small amounts of water to maintain a muddy consistency until mason bee nesting season is over in mid to late May.

Managing mason bees:  Mason bees stop laying sometime in mid to late May.  Pull nesting materials out of houses and store them (inside plastic boxes with breathing holes if mice are a concern, wrapped in cheesecloth or a couple layers of mosquito netting to protect from wasps if not) in your tool shed, wood shed, or garage for the rest of the summer.  In October, open the nesting materials and harvest the cocoons.  Inspect for parasitic wasps, pollen mites, and chalkbrood disease and clean.  Clean your cocoons by stirring them gently around in a 0.5% bleach solution (1/4tsp bleach in 1 cup water, or 1 TBLS bleach in a gallon of water) for a minute or two, then rinse them in clean water and let them dry.  When dry to the touch, store them refrigerated until time to release again next spring.

For our native plants, and for our early blooming crops, help provide for and increase our native mason bees.  It’s easy, and can be rewarding on many levels.