Pollinator Profile: Mason Bees

There are many species of mason bees, 120 in North America alone. Mason bees do not produce honey, instead females gather pollen for their larvae. Mason bees are small and easily confused with flies, however they have longer antennae, smaller eyes and four wings. Females are larger than the males, about 3/4″ and 1/2″ in length respectively, and the females have shorter antennae. They are incredibly gentle bees and although the females can sting, they very rarely do and do not even act aggressive at their nests. A Blue Orchard Bee’s sting has very low venom and produces the equivalent of a mosquito bite.

They have a very short active life, only foraging for 6-8 weeks in the spring, usually sometime during March, April or May. Males emerge first and do a little pollinating before waiting for the females to emerge. Once the males mate they die shortly thereafter. The females emerge two weeks after the males, mate and start to build their nests in holes. Mason bees do not make their own holes, in a natural setting they’ll use snags or old trees and will also make use of human structures such as shake siding and even outdoor furniture.

They lay eggs in their nests, upwards of 9-11 in each hole, leaving pollen with the eggs and then use mud to seal the egg and pollen in an individual chamber. The eggs hatch into larvae and feed off of the pollen before creating cocoons which the adults will stay in as they overwinter.

In the Pacific Northwest the Blue Orchard Bee (Osmia lignaria), has certain benefits over the honeybee such as foraging in the cold weather or light rain. They also stay close so it’s easier to target certain plants such as fruit or nuts trees. Blue Orchard Bees do not compete with the honeybee and they can even work alongside each other. In addition, the Blue Orchard Bee is not choosy about their pollen and will visit a wide range of flowers. They are messy pollen gatherers unlike the honeybee who packs wet pollen neatly onto their leg. The Blue Orchard Bee gathers the pollen, but being dry, much of it falls off and as a result, they pollinate nearly every flower they visit. One Blue Orchard Bee can pollinate 2,000 blossoms in a single day, the equivalent of what 100 honeybees can do.

Like most other pollinators, landscaping can help attract mason bees. Native spring flowers are perfect for the Blue Orchard Bee because they emerge in the spring and don’t have a strong pollen preference. Composite flowers with open petals are easier for the mason bees to harvest pollen from, and deep flowers, such as lilac are more difficult. Because they don’t forage too far from their nest sites, the flowers need to be within 300′ of a nest structure.

They also require a source of mud which they use to divide the chambers of their nest, and the mud needs to consist of more clay than sand. While this isn’t usually a problem in western Washington, if there is no close mud source, one can be created.

The third essential is to provide nest shelters. The basics of a mason bee nesting hole is a 5/16″ hole at 6″ deep. There are many materials for making holes, paper tubes with a paper insert are perhaps the best solution, but reeds also work well. Plastic straws are not good because the dampness of the pollen in the nest compartments is trapped, and can end up killing cocoons where paper pulls the moisture out. Wood blocks are not ideal either because they can be difficult to clean and inspect, although using the paper inserts in wood blocks is an option. Mason bees require a clean hole and if a block of wood is reused it can contain a build-up of pests.

The paper straws or reeds need to be protected in some type of shelter that will keep them dry. Anything from manufactured structures to a plastic soda container can work for this. The shelter should be placed where it gets morning sun to help the bees warm up in the morning. It should also be placed so that the open front is out of the direct rain and wind. Also avoid putting it too near bird feeders or over water. Mason bees identify their hole with scent, but they have to get close to smell it. Stacking the straws so that some stick out further than others helps the bees to identify their own hole.

For more information about attracting and raising mason bees visit The Metropolitan Field Guide.