Red Mason Bee Encounters

Until very recently, I didn’t know much about red mason bees, apart from them being employed commercially for apple pollination. I didn’t even know that this solitary bee can be found in Scotland as far north as Nairnshire. It was on World Bee Day (20^th May) that I found four eggs under the roof of hive six.

Louise’s little three- year- old daughter called to her mummy that there was a honey bee on the climbing frame at Brodie castle in Moray on World Bee Day. It’s quite remarkable that Charlotte could recognise and correctly identify a honey bee. Louise was delighted. She looked around and discovered another little bee on the climbing frame and identified this as a red mason bee (Osmia bicornis) using a nature app. You can see from the picture above that this was a male mason bee by his long antennae.

Coincidentally, on the same day I was having a tricky time checking my colonies for swarm prep so I didn’t spend too much time pondering when I found the four little mud-walled cells packed with pale greenish yellow pollen on top of the crown board. The weather was not great and I had to get all the colonies inspected before the next shower so had no time to collect my camera. What I did was carefully remove the mud cells with my thinnest hive tool, the J tool, and place them all in a honey comb container and leave them on my desk indoors. I saw a yellowish fluffy bee flying toward the hive when I was replacing the roof but I couldn’t instantly identify it. It was flying fast and had a similar posture to a leaf cutter bee with a slightly up-curved abdomen in flight. I know leaf cutters bees. I’ve watched one pull a leaf into the hole on the side of a metal tube. Indeed, mason and leafcutter bees belong to the same family of Megachile. Mason bees are in the genus osmia, while mason bees belong to the genus megachile.

I was fascinated to see that the eggs were ‘glued’ by their tail (dorsal) ends to the lumps of pollen that mother bee had carefully collected from around 1875 flowers (for each egg) —according to one online source. How did I know that this was the tail end? Well, when the eggs hatched a few days later the larvae bent over to feed on the pollen ball while still safely anchored to it by their rear ends. Observing closely under my magnifying glass I could see them move and feed. A couple of days later they fell off the pollen balls and moved around a bit.

On day two, I was disappointed to see that one larva was discoloured and grey looking. I thought it was dying. I’m used to checking for pearly white shiny healthy honey bee larvae. However, I needn’t have worried because the mason bee larvae all changed colour when they started feeding but they are still healthy, moist, shiny and defecating. One larva is larger than the others and was the first to hatch. I might have called him Charlie but resisted the temptation to anthropomorphise these little insects. By the fifth day, three eggs had hatched and the fourth appeared to eclose on the seventh day but failed to thrive and dried up over the next few days.

So, what next? I started researching to find out more about these insects. Firstly, I wanted to know what flowers mother bee had been foraging on, and if I could feed them pollen collected from my honey bee pollen that fell on the bottom boards through the open mesh floors. Knowing that red mason bees are used to pollinate apple orchards and oil seed rape fields, I felt sure that I would find OSR pollen grains in the sample taken from one of the pollen balls. However, on reflection that was not likely since these solitary bees do not forage far from home.  It was a bit late for apples. This exercise took me the best part of a day to achieve as I searched through pollen data bases and textbooks. Making the pollen slide was easy but identifying the pollen grains was not. I discovered that each grain was around 28µm in size and triangular in shape. There were three apertures and a distinctive pattern on the surface that convinced me that this was birch pollen.
The timely arrival of June’s BBKA News (The British Beekeepers’ Association Magazine and top-class source of information) was my salvation. I couldn’t believe my good fortune to find the reference to a piece of recent research that looked at the forage preferences of managed pollinators such as bumble bees, honey bees and solitary bees for particular crops. https://doi.org/10.1016/j.agee.2025.109518

I discovered that mason bees do not actually deliberately collect apple and OSR pollen because they prefer pollen from the beech and oak families. When they visit apples and OSR they are probably attracted by the nectar, but, since pollen is collected on their body hairs they collect it by default and are good pollinators because of it. To discover that mason bees have a wider taste and preference for pollen reassured me that it would not harm them to have a varied diet (just like honey bees) so I added some pollen grains that I’d collected recently from fallen pollen loads in my apiary. They are all chomping away busily on those I’m pleased to report.

But what next? I checked out Bug Life and Bee Life  https://www.buglife.org.uk/bugs/solitary-bee-week/  https://www.beelife.org/mason-bees/

to learn more about the life cycle of mason bees so that I can care for them appropriately. I discovered that they will pupate in a few weeks and spend the winter in cosy cocoons to emerge next spring. I learned how to make mason bee houses and that many people spend a lot of time on conservation projects to increase these pollinators. You can actually buy red mason bees online which is not a surprise really as they are used commercially. I’m presuming that the spread to Scotland may be through this avenue rather than through a natural migration north.

It is not just forage that is in short supply, it is reduced safe nesting sites that is the key problem which is why setting up mason bee houses is so useful— and popular. There are ways to do this hygienically with bee welfare in mind. This video gives good advice on how to look after mason bees over winter in a shed and how to set up safe places in spring for them to emerge from and come back to nest in themselves. https://www.youtube.com/watch?v=3eDGDYGHXLk

I will keep you posted on progress.