Mason Bees
Please be aware we are not specialists in regard to mason bees and this page contains a collection of information taken from various sources on the internet, combined with our own thoughts on how they can be best dealt with.
The page is under construction and will change & grow over time as we learn more.
About Mason Bees
Mason bees or mortar bees is the common name for a species of bees in the genus Osmia of the family Megachilidae.
In the UK there are about 20 species that belong to this group of which the most common is the red mason bee (Osmia rufa).
Mason bees are common throughout most of the UK, all be it in declining numbers.
They get their common name from their habit of nesting in cavities in limestone mortar joints on south facing walls of older properties.
But as per most fauna they adapt to our modern way of building and can be found in window frame surrounds, beneath roof tiles, behind guttering and many other locations.
Nor do they stick to the south facing aspect, which in changing times is becoming too warm a location and they now seem to prefer a more westerly aspect.
In the early part of their flying season males will be found on flowers sapping up nectar which provides them the energy to fly from nest to nest within their local vicinity looking for the opportunity to mate with a female.
Females will be seen toing & froing from their nesting site carrying pollen or mud which they use to create individually sealed chambers for each of the eggs they lay.
A good environmentally sensible solution for Mason Bees
This page is aimed at identifying a good environmentally sensible solution to resolving any problems that Mason bees may be causing.
This post has come about after I recently attended a short webinar for pest controllers “Bees in places they ought not be” and one of the species mentioned was the mortar bee which is another name for something I would usually refer to as a Mason bee.
The solution advised was in my opinion incorrect or rather, overly simplified for the stupid, and led me to look into it further as this advise seems to be widely offered as being the solution for Mason bees pretty much everywhere you look.
Knowing what little I do about the lifecycle of the Mason bee I don’t believe it is the best advice at all, especially considering the decline of native bee species within the UK.
Discussing what to do with Mason bees with an expert
So I wrote an email to someone I figured would be an expert on the matter being that they supply mason bees and nests, in the hope that between us we could identify the most appropriate way to deal with Mortar bees. Unfortunately I have not heard back.
So what you find below is purely my own thoughts on the matter without the benefit of any experts opinion unfortunately.
Repointing walls to prevent Mason bees
In the webinar the solution to mortar bee issues was to repoint the wall, this is something I am not convinced upon at all, or certainly not without carrying out additional work prior to.
In the very short Q&A section at the end of the webinar I asked whether this would kill off the next generation of mortar bees. I was told otherwise and that rather than repointing the wall the holes that were not sealed with a capping could be filled in.
So at least on being questioned they clarified that only holes that had not been caped by the Mason bees should be filled.
Unfortunately I did not get the opportunity to ask further questions on this subject, but one of them would have been to ask them with what should the holes be filled and whether this was a structural or aesthetical solution to the problem.
What struck me was that clearly all these people offering this kind of advice are doing so because its easy to say and makes it appear that they have a solution to a problem or perceived problem. But who has actually thought it through?
Mason bees are they a problem?
Do mason bees sting
In short, generally speaking probably not.
Mason bees are noted for their lack of aggression and are happy to live and forage in close proximity to humans so you are very unlikely to be stung.
Like pretty much all female bees a Mason bee can sting – but in the rare case one does sting you it causes hardly any discomfort and is nothing like a wasp or bee sting, or so I have heard – I have never been stung by one and happily handle them with bare hands.
So what is the cause for concern?
Do mason bees cause structural damage
The fact that mason bees can make burrows or utilise gaps and crevices in weak mortar?
Does this cause cause possible long-term damage to a building?
I’m not entirely sure it depends upon the building and what’s going on.
But if the answer is yes before a solution is finalised we should look at the lifecycle of a Mason bee.
Mason bee lifecycle
Understanding the mason bee life cycle is important to understand so that an environmentally friendly way of dealing with them can be found.
The female mason bee will on average live for about 20 days (just short of 3 weeks).
During this time she will usually create between two and four nests (although some may make as many as seven).
Each of these nests will have on average 5 chambers with an individual pollen store and egg in each chamber.
The first few chambers of a Mason bees nesting burrow will be for the female eggs, and the last two or three chambers will be for the males (unfertilised) eggs.
This allows the males to emerge first in the spring once temperatures are over 10⁰C, allowing them to fly constantly between the various nests nearby and be the first to meet and mate with the emerging females.
As soon as the female emerges she will mate.
Unlike the honey bees, mating does not lead to the end of life for the drone (male bee), so the male bees can mate with multiple females during the season.
But just like honey bee females she will mate with multiple drones storing the sperm in a sac called the spermatheca.
Most of this mating will happen on the first day post-emergence.
Once mated she will move away from the area for a couple of days to rest up & further mature, before
Having mated the female mason bee will leave the nest site for two to five days resting up in crevasses of bark or shelter amongst clusters of males. During this pre-nesting period their ovaries mature, and they complete their development.
After a few days and on completion of their development most females will head back towards the nesting site they emerged from. But this is not always so, some females will disperse to find new sites (this dispersal rate generally increases during periods of heavier rain and wind). Interestingly dispersal rates are also generally higher when floral resource availability is low relative to the population size, which ought to be considered if changing planting schemes or introducing honey bee colonies to an area. In the case of released bees, how they have been released has also been seen to influences the dispersal rate, with greater rates of dispersal observed in populations released as loose cocoons rather than as in nest tubes.
Once they decide where to build their first nests, the female mason bee they mark the entrance with a glandular odour and fly an orientation flight to memorise the location.
She will then start to build a new nest constructing an inner most wall at the very end of the chamber.
After building this end wall they initially return, entering the nest headfirst and regurgitate the nectar they collected onto the mud partition. They will then back out of the nest, turn around, and back into the nest to deposit pollen by scraping it off of their abdominal scopa with their hind legs.
Each bee repeats this process an average of 25 times to build up a sufficiently large pollen provision for the larvae. The number of trips varies depending on the sex of the egg, with male offspring requiring smaller provisions than females.
On the final provisions trip she does not collect pollen, only nectar which she then regurgitates onto the pollen provision’s surface. She then turns around and lays an egg (oviposits) on the pile of pollen.
She will then collect mud to create another partition and seal the brood cell chamber.
She will repeat this process creating new chambers until the burrow length is full of chambers, at which point she will finish of with a thick mud cap/plug spaced away from the last cell partition creating a vestibule space to disguise the entrance and to keep predators and weather at bay.
Each female mason bee will build several nests, most commonly between 2 and 4 over their short (20 day) lifetime, laying between one and two eggs per day with males making up a greater proportion of the offspring in the later nests as the her foraging efficiency declines.
Each of the pollen stores require approximately 25 loads of pollen, with approximately 75 flower visits per pollen load.
Doing the maths you can see that over her lifetime she will make over 28 thousand floral visits.
But not only do they need to carry out pollen collection they also need to collect mud (it takes approximately ten trips to build the nest’s innermost walls), lay eggs, protect their nests against parasites, and find and inspect new nest locations.
So these girls really are busy.
The larvae hatch after a few days (3 days), and will feed on the stored pollen and nectar for the next 10 days. They will then spin a cocoon and pupate within this cocoon inside of the chamber in the nesting tube.
Towards the end of summer, the bee transforms into the adult stage called an imago but will remain in the cocoon throughout the winter.
The mason bees will then emerge in the early spring once temperatures have risen to being consistently above 10⁰C.
The males have shorter lives.
Why the wall should not just be repointed
Because of the life cycle of the Mason bee it’s very difficult to identify the correct time to carry out any pointing work without killing off the next generation. It was suggested that these chambers could be safely repointed without detriment to the mason bee population, I would suggest this is not the case.
Yes individual holes could be filled later in the year after all the flying bees have disappeared, but this is not a very practical solution, and how many of the burrows would be open & visible as a percentage of the total number of burrows in the wall?
As for repointing this should definitely not be done before further action has taken place as repointing involves the removal of mortar for a depth of a minimum of 1.5 x the width of the joint, with a minimum depth of 25mm but probably more if its being repointed for structural stability.
The SwarmCatcher solution to Mason bees
We believe that this should be remedied overtime and the process relies upon a few different steps.
Initially alternative nesting sites for the bees need to be created, each location may require different ideas dependent upon the structures and the numbers of bees.
If for example they are in a thick masonry wall then large framed sections of 75mm to 150mm (ideally 150mm) thick mortar with holes about 1/4 to 3/8 inch in diameter (slightly larger than the diameter of the females body) drilled into the mortar in a slightly upward fashion (to prevent water collection being an issue), should be made up and placed into position.
The holes should only be open at the entry point, not at the other end.
Alternatively mason bees will use wood as long its not cedar or pressure treated.
If relocating mason bees they should ideally be relocated to a dry, protected site, preferably with east or southeast exposure.
If a paper straw liner is inserted into each hole the cocoons can be retrieved in the autumn and then washed to get rid of most mites and diseases and the cocoons can be safely stored over winter for release next year.
Then the wall should be covered with a grid pattern of wires approximately 1m x 1m in size, and sections of insect proof netting attached to each square.
When the bees emerge they can find there way out of the netting between the section spaces, but will find it difficult to find their way back to the nesting point, finding the alternative framed nesting sites.
The site should be monitored to ensure the netting is doing what its meant to, and if so, will allow for the wall to be repointed at the end of summer.
Bee Hotels
Mason bees can often be found in solitary bee hotels, but this is not a guarantee as they have fairly specific environmental demands requiring close proximity to a mix of certain food sources. So these hotels are good for bees utilising an area but not necessarily good at attracting them into an area.
Parasites, Predators & Pathogen Control for Mason Bees
Several parasites, predators and pathogens can injure or kill mason bees.
Small Chalcid wasps pierce mason bee cocoons and lay eggs in them. These wasps emerge later than the mason bee, so you can decrease the risk by taking down mason bee nests once they have filled the opening. The Krombein Mite, Chaetodactylus krombeini, reproduces within a mason bee cell and feeds on the pollen stored for the young bee larva.
By the end of summer, the cell is packed with empty pollen grains and thousands of mites.
You can rid of the mites by cleaning the cocoons between October and December. Adults are fully developed at this time and can withstand this process.
Use water no warmer than 10⁰C so as not to ‘wake up’ the bees.
First, soak cocoons in cool water to soften and remove mud. Using a sieve, gently roll and move cocoons through the water.
Discard debris.
Then soak them, no more than 10 minutes, in cool water with a mixture of .05 percent bleach (1 tablespoon bleach per 1 gallon water) to kill adhering bacteria, fungi and most mites.
Rinse well under cool water to remove all traces of bleach.
Dry on a clean paper towel for 1 hour.
Sort and discard damaged, diseased or parasitized cocoons.
Put clean, air-dried cocoons in a small container with air holes and store in the refrigerator.
References
The Orchard Mason Bee, Brian Griffin, 1999, Knox Cellar Publishing.
Pollination with Mason Bees, Dr. Margriet Dogterom, 2002, Beediverse Books
All About Mason Bees, DVD, Dr. Margriet Dogterom, 2007, Beediverse
How to Manage the Blue Orchard Bee, Jordi Bosch and William Kemp, 2001, Sustainable Agriculture
Network
Orchard Mason Bee, Washington State University publication, PLS -112
