Mason Bee Problems and Possible Solutions

By Judith Paul | April 15, 2019
Osmia lignaria
Figure 1. Osmia lignaria, mason bee or blue orchard bee


This paper was written by Judith Paul for the Oregon State University course Entomology 331: Pollinators in Peril in the winter of 2017.

In the Pacific Northwest, a busy solitary bee unknown to many people is quietly pollinating orchards and early-blooming plants. Metallic blue-green and about the size of a honey bee, it might be mistaken for a housefly. It is Osmia lignaria (Figure 1), commonly known as orchard mason bee or blue orchard bee. These bees are native to North America. There are two subspecies—Osmia lignaria propinqua (western) and Osmia lignaria lignaria (eastern).

While there are approximately 500 species of Osmia worldwide, most are found in North America, Europe, and the Middle East (Wilson and Carril 2016). Multiple sources indicate there are approximately 130 to 150 species In the United States and Canada alone.

This paper will explore Osmia l. propinqua and whether or not it can benefit other pollinators in providing the pollination so necessary to the world’s food supply and whether or not the potential threats to this wild native bee can be addressed through management and, if so, how.

Osmia l. propinqua are found west of the Rockies in North America. These mason bees are generalists and will forage only within 300 feet of their nest sites, collecting pollen from a variety of flowers, although it is thought that they prefer nectar and pollen from fruit trees like cherry, apple, and pear. Native pollinators are naturally attracted to native plants. Because activity for an adult Osmia l. propinqua begins in early spring, it is necessary for early-blooming plants to be available for forage and pollen collecting. In the Pacific Northwest, the Bigleaf Maple tree (Acer macrophyllum) and Indian Plum (Oemleria cerasiformis) are great examples of available plants that are mason bee-ready in early spring.
Mason bees are solitary bees and may live in close proximity with other mason bees. Nests used by mason bees can be existing cavities, such as crevices, holes bored in wood or trees by beetles, hollow stems of perennial plants, or manmade nests set in a block of wood.

Adult male bees are the first to emerge from cocoons in early spring when temperatures are consistently in the 50-55 degree-F range. The males only live for about two weeks and are preoccupied with reproduction. They also pollinate, but their foraging is solely to provide them with nourishment. As the females emerge, the males are ready to mate with them. The females then look for nectar, pollen, mud sources, and nesting sites. If these are available, she will begin gathering pollen on the hairs of her abdomen to take back to the nest site. She mixes the pollen with nectar to form a small ball on which she will lay her egg. (Figure 2.) It can take up to twenty trips for the female mason bee to bring back sufficient amounts of pollen and nectar to make a ball large enough to provide the food source for the larva once the eggs hatch. The female mason bee’s lifespan is around four to six weeks.

mason bee egg cells
Figure 2. Mason bee egg cells

The female mason bee will compartmentalize each egg she lays by walling it off with mud, making a linear series of cells. The first eggs she lays are fertilized eggs and will become females. The last eggs are unfertilized eggs and will become males. She will lay approximately one to two eggs per day.

The Issue/Threat/Problem

Pollinators are necessary for the production of the food we eat and for maintaining natural habitats that are essential to wildlife and pollinators. Pollination helps to create a diverse plant population. While some pollination takes place by wind, many of the vitamin-rich food crops rely on animal and insect pollinators.

As a group, mason bees are not in danger of becoming extinct. The conservation status of O. lignaria is ranked “secure” by the NatureServe Conservation Ranking System. However, there is an overall increase in concern in the decline of native bees and other pollinator populations (Young et al 2015). Habitat loss, climate change, presence of pests and pathogens, use of pesticides and herbicides, and other factors contributing to the decline of pollinators in general are potentially troublesome to mason bees.
Pesticides can wipe out mason bees. Because each individual female mason bee works alone to provision her nesting cavity and lay eggs, there will not be any progeny coming from her if she is overcome by pesticides. Furthermore, mason bees can be affected by herbicide use. There are two instances reported in Mason Bee Revolution: How the Hardest Working Bee Can Save the World One Backyard at a Time where the numbers of mason bees that were being kept in backyards declined dramatically when fertilizers and an herbicide were used on the homeowners’ lawns. No insecticides were used and the mason bee nest boxes were not close to the areas treated. Even though there were apple and pear trees present for pollen and nectar, and the trees were not treated, the mason bees left. No dead bees were found. A theory proposed that the bees did not like the odor of the sprays and moved to more pleasant air (Hunter and Lightner 2016).

An additional threat to mason bees is a lack of a food source when the bees are active in early spring.

Because these threats to pollinators have a potential to affect the world’s food supply, it is crucial that we work to alleviate or eliminate them. Mason bees’ usefulness, industriousness, and efficiency at pollinating are not sufficiently known. They are less work than maintaining honey bee hives, do not require food or water in the winter months, and any pests or pathogens can be treated without pesticides by the monitoring and cleaning of their cocoons and nest tubes.

Could Osmia l. propinqua be a benefit to other pollinators such as honey bees? A study published by Pensoft Publishers and printed in the January 14, 2013, issue of ScienceDaily, found that the presence of wild bees altered honey bee behavior in almond orchards, increasing the effectiveness of pollination, and the orchards had greater fruit set than those without wild bees. In extreme weather conditions, such as high winds, honey bees were not active; however, the wild bees were actively pollinating the flowers.

The Xerces Society Guide to Attracting Native Pollinators states that native bees can increase the effectiveness of honey bees in pollination by causing them to fly more frequently between rows of male and female plants, rather than staying methodically on one tree and thus avoiding cross-pollination (Mader et al 2011).

Would managing wild native mason bees such as Osmia l. propinqua be a benefit in orchards?

After it had been determined that O. l. propinqua was active in collecting nectar and pollen from an apple-prune orchard and had successfully nested in man-made materials, a research program was established at the Bee Biology & Systematics Laboratory at Utah State University. A field experiment was carried out to determine whether or not O. l. propinqua could be managed as a pollinator of almond orchards. The bees, which had been field-trapped, were raised in the laboratory and then transported to an almond orchard in California. Nesting materials were also set up in the orchard. The almond orchard was selected because it had both early- and late-blooming almond varieties. The experiment proved successful, indicating a high potential that O. l. propinqua would be an effective pollinator of almonds (Torchio 1980).

If O. l. propinqua has potential to be an effective pollinator of almond orchards, how would it fare in backyard, urban school, and community gardens? Could the threats to mason bees be addressed by managing them? If they are managed, will they be subject to pesticide use in adjoining areas? Will there be more susceptibility to pests and pathogens? Could managing mason bees, rather than leaving them in their wild state, actually initiate or exacerbate threats? All of these questions require further study and research, collecting data from these various situations.


The potential threats to O. l. propinqua discussed above can, and need, to be addressed through management and education. Without careful management of the mason bees, they will be susceptible to pests and pathogens (pollen mites, invasion of parasitic wasps, chalkbrood disease, etc.). They will also be likely to spread parasites to wild populations. Education on the requirements of keeping healthy, productive mason bees will do much to ensure the viability of O. l. propinqua.

Because O. l. propinqua is a gentle solitary bee, it is easy to manage. O. l. propinqua is in an egg, larva, and pupa stage for about ten months and active as an adult for about four to six weeks. If they have nested in managed, artificial nesting tubes, O. l. propinqua can be moved around to pollinate targeted crops such as fruit trees. The materials they need in order to survive and reproduce can be readily supplied for them.

It has also been found that carefully maintained and managed O. l. propinqua and other mason bees will increase their populations each year. Wild (unmanaged) mason bees are susceptible to pests and pathogens, as they may be likely to nest in previously used nest sites which have not been cleaned, thereby reducing their populations. They may also have nested in a fruit or nut orchard that is subject to spraying of pesticides and herbicides, whereas the use of pesticides can be controlled when managed mason bees are brought in to pollinate the orchard. If changes in weather patterns occur, managed mason bees can be moved to areas where there are blooming plants, whereas unmanaged mason bees may not find the forage they need to survive.

While unmanaged native mason bees are apparently doing fine, they are subject to the same threats that are affecting other pollinators. By managing them, O. l. propinqua can be kept healthy, increase its population, and be an effective pollinator to maintain the biodiversity of natural ecosystems and our food supply.

I look at the management of mason bees as a grassroots effort, without the involvement of government agencies. Education is imperative. For O. l. propinqua, it is important to protect nesting habitat. If artificial nest houses are used, they must be appropriately placed to protect them from the weather and predators, and a mud source must be available. Early-flowering plants and diverse plants that successively flower from spring through June and that are within 300 feet of nesting sites are critical for supplying pollen and nectar for these mason bees. The avoidance of systemic and other pesticides is also critical. And acquiring bees from local sources and not introducing bees to areas where they are not native is crucial to preventing the spread of pathogens.

Without responsible monitoring and maintenance of O. l. propinqua, management of them could prove detrimental to their health, promote the decline of their populations, and introduce pests and pathogens into unmanaged or wild native bees and pollinators.

In order to alleviate or remove threats to mason bees, I would propose some type of database collection or organizational effort be made. A community science project would be a way to start collecting data based on monitoring of mason bees at the grassroots level. Information that would be important to share would include the reporting of the presence of unmanaged mason bees; the success rate of managing and maintaining mason bees; number of cocoons harvested from managed mason bee nests and the number of cocoons harvested from wild (unmanaged) mason bee nests; types of pests or pathogens found in nesting tubes; effectiveness of mason bees in pollinating orchards; and reminders on when to carry out the necessary storage and maintenance procedures during the year.

This information could also be shared through a listserv, an electronic mailing system that anyone interested in mason bees could subscribe to in order to learn what others have observed. The listserv could also be used by those keeping lesser-known managed bees to share data and observations that could be helpful to mason bees.

I’d also be interested to find out where unmanaged mason bees are nesting and foraging. This could be part of the community science project, having gardeners, orchardists, and farmers set out nesting tubes in early spring and checking these tubes for evidence of mason bee use in early summer.

Just as farmers benefit their crops by providing pollinator habitat in the form of hedgerows, gardens can benefit by becoming pollinator habitats that in turn mitigate the loss of habitat and biodiversity that comes through urbanization.


There are many stakeholders–from the land owners, orchardists, and farmers who need crops pollinated; to families and homeowners who want to enjoy and protect these mason bees; to all who want to eat fruit, almonds, or enjoy native plants; and to the mason bees themselves. There are also businesses that are selling and renting mason bee nest boxes and cocoons who are stakeholders.
One such business is Crown Bees, which has organic farm pollination programs designed for CSA farms, encourages backyard gardeners to keep mason bees and help provide them to pollinate commercial crops and orchards, and actively educates the public on the benefits of these native bees. Besides providing information about mason bees, they sell mason bee cocoons and make sure they are the right bee for the area. They have a stake in the bees they are selling. They want the bees to do what they are meant to do, and they want people who purchase them to have successful pollination. They are a stakeholder and a responsible party for addressing the problems that may arise from homeowners and backyard gardeners who keep mason bees.

Other stakeholders in the business of promoting and supplying mason bees include and Watts Solitary Bees.

Another stakeholder, Benton Soil and Water Conservation District (BSWCD) is initiating a Bee Buddy program where homeowners demonstrating that they have a safe environment for the mason bees will be provided with cocoons and a mason bee nesting box in March. They will be able to watch the bees emerge and pollinate their fruit trees and native plants. In June, BSWCD will collect the nest box from the homeowners and store the cocoons through the summer, fall, and winter. In October, BSWCD will hold a cocoon harvesting class to train the Bee Buddy participants and other interested parties on how to properly clean the cocoons and nest boxes. It will be interesting to discover if crop yields have notably increased through this program.

Similar to BSWCD, Rent Mason Bees is a small business focused on providing gardeners and fruit growers with mason bees and leafcutter bees native to the Pacific Northwest. They provide this service because they care about the environment and future food production. They only service customers in Washington and Oregon, and customers must pick up and return the bee kits to Rent Mason Bees, which does the cleaning and storing of the cocoons. To ensure the continuation and safety of the bees, no shipping of them is allowed.

Linn County Master Gardeners is a stakeholder that does its share of educating the public about pollinators. They sell mason bee supplies, hold an annual BeeVent Pollinator Conference, and have a listserv called Bee Notes that provides helpful information and tips about pollinators.

More information is becoming available on mason bees and how to manage them. In the Willamette Valley area in Oregon, mason bee talks and workshops have become popular and are often combined with those for honey bees. While not as prevalent as for honey bees, there are associations and organizations that provide information, resources, events, and support for encouraging mason bees as pollinators in commercial orchards and backyard gardens. Some of these include: The Orchard Bee Association, located in California, and the Home Orchard Society, located in Oregon.

Global Context

Because O. l. propinqua is native to the western region of the U.S., the bee itself is not in need of international cooperation or aid. But, on a global scale, the importance of mason bees as pollinators in orchards, both fruit and nut, is highly important, both economically and nutritionally.

The following export information of various crops indicates that there would be a great loss if honey bees continue to decline and mason bees disappeared. These crop industries are also stakeholders in the mason bee pollinator network.

  • According to the Almond Board of California, California produces about 80% of the world’s almonds and 100% of the U.S. commercial supply. Almonds are California’s #3 agricultural crop valued at $6.4 billion in 2013. Almonds are the U.S.’s #1 specialty crop export, with 67% of the crop exported to more than 90 countries worldwide. (I found it interesting that nowhere on the California Almond Industry fact sheet was mention made of the efforts and contribution that honey bees and mason bees make to the almond crops.)
  • And according to an article on USDA Economic Research Service in March 2016, by 2015 the U. S. was the world’s third-largest apple exporter.
  • Per Agricultural Marketing Resource Center, in 2014, United States cherry exports, sweet and tart varieties combined, were valued at $536.84 million.
  • The United States is one of the largest exporters of cherries in the world, and the second largest producer after Turkey.

Those who depend on the honey bee to pollinate their crops or orchards, either by renting honey bee hives or managing their own hives, should look at diversifying their pollination practices. The mason bee is a perfect complement to the honey bee!

Works Cited

  • Bosch, J. and W. Kemp. 2001. How to Manage the Blue Orchard Bee. Sustainable Agriculture Network, Beltsville, MD.
  • Edmunds, Brooke, Richard Little, and Ramesh Sagili. 2016. Nurturing Mason Bees in Your Backyard in Western Oregon. []
  • Hunter, Dave and Jill Lightner. 2016. Mason Bee Revolution: How the Hardest Working Bee Can Save the World One Backyard at a Time.
  • Mader, Eric, Matthew Shepherd, Mace Vaughan, Scott Hoffman Black, and Gretchen LeBuhn. 2011. The Xerces Society Guide to Attracting Native Pollinators: Protecting North America’s Bees and Butterflies
  • Pensoft Publishers. 2013. Two new studies show why biodiversity is important for pollination services in California almond. ScienceDaily.
  • Torchio, P. F. Field Experiments with Osmia lignaria propinqua Cresson as a Pollinator in Almond Orchards: I, 1975 Studies (Hymenoptera: Megachilidae). Journal of the Kansas Entomological Society, Vol. 54, No. 4 (Oct., 1981), pp. 815-823.
  • Wilson, Joseph S. and Olivia Messinger Carril. 2016. The Bees in Your Backyard: A Guide to North America’s Bees.
  • Young, B. E., D. F. Schweitzer, N. A. Sears, and M. F. Ormes. 2015. Conservation and Management of North American Mason Bees. 21 pp. NatureServe, Arlington, Virginia
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