Mason Bees

MASON BEES AS AN ALTERNATIVE - by Bill Anderson - revised 4/10/98


PART I: Decline of the wild honey bees
Since 1994 wild honey bee populations have been devastated by two species of parasitic mites. Many domesticated honey bee colonies have also been lost.

This is a nationwide problem. Recent estimates are that 90% of wild honey bee colonies have been lost. In previous years wild honey bees have done most of the pollinating of our fruit trees and gardens. It has also been estimated that 30% of our food supply depends on pollination of a wide variety of plants by bees. Douglas County, Oregon was considered to be an under pollinated area even before the honey bee/mite problems arose.

PART II. Mason Bees:
The Orchard Mason Bee, or Blue Orchard Bee (Osmia lignaria), usually referred to as Mason Bees or Osmia, is native to Douglas County. It is a very good pollinator as it collects much more pollen than nectar. It is smaller than a honey bee, black in color with a dark blue irridescent sheen on it's abdomen (or rear end). A closely related bee (Osmia ribifloris) has an emerald green sheen on it's abdomen. Mason bees are very gentle and will sting only if roughly handled.

They are a solitary bee (no hive or queen) and usually nest in holes abandoned by a variety of wood borers. In the deepest part of their hole they form an egg chamber, collect a loaf of pollen and nectar, lay an egg on the pollen, and seal up the chamber with mud. They repeat this process until the hole is full of egg chambers. These hard working little bees only live for six to eight weeks in the spring, usually from the middle of March to the middle of May. This timing is perfect for fruit trees and berries.

The eggs hatch within a week and the larvae eat the pollen in 4 to 6 weeks, then transform into pupa. In another 2 to 4 weeks, or by mid-July, the pupa changes into the adult stage and remains in the cell until the following spring. The inner cells contain females and the outer cells contain males. Obviously, they emerge in sequence.

During their 4 to 6 week life, the female may occupy 4 to 6 tunnels and may lay up to 36 eggs. One of the limiting factors of Mason bee population is the lack of suitable nest holes with a mud supply nearby. With a minimal amount of effort, we can provide nest blocks that are a proven method of increasing bee populations and assure better pollination of our fruit trees and gardens.

PART III: Building nesting blocks. A mason Bee nest block can be made by drilling a series of 9/32 or 5/16 holes into almost any piece of sound dry wood that is free of cracks, providing it is not treated wood. The holes can be a much as 10 inches deep, but must not go all the way through the wood. A typical nest block could be made out of a 12" long piece of 4" x 4". Since 4" x 4" is actually 3.5" x 3.5", the holes should be only 3" deep. The holes can be all on one face, spaced 3/4 inch apart on center minimum. Our layout would be 4 holes across and 12 holes vertically. This would give a total of 48 holes. An alternate hole arrangement would be to drill row 1 on the left side of the block, row 2 on the front, and row 3 on the right side. And so on down the block. This will spread the bees out a little.

Cleanly drilled holes are desirable. A brad-point bit will give somewhat better results than a steel bit. The blocks will also need a small eye screw or metal strap to hang the up with.

PART IV: Placing nesting blocks.
A nest block should be preferably hung on the south side of a building (with east and west sides also being acceptable), high enough so the eaves will protect if from the weather. A block could also be placed on a covered porch. If a dry, protected location isn't available, a small roof could be fastened to the nest block itself. The nest block needs to be fastened in place so it won't be jared or jolted, or knocked over, as this could dislodge the larval bees from their pollen food supply, thus killing them

Once you have put up your empty nest block, the Mason Bees will find it and begin to make their nests in the holes. A nest block in a well-protected location can be left in place all winter. If it's in a less protect area, it can be moved indoors and stored in an unheated building in the fall (by then the larval bees will be in the cocoon stage and wont be harmed by movement.) The nest block would need to be put out the following year about the first of March.

Because you bee population is expanding, you will need to put out additional blocks next spring. The female bee does all the nest building. They identify their hole by marking it with their scent. When they finish filling a nest hole, unless there is another unmarked empty hole nearby, they will fly away. We can further assist them by planting early blooming plants to assure a continuous pollen and nectar supply, and by providing an artifical mud supply if needed.

Another way we can benefit all species of bees is to consider bee welfare when spraying pesticides. Try to follow the general guidelines of not spraying plants when they are blooming, don't spray when the wind is blowing, and mow flowering weeds before spraying them. Try to spray in the evening when the bees have returned to their nest.

Large numbers of Mason Bees are being used in commercial agriculture in Douglas County and elsewhere. The need for more efficient methods, and concerns about buildup of molds and disease organisms in wooden nest blocks after several years use have led to the development of more sophisticated materials and products for Mason Bee nests. Six inch long reusable cardboard or fiber glass tubes lined with replaceable paper straws are available from commercial supplies. For those who don't wish to wait for their Mason bee population to build up gradually, bees can also be purchased.


Most of the material on this informatin sheet was condensed from Washington State University Extension Bulletin 0922 and information from Oregon State Univeristy, fact sheet #10. These fact sheets can be obtained from you local Extension Service office. For more information or to make comments, call the Douglas County Office of OSU Extension Service, phone number 541-672-4461 or 1-800-883-7568.

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