Orchard Mason Bees

With the decline of wild bee populations, the orchard mason bee (Osmia lignaria) is becoming increasingly popular in the home garden. The orchard mason bee, native to our region, is a gentle, beneficial pollinator of native and cultivated tree fruit, related plants in the Rosaceae (rose) family, some berries, and a few other crops. They are only active for about two months in mid-spring, so will not pollinate mid-summer vegetables. They do not produce honey, but rather the female gathers pollen/nectar for its young and places it in nesting holes prior to laying eggs.

They do not nest in hives, but use existing holes in wood such as insect holes found in trees or wood, woodpecker drillings, etc. They may also nest in hollow stems. Some homeowners may become concerned if they see these bees in the spring entering gaps under siding, or investigating nail holes or other openings in building structures. They do not make holes nor are they destructive insects, so no controls are recommended; however, the holes can be filled with caulking to prevent nesting in siding.

In appearance, the orchard mason bee is slightly smaller than a honey bee, shiny, and dark blue, almost black, in color. Males are smaller than females, have longer antennae and have an additional tuft of light-colored hairs on the face. Females have hairs on the underside of the abdomen for carrying pollen.

Cultivating Orchard Mason Bees

A complete kit for cultivating orchard mason bees can be purchased from local nurseries, or one can build his/her own nesting block.. Regardless of method, placement of the nesting block is the most important consideration in successful mason bee culture. The bees require a warm, dry, wind-protected place for their nests. The best place is usually on the side of a house, shed, or other large structure,at least 3' above the ground, ideally facing east or south to catch the morning sun, and under an eve to keep off rain. A second story deck or balcony works well. They will avoid nesting blocks placed out in the open.

The bees divide their nest cavity into cells and cap the finished nest with mud collected near the nest. In western Washington, mud is usually available, but in drier areas, provide mud by creating a small trench or tub lined with plastic, and keeping it filled with mud near the nesting box. Mud should not be highly organic or sandy. Clay soils work well.

The nests can be made from 4" by 4" or 4" by 6" pine or fir untreated wood. posts. Holes are drilled in the post, ideally 5/16" in diameter, at least 3" deep on 3/4" centers. A hole smaller than 5/16" encourages higher production of male bees which reduces the reproductive potential of the population. The bees will utilize the posts for a year or two but eventually the holes will become contaminated and the bees will search for other nest sites. The easiest thing to do would be to just replace with new blocks.

The life cycle of orchard mason bees is only about 2 months. By mid-June nesting activity is usually finished. Once all nesting activity has stopped, the nesting block may be moved to a shelter such as a shed or unheated garage, gently moving the occupied block and keeping it upright. This will give the bees added protection from predators and parasites, yet will allow them exposure to the cold temperatures they need to break hibernation, usually in March and April. If desired, bee emergence can be delayed for a short period by refrigerating the block in the spring until flowers have appeared on plants to be pollinated. Bees will need three days to warm up following refrigeration before emerging.

By mid-June, the process is repeated.

- Sharon Schlittenhard