Red Mason Bee (Osmia rufa)-life cycle part (1)

March 4, 2012

in Red mason solitary bees, Solitary Bee Observation Box

The Red Mason Bee (Osmia bicornis) one of many solitary bees

Unlike bumblebees and honeybees, which live in a social nest together as a family and work as such for the benefit of their siblings, Red Mason bees (Osmia rufa) are just one species in a much larger family of numerous solitary bee species. In the UK there are more than 250 different species of wild native bees! Solitary bees such as O. rufa, work alone, searching for suitable cavities in which to create cells, provision them and lay their eggs.

Updated 2016 life cycle video


To survive and prosper these bees need three different habitats within a relative small area as they do not travel long distances unlike bumblebees and honey bees. Such habitats would have to provide an area for food foraging, an area that has a nest site and lastly but just as important is an area that can provide material for their nests, i.e. mud. The further away a source of mud is, the more likelihood that her egg will be parasitised by a wasp as it remains vulnerable without the mud walls to seal it in.


Males and females overwinter as adults in their silken cocoons. As temperatures rise and remain above a certain temperature for a few days, the males are the first to wake from hibernation and chew their way out of their cocoons, as shown below.


Male mason bees

Male bees are smaller than the females and require less food and cell space. They are laid near the entrance of the cavity as they appear a few days before the females, who remain safely inside the cavity a little longer, further towards the rear. The female, just before she lays an egg, if she wants a female to be produced, fertilises the egg from the sperm she has stored. from her mating. If she wants a male, she simply bypasses the sperm thereby laying an unfertilised egg which becomes a male. The males have a small but dense tuft of light coloured hair on the front of their heads, with females having much darker hairs on their much larger heads. Emerging earlier than the females gives them time to feed, fight in a free for all near to nest cavities eagerly waiting to pounce upon the emerging virgin females. Like all male bees, they have no sting so grapple with other males for the best vantage points and competition is fierce.


A red mason bee feeding up before the females emerge.

Nest sites

In the wild, O.rufa is catholic in its choice of nest sites. Cavities in sunny sandy banks, fallen dead logs, small holes in mature hedgerows bushes, hollow plant stems such as hogweed, beetle borings in dead wood.and unusual site such as a hole in a flint stone, a lock and even a fife flute in a garden shed! I have seen them use a nail hole in an old wooden fence posts and a leaf cuter bee using an old park picnic table where a bolt had fell out! They will also use pre existing holes in the mortar between brick walls, usually made by nails or holes in soft eroding. Although some people may believe that the mason bees get their name from this practice, actually it is because they use mud to build walls, Do they get the name mason bee from this practice or because they use mud as a building material? I have been called out numerous times because people have seen wasps or bees making holes in their walls and ‘digging’ out the mortar, (which is not true they just cannot do this physically).  Once she has laid her eggs she dies, living between 10-12 hard working weeks, has only one generation a year, never seeing her offspring, which are hopefully alive and well in their cells. They are spring bees and their activity begins in the UK, from mid March onwards, depending upon where you live.

Updated 2016 video

To this female red (ginger!) mason bee this wooden block is just as good as a beetle bore hole in a dead log! After inspecting the cavity and if necessary clearing it out, she will seal the back end of the cavity with mud to stop any mites, predators or other undesireables etc., from entering.


By releasing pheromones, females attract males who embrace the females by standing on her back and engaging in a series of behaviours to persuade her to mate with him. During the courtship behaviour, the female assess the male and being larger and stronger, she can reject the male by physically pushing him off her back. By eliciting this behaviour, the female evaluates the male’s suitability for mating. Recent research has found that females select males using four traits:

Male odour – accepted males had a higher concentration of certain chemical compounds over the rejected males;

Vibrations produced by males – using their thorax flight muscles, males producing longer or more intense bursts were preferred which may be a sign that the male is active, healthy, and has strength and vigour over weaker or older males;

Size of the male  – average body size males attracted females, large bodied males may be strong but over a certain size can become a disadvantage having negative impacts on other determining factors;

Genetic relatedness – females preferred males from their own populations.

Females only mate the once, but males may still try to mount her. Males are competing with an ever diminishing number of virgin females. One mating gives the female enough sperm, which she stores in a sperm sac, the spermatheca, to fertilise her eggs, usually about 40 eggs. After one or more matings, the now tatty, tired and worn out males die, leaving the fertilised females to get down to the serious business of finding, provisioning and making nest cells.

Red Mason bee (Osmia rufa) leaving the cavity to collect more pollen, nectar or mud

When my children were younger, we used to watch the mason bees entering the nest cavities, covered in pollen or carrying mud in the jaws. Each nest cavity was numbered and we made note as to how many times a particular nest hole was visited and what the females were carrying! Mud was always first whenever a new cavity was to be used. It can take between 8-15 visits to provision a cell which can be done over a few days depending upon weather conditions and the distance to the pollen source and quantity.

Red Mason bee (Osmia rufa) guarding her nest cavity from intruders

Acceptance and orientation

After a thorough inspection and a decision that the cavity is suitable the female will, like female birds when deciding upon a suitable nest box, roost overnight in it. On cool days and after a hard days work, you can see the female sitting on guard just inside the the cavity entrance, ready to repel intruders, including other females looking for a suitable nest site. Once accepted and as soon as it warm up, she will need to feed upon nectar which she has already scouted out, hopefully growing nearby and certainly within a few 100 metres or she will not accept the nest site. After leaving, like bumblebees, she performs an orientation flight around the nest cavity, making a mental note of certain details around the nest hole and landmarks next to the nest site. Like taking mental photographs. She flies close to the entrance hole and slowly zig zags, moves back and forth, up and down like figures of 8. Then she gradually widens the distance from herself to the nest entrance continuing with these actions. With solitary bee boxes, there could well be several females learning the exact hole in which to make their cells, at the same time and one using the hole above or below its next door neighbour! She has to find her way back to her own nest cavity and not her next door neighbours or sure enough a squabble will ensue!

Supersedure and competition

I mentioned a squabble above. Scientists call this supersedure, the act of taking over by a second individual of the same or of another species the cavity or boring that has been partially provisioned by a first individual. In many cases the first bee may have abandoned the nest, fallen prey to a predator or parasite or simply died before completing her cavity, as in the case below.


The cavity above was completed. Several days of poor weather may have killed the female in this case, or she died of old age.


Red Mason bee (Osmia rufa) backing into a partially completed cell to deposit more pollen

Notice she does not have pollen baskets like bumblebees and honeybees. Instead she collects pollen, rather inefficiently, all over her hairy body. This makes her an efficient pollinator of numerous fruits and flowers as the pollen is more easily dusted off as she visits flowers and also falls off as she grooms herself when moving the pollen underneath her body to a dense pile of bristle like hairs called the scopa. The darker material in this photograph is regurgitated nectar for her offspring. When constructing the individual cell walls she carries mud with her jaws and enters the entrance hole head first. When she is depositing pollen it makes sense for her to reverse into the cavity making it easier to brush of the pollen on her underside rear scopa.

Red Mason bee (Osmia rufa) cells complete with pollen, nectar, egg and capped either end by mud

Cell provisioning

After inspection, she seals the end of the chosen cavity against unwanted guests, by using damp mud which hardens. Other solitary bees may use leaves, such as the leaf cutters. She will now to make a cell. First comes the inner cell wall. Then she lays a marker of mud where the next cell will be made. The resulting cell is then provisioned with pollen and a little regurgitated nectar. She always lays her egg on top of this food pile. If she laid it on the floor the helpless grub is unable to crawl, just like a newborn human baby cannot crawl, to reach the majority of the food pile and it would starve.

Cell construction
But why go through all that trouble to make individual cells? Cells have several purposes. They offer a degree of protecting the individual larvae from predators and parasites, ensures the larvae have an adequate amount of provisions, ensure that the egg is not knocked off the food pile by a neighbouring larvae as the larvae grow and move and although not applicable in the case of solitary bees, solitary wasp larvae would cannibalise their neighbours!


Red Mason bee larvae in bottom cells with eggs in top cells. It could be the offspring of the same female or another female

Within the cell the larvae consume the pollen and nectar and are able to move, albeit sluggishly around the cell.

Leaf cutter bee cells, with opportunistic Red Mason bee (Osmia rufa) cell, provisions and egg

Leaf cutter bee cells, with a Red Mason bee (O. rufa) cell, provisions and egg on the left. The leaf cutter female may have been killed or died and did not complete her cavity leaving it unsealed which was soon used by a opportunistic Red Mason bee (O. rufa). Being near the entrance to the cavity, is this a male or a female egg she has laid? When the female has completed the cavity she finished off the last cell, leaves a gap called the vesicular cell and plugs up the cavity entrance with a thicker mud wall. Sometimes she is not around to undertake this task and another female or another species may try to squeeze in a a cell of her own, provision it and lay an egg. The walls of these cells are not as thick, there being not enough room in some cases and are easily attacked by birds, such as wood peckers and great tits, who eat the last grub laid. Definitely a case of last in, first out! The vesicular cell may well be another defensive barrier against pests and predators.

Red Mason be (Osmia rufa) cells containing cocoons, with female depositing pollen just to their right

Even though this tube has been blocked by a small sponge, this red mason bee female was taking no chances and still made a mud wall against potential pests. When the larvae are fully grown having consumed all the food provisions and just prior to entering the next stage in their life, they defecate, allowing their bodies to be free from any waste products that has the potential to cause problems before they enter the next stage of their life. The larvae spin a brown tough silken cocoon inside of which they enter the pupal stage, turning to adults inside the cocoon and spend the winter as adults inside the cocoon, awaiting spring when they wake up and chew their way out of the cocoon.

See my new Registered Design award winning solitary bee box and bumblebee nest box both of which are radical, practical and educational, offering them a safer nesting environment in which you can observe the bees. Great for schools!

For more information about solitary bees and wasps visit BWARS

For more bumblebee information and to help save bumblebees join the Bumblebee Conservation Trust at Stirling University

Refs: Besides my own observations, photographs and experiences;

Conrad et al, (2010), “Female choice in the Red Mason bee (Osmia rufa)” J Exp Biol 213, 4065-4073

Krombein, K V, (1967 ) “Trap-nesting wasps and bees: life histories, nests and associates” Smithsonian Press, Washington DC, USA

O’Tool, C .(2000) “The Red Mason Bee”,Osmia Publications, Banbury.

Raw, A. (1972) ” The biology of the solitary bee Osmia Rufa ”  Transactions of the Royal Entomological Society of London, Vol. 124 Issue 3, pages 213-229






{ 13 comments… read them below or add one }

Brian Prudden June 14, 2015 at 12:53 pm

Hi Team,

I regularly make nesting tubes from last seasons “teasle” growth & cut them to length accordingly to fit an old plastic lemonade bottle cut to size to allow rain drainage. This method seems to work incredibly well to the extent that all tubes are fully utilised within a season, but we are now realising that this year the majority have not hatched out at all, when cut open the fully laden tube is still as initially laid with egg sat on pollen food source. Any ideas as to why?, are the tubes getting overheated as they are facing south & do not have any shade? They are not getting water logged. Please indicate your thoughts by return.




nurturingnature June 15, 2015 at 8:41 pm

Hi Brian, It not a team its just me!! It would be easier for me to see photos of what exactly you have made and the contents thereof! Cheers, George


Kev Lamb May 7, 2016 at 1:50 pm

Hi George,

This is my second year with Mason Bees using the bee house, the first year I had approx 5 tubes used where as the second year I had approx 30 used, this years bees are hatched and mating at the moment but I am concerned about the bees reusing the tubes as the house I originally started with is bamboo and I wanted to clean this as it cannot be taken apart, I have purchased a new house which can be de constructed for ease of cleaning.

I assume as the bees are now still popping in and out of the old house I would be best to leave this until Sept/Oct and try and remove the cocoons from the old house and destroy it as I would need to saw the bamboo to remove the cocoons.

Just wanted to know if the bees will actually reuse the tubes that were already used or will they seek out unused one?


nurturingnature May 10, 2016 at 8:31 pm

You have nothing to loose Kev by putting the new doc right next to the old one. This may persuade a few to use the new one. I would do as you suggest though re the old one. You could also completely block the old one, and loose a few bees but only those ones that are inside their cell without the sealed mud wall. This may cause the females a little stress and hopefully force them to restart in the new one. When they have take out the old one and put it somewhere else out of harms way and from mice. Cheers George


Kev Lamb May 11, 2016 at 5:37 pm

Many thanks George, it looks like they are now using both houses so I will look to remove the cocoons in Sept/Oct from the bamboo one and then replace that with another one that can be opened and cleaned.

On another note I have seen a few Cacoxenus indagator around the houses….



nurturingnature May 11, 2016 at 9:48 pm

So have I, 7 over 2 days! Cheers, George


Caroline March 16, 2017 at 8:27 pm

Hi George , when should I wake my Mason’s up in Scotland ?


nurturingnature March 16, 2017 at 8:41 pm

What type of nest box do you have Caroline?


Caroline March 17, 2017 at 3:11 am

Hi thanks for the fast response, I have the wooden hive that u can take apart . Also the green one with the straws that came with the bees and I have lots of other wood ones with holes some have wax paper inside and even a small one with cut aquarium pipe . I saw my first bee other than a bumblebee , last year A bee with a horn lol never seen anything like it , so the years research began. Lots of reading and your lovely videos have kept me busy .


Louise Godfrey May 5, 2017 at 8:30 pm


My friend has Red Mason Bees in her bug box, it is a little house with bamboo tubes, they started nesting last year, and there is lots of activity this year too. My question is, should she replace the tubes, or will the bees just naturally use ones that are vacated? If she needs to replace them, when should she do this? A really informative site, and I read the questions, but was still not sure. Many thanks. Louise


nurturingnature May 8, 2017 at 10:05 am
nurturingnature July 7, 2013 at 6:43 pm

Assuming they are red mason bee larvae, you should be able to feed them with freshly collected pollen. I would keep them in a container, darkened and with air, though not holes larger then 1mm as there are specialist solitary wasps than will seek them out to lay eggs into them -(. If you are lucky and they survive they may spin a silk cocoon each and you have your next generation. Cheers George


nurturingnature May 29, 2014 at 3:51 pm

Thanks for sharing 🙂 Cheers George


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