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Majestic mason bees!

Updated: Apr 13, 2020

"Cocoon" by Jasna Guy, artist and board member of Native Bee Society of BC (select full screen mode for optimal viewing).

"One of the most fascinating aspects of exploring the life cycle of bees is learning about how certain bees pupate. Mason bee larvae spin lovely cocoons around themselves as they prepare for the final part of their development. A cocoon is essentially a little room in which the larva undergoes its magnificent and mysterious transformation from larva into adult fully-formed bee.

In this time of COVID-19, so many of us are cocooning too, as our worlds have shrunk by necessity into the isolation of our homes. Like the mason bee, I hope that we will all soon emerge into a healthy and productive life." - Jasna Guy


A male Osmia sp. visiting Ribes sp. flower on a beautiful Spring day. Photo by Sarah Johnson.

The beauty of Spring has been a wonderful reprieve from all the stress and anxiety many of us are experiencing during these uncertain times of COVID-19. Here in British Columbia, provincial health authorities have communicated that it is still safe to enjoy time in nature if we’re able to maintain social distancing protocols. Thankfully, observing bees is an entertaining and low-risk activity to enjoy in the great outdoors. Though there are many kinds of native bees already out and about, one group of native bees that receive a lot of buzz around this time of year are... mason bees! Due to their gentle temperament, attraction to human-made homes and commercial distribution, Osmia are very engaging and charismatic microfauna. Through this article, we will dive into the world of these fascinating wild pollinators- who they are, why they’re special, and how we can support them in our day to day lives.

What are mason bees?

Osmia, commonly referred to as mason bees, is a genus of solitary bees in the family Megachilidae. There are close to 500 species of Osmia have been identified, primarily in the Northern Hemisphere (1), with about 75 species discovered to date in British Columbia (2).

Similar to many other bees in the family Megachilidae, Osmia are ‘cavity nesters’. Cavity-nesting bees seek out empty cavities in nature-hollow stems, crevices between rocks, fallen logs, empty beetle burrows- rather than excavating their own.

The common name “mason bee” comes from a shared behaviour of most female Osmia spp. While constructing their nest, females will gather ball of mud and carry them back to their nests, using them to create partitions (walls between cells) for each individual egg laid in her selected cavity. The term Osmia comes from the Greek word, osme, meaning “odour” derived from the habit of these bees of marking the entrance of their homes with a distinctive scent that helps them locate them.

These beautiful bees are distinctively metallic - often blue, green or blackish in colour. Like many other bees in the family Megachilidae, Osmia females carry pollen on dense scopa located on the underside of their abdomen. Foraging females can often be observed in flight with a dusting of white or yellow on their “bellies”. Another characteristic shared by Osmia and fellow Megachilids is a rounder and wider ("bullet-shaped") abdomen relative to other native bees.

Click the arrows to slide through this photo gallery.

In order of appearance: (1) A metallic blue Osmia sp. visiting Rubus ursinus, photo by Jasna Guy; (2) A metallic green Osmia sp. visiting Phacelia hastata, photo by Lincoln Best; (3) A metallic black Osmia sp. visitng Ribes sp., photo by Sarah Johnson; (4) An Osmia sp. female visiting Phacelia hastata with her pollen-packed abdominal scopa on display, photo by Jasna Guy.

Why should we care about them?

Osmia are voracious spring pollinators, many of whom are specialists. The term specialist refers to bees that have co-evolved with a plant to rely on the nutritional constituents of a particular species, genus or family of plants. These co-evolved specialist relationships are often critical to the survival of both parties (plant and pollinator) involved.

Generally, many Osmia have a foraging preference for flowers in the Rose family (Rosaceae). In agricultural settings, this includes many of our common orchard crops including apples, almonds, cherries and plums. In natural habitats, many Osmia have specialist relationships with wild fruiting shrubs. Osmia laticeps, for example, is a huckleberry specialist found in northern, and cool montane ecosystems in British Columbia (2).

Mason bees are also highly efficient pollinators. In studies comparing floral visitation rates of different bee species in cultivated apples (3) and wild raspberries (4), flower visitation rates of Osmia were close to double that of the European honey bee (Apis mellifera).

How can we support mason bees?

Habitat loss is one of the greatest threats to our wild native bees. So first and foremost to conserve pollinators, we need to invest time, resources and people power into conserving remaining wild pollinator habitats.

Those who manage land or have backyards have the opportunity to restore habitat for native bees. To welcome native pollinators to these areas, we need to think about two things: flowers and nesting habitat.

If you build it, they will come. Creating habitat with flowering resources that bloom throughout the spring and summer season will attract a diversity of bees and other wild pollinators to backyards and managed habitats. Osmia are some of the first bees to emerge in late winter. Depending on annual temperatures, this can be as early as late February in southern British Columbia. Thus, to support mason bees and other emerging pollinators it’s important to include early blooming flowering plants for foraging resources. Native species are typically the best for supporting our more specialized native bees, so it’s important to do a little research into what spring blooming plants naturally occur in your area. Some early-blooming native species that can easily be sourced from nurserys in the Pacific Northwest include willows (Salix spp.), currants (Ribes sp.), Oregon grape (Mahonia aquifolium), salmonberry (Rubus spectabilis), sea blush (Plectritis congesta), camas (Camassia spp.) and elderberry (Sambucus spp.).

A low maintenance approach to provide nesting habitat for Osmia and other cavity nesting bees in managed habitats is to include perennial shrubs that give rise to hollow stems. Canes of blackberries, raspberries and their relatives (Rubus spp.) work great for this.

Since Osmia nest in found, empty cavities, it’s also relatively simple to design human-made homes for them by drilling holes into wood, or using natural or recycled materials. Witnessing a mason bee emerge from a cocoon, as she feels the warmth of the sun, and stretches out her new legs and wings for the first time is a breathtaking experience. Depending on the availability of flowering resources in your area, it's very possible to have a high level of nesting success in houses placed in backyards and gardens.

A newly emerged male Osmia lignaria stretches out his legs and antennae for the first time, Video by Marika van Reeuwyk.

Native Bee Society of BC member Pat Homes, who lives in North Vancouver has been gradually building her mason bees bee operation for 20 years. In 2019, she reared over 800 cocoons in her backyard in North Vancouver, and over 1100 cocoons at her operation on Hornby Island.

The Environmental Youth Alliance (EYA) in Vancouver constructed a new cavity-nesting bee hotel this past Fall with youth from the Urban Native Youth Association. They introduced over 300 mason bee cocoons (some donated by Pat Holmes!) to the hotel this Spring, and will be giving out new cocoons from this season out next year.

Mason bees have a wide diversity of nest predators. Kleptoparasites are pollen “thieves” who consume the pollen left by a female mason bee for her offspring. Examples include pollen mites (Chaetodactylus krombeini), carpet beetles (Anthrenus verbasci), and the newly-introduced houdini fly (Cacoxenus indagator). Parasites of mason bees enter cells or cocoons of mason bees and consume young bees during development. Examples include parasitic wasps (Monodontomerus spp, Sapyga spp.) and chalkbrood fungus (Ascosphaera spp.). In the absence of effective management, human-made homes with a large aggregation of cavities can build up populations of these pests quite quickly, even within a single season. Cleaning and/or replacing materials in cavity nesting bee homes on an annual basis will help to mitigate this problem. Depending on the set up, cleaning a bee hotel is a relatively simple process. By taking this small action each year we can ensure our bee homes are having a positive effect on local native bee populations.

Click the arrows to slide through this photo gallery.

In order of appearance: (1) The new EYA cavity-nesting bee hotel, photo by Marika van Reeuwyk; (2) A newly hatched Osmia lignaria male reared by Pat Holmes and released by EYA Strathcona Community Gardens, photo by Marika van Reeuwyk; (3) Pat Holmes backyard mason bee operation, photo by Pat Holmes; (4) Inside an Osmia cavity nesting tray, photo by Lincoln Best; (5) A newly emerged Osmia lignaria male covered in pollen mites, photo by Marika van Reeuwyk.

Whether it’s by heading out to a local green space to watch action on spring flowers or by putting out cocoons in a bee condo in your backyard, we encourage you to brighten your day by observing mason bees up close this Spring…. If you find one, why don’t you snap a photo and submit it to the NBSBC Native Bee Photo Contest?

Questions or comments? Please get in touch with us at or on Facebook, Instagram or Twitter.

A friendly Osmia lignaria male poses for a selfie on a finger. Photo by Marika van Reeuwyk.


(1) Wilson, Joseph, and Olivia Messinger-Carril. The Bees in Your Backyard: a Guide to North America's Bees. Princeton University Press, 2016.

(2) Best, Lincoln (2020, April 5). Email Interview.

(3) Vicens, N. and Jordi Bosch, B. (2000). Pollinating Efficacy of Osmia cornuta and Apis mellifera (Hymenoptera: Megachilidae, Apidae) on ‘Red Delicious’ Apple. Environmental Entomology. 29(2), 235-240. (4) Yokoi, T., and Kandori, I. (2016). Comparison of Foraging Activity between mason bee Osmia orientalis (Hymenoptera: Megachilidae) and honybees for wild easpberry Rubus hirsutus (Rosales: Rosaceae). Entomological News. 125(5), 363-367.

Article written by Marika van Reeuwyk

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