Red Mason Bees - Osmia bicornis

Updated: 26th February 2021

I love mason bees, and have spent much time observing the red mason bees (Osmia bicornis) in my garden.  I have captured quite a lot of video footage, including footage of the bees mating, and females emerging from the nest.  I share one such video below.

Mason bees are so-called, because they commonly use mud in the construction of their nests, however, it is actually the common substitute name used to describe bees belonging to the genus ‘Osmia’ which are part of the family ‘Megachilidae’.

Osmia means 'odor', and actually refers to a faint lemony scent used by these bees to mark their nest entrances.  It is believed that each individual Osmia produces a unique scent of its own! 

How do we know?
In an experiment using straw nest inserts inside a block of wood, it was found that once bees had begun building nest cells inside the straws, each individual bee correctly located her own straw even when it was moved from its original place!

Female red mason bee, Osmia bicornis resting on stoneFemale red mason bee, Osmia bicornis.

Approximately 500 species of Osmia have been identified around the world, with more than 130 species found in the US and Canada - some of these species are found in vibrant metallic blues and greens.

Mason bees are a solitary species and are non-aggressive. They are a friend to gardeners and farmers, and may be used alongside honey bees for pollinating crops.

Mating Behaviour And Life Cycle Of Red Mason Bees

Similar to the leafcutter bee life cycle, adults emerge in spring following a period of development and hibernation in a cell. 

Males appear first, waiting eagerly for sight of the females with which they will mate.

Female Red Mason bee peeping out of the entrance of a nest tunnelFemale Red Mason bee.

Below is a video taken of Red Mason Bee (Osmia bicornis - previously named Osmia rufa) activity around my own bee house. 

Males patrol and wait around the nest, watching, sometimes 'dive bombing' other males as they wait for the females to appear, whereupon they will pounce immediately and mate.  Note males have a pale "moustache" or tuft of hair on their faces, and they are smaller than the darker-faced females.

If you watch the video carefully, right at the very beginning you will notice to the top left, 2 males (note pale 'moustache') - one is inspecting the inside of one of the holes, checking for a female, whilst the other male can hardly get a look in!  Watch as the female emerges, and quick as a flash, the male has caught her!

Meanwhile to the top right, and on top of the bee house is an amorous pair, already mating for quite some time!  With some bee species, the mating process is completed very quickly, but not with this species.

As the camera moves to the hollow canes below you'll see a lovely female, with her sweet little face peeping out of her chosen cane!

mason bees mating on a leafPair of mason bees mating

After Mating....

After mating has taken place, the males will soon die. The female will seek a suitable location for her nest. She then constructs the nest, providing pollen and nectar in each cell for a single larva.  

It may take many trips for bees to provide enough pollen for just one egg, and therefore, visits to many flowers are necessary.

Eggs destined to become females are laid toward the back of the nest (as stated, they emerge after the males), whilst males are laid toward the front.

They will usually make between 4 and 6 cells, although sometimes as many as 10.  The larvae develop and pupate, emerging as adults the following Spring.  Read about the bee life cycle.

Four or five similar nests may be completed in a season.  There will be one generation per year.

Red Mason Bee Nests

Osmia are cavity nesting bees.  The females make their nests in existing cracks and crevices in walls, but they also favour hollow plant stems.  Some mason bee species make use of empty snail shells - read about the red-tailed mason bee, Osmia bicolor.

Mason bees may also take up residence in holes created by other wood-boring insects. They they are unlikely to cause any damage where their nests are made.

entrance to a hollow cane nest tunnel sealed with mud

Once a brood cell has been made, mud is often used (see image left) to make partitions between each individual cell and.

The female uses a pair of ‘horns’ on her face to pack mud to close the cell.  The mud they use may be gathered from the edge of a pond, or puddle from where they take small pellets to their nest.

Pollination And Mason Bees

Mason bees as a group are super efficient pollinators.  Increasingly, as with bumble bees, mason bees are being reared commercially, especially for fruit crop pollination.

Did you know?

It takes Osmia Lignaria - the Blue Orchard Mason bee, 15 to 35 trips and around 75 flower visits each trip (that's up to 1875 individual flower visits) to provide sufficient pollen for one egg! (1)

In the Americas, the 'Orchard Mason Bee', Osmia lignaria -  has been found to significantly improve cherry crop yield, even greater than honey bee colonies (2).

Mason Bee Houses

For several years now, we have had the pleasure of watching these wonderful little pollinators establish nests around our garden, providing excellent entertainment!

Below is a photograph taken of the bee house featured in the video.  I received this bee house as a gift.

A bug hotel made from a segment of tree trunk and filled with canes, and occupied by mason bees.  Some of the nest entrances in the canes have been sealed with mud by the bees.

You can also make your own bee house very easily!  To encourage them to set up a home, site bundles of hollow canes or a log drilled with holes (do not use chemicals on the log), in a sunny position.

  • Ensure the holes are around 1cm in diameter or a little less, and about 15cm long. Ideally, there will be a source of mud nearby, and plenty of opportunities for foraging. 
  • Please do not use plastic materials, glass or metal.  Such materials can cause overheating, moisture, and related fungal and bacterial diseases, and ultimately death.
  • Also, I advise against cedar wood - it has insect repellent properties.

If purchasing an insect or bee house.....

Some solitary bee houses are especially made to allow ease of cleaning, with replaceable carboard tubes. This is helpful because it helps to preserve hygiene and protect from mites (see below), which can be a problem for mason bees. 

You will also find that some are designed to look attractive in the garden, and these do make nice gifts, but I recommend that you do not sacrifice a sturdy, quality build, and think about the bee's needs first. 

Mason bees and pollen mites

Red mason bees mating, but unfortunately infested with mites all over their bodies.  Side view.Red mason bees mating, but unfortunately infested with mites.

Pollen mites pass from bee to bee, dropping off onto flowers, hitching a lift on the next bee, and so being carried to different nest cells, where they breed and eat the bee larvae's food.  It is also possible they eat the bee larvae itself.

As mentioned above, you can make or purchase protective card inserts for use inside a mason bee house, and they are recommended, as they are shown to reduce pollen mite infestation. 


(1)Bosch J, Kemp WP. 2001. How to manage the blue orchard
bee. Sustainable Agricultural Network, Beltsville Maryland.
98 pp.

(2) Bee Population Returns and Cherry Yields in an Orchard Pollinated with Osmia lignaria (Hymenoptera: Megachilidae)
Jordi Bosch, William P. Kemp, and Glen E. Trostle
Journal of Economic Entomology Apr 2006 : Vol. 99, Issue 2, pg(s) 408- 413
Abstract & References

Learn more about solitary bees.

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