Native Mason Bees for Early Crops – Part II

In part one, we covered what a mason bee is and why they are important.  Now let’s dive into what mason bees need (all within an area of only 100 to 200 feet) and how to encourage and manage your own populations.

Reproduction

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 nectar and pollen inside the nesting cavity, lay an egg beside it, then seal off that egg chamber 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. The female mason has control over the sex of the egg she lays and she intelligently lays the female eggs 3 inches or deeper from the entrance of the nesting cavity, laying the male eggs within the outer 3 inches of the hole – thus nesting tubes should be about 6 inches deep.

Food

For mason bees to stick around and reproduce, they must have flowers that provide pollen for larvae (and nectar to feed the adults!) and bloom during the March to May adult life cycle. 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. Having plants that bloom every season is helpful to all pollinators and native plants are the best choice for native pollinators.

Nests

You must provide available nesting cavities, be it cardboard tubes or hollow sticks stacked in protective “houses”, for the bees to lay eggs in.  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 (you can go higher), 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. Mason bees need ready access to mud for wall and plug building, and moist clay is their soil of choice. They require mud for the construction of their nests so you must ensure there is a good mud supply near a nesting area.

You can dig a shallow hole into native soil and mound around the rim, or set soil that is high in clay in a container near the nesting house. 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. If clay is not available, they will move on to a different site where clay can be readily found.

Managing mason bees

Mason bees stop laying sometime in mid to late May. When June 1st hits, pull nesting materials out of houses and store them inside plastic boxes with breathing holes if mice are a concern, inside a folded-over and staple-sealed paper bag, wrapped in cheesecloth or a couple layers of mosquito netting to protect from wasps. Whatever container you choose to keep the nesting tubes in, place the package 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 (1/4 tsp. bleach in 1 cup water, or 1 tbsp. 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 at temperatures at or around 39°F and 60-70% humidity (a household refrigerator generally does the trick) until time to release again next spring. In the ventilated container the cocoons are being stored in, maintain a moist paper towel (moist, but not dripping wet) within the container to prevent the cocoons from drying out.

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.