The Secret World of Mason Bees

By Charlie Narewski Scullion

After solstice, my mind focuses on the coming of spring -  having spotted Lesser Celandines and violets already by late December, it's not so hard to imagine.

One of my jobs to prepare for spring is to open up the observation drawer and nesting tubes on my solitary bee nesting box. We got the box three years ago along with a tube of 30 Red Mason Bee cocoons; brown, soft and papery. The males are smaller than the females, as are their cocoons. This year I harvested 29 females and 59 males.

Red Mason Bees (Osmia bicornis) are cute, stingless solitary bees, with fluffy blonde body hair, a ginger bottom and a broad black head. They nest naturally in the crumbling mortar of wall cavities and insect burrows in wood. They are distinctive and beautiful, but like so many insects they are in serious decline.

Around March we attach the nesting box, with its cardboard nesting tube and wood/plexiglass observation drawer onto our allotment shed. Essentially, it's south facing. We place the cocoons into a hatching box and around April (a little later in cold years like last year), they chew a hole through the cocoon and emerge from holes in the box. The little males hatch first, ready to mate with the females when they emerge. Immediately, the females begin to work on their nest. They enter the tubes, adding a thin wall of mud then  packing it with a bed of bright yellow pollen onto which they lay a single egg. They close this off with another mud wall and continue this process making chambers all the way down the tube. In the observation drawer you can see this happening, and sometimes you will find a female in there, sheltering from the weather.

The eggs hatch and the larvae begin to eat the pollen, growing bigger until they fill their chamber, at which point they spin a paper cocoon around themselves and the cycle is complete for another year. Some chambers are clepto-parasitized by Houdini Flies (Cacoxenus indagator), who sneak in to lay their eggs before the chamber is sealed off by the mother bee. They outcompete the bee larva for the pollen in the store.

Red Mason Bees will typically seek pollen within a tiny 30m radius of their nesting site, so it is vitally important to support them with as much pollen as possible. They are thought to be as much as 200 times better at pollinating than honey bees, so there is a mutual benefit to their presence in the garden. On our wildlife-focused allotment the early apple and cherry flowers are ideal, as are the early dandelions, forget-me-nots and willow, followed a whole host of wildflowers such as buttercups and spurges.

They are often followed around July by Leafcutter Bees (Megachile centucularis) which use a similar diameter of hole. They have a similar life cycle, but instead of mud they line their nests with chewed up green leaves - the cocoons within look similar to the Red Masons'. We attach a little gauze over the capped-off Masons' tubes as the Leafcutters will sometimes excavate their tubes, even when other empty ones are available. On our allotment, the Leafcutters collect their pollen from summer blooms such as the dog-roses, sunflowers, mints, ash and lavender.

All gardens are work-in-progress and as I lay on the grass last summer looking up, I noticed that the canopy tips were almost touching and that our plot, which used to be a baking hot suntrap, was beginning to be more of a forest glade. This loss of light had a noticeable effect on the abundance of flowers on the plot last year and our cocoon harvest showed that for the first time, no Leafcutters came - what was still a reasonable south-facing plot in April was too shady by July. So at the end of summer we set to work opening up the canopy and letting much more southern light through. We did this before leaf-drop so we could gauge how much light difference we were making. Now the plot is sunny for most of the day again (which the four-year-old grape vine below our nesting box should enjoy too, and with luck our endeavours might bring our first grapes).

Being invested in the life cycle of a declining native insect is hugely rewarding and to see that first little package of bright yellow pollen makes the heart sing with excitement. Go to www.masonbees.co.uk to see images of the type of homes you can provide for them; sadly this company have recently gone bust, but you can make your own boxes or cautiously source alternative accessories elsewhere online, or just support these wild species by filling your garden with pollen-and-nectar-rich flowers! Happy earliest visions of spring!

Text and photos © Charles Narewski Scullion, 2025